Fernando Torres-Canas
email: fernando.torres@crpp.cnrs.fr

Ahmed Bentaleb

Marie Fllmer

Julien Roman

Wilfrid Neri

Isabelle Ly

Alain Derré

Philippe Poulin
email: philippe.poulin@crpp.cnrs.fr

Marie Fӧllmer

Improved structure and highly conductive lignin-carbon fibers through graphene oxide liquid crystal

come

Introduction

Highly conductive carbon fibers (CFs) are used in diverse applications from reinforcement of composites to microelectrodes used in bio-electrochemical applications.

They are lighter and chemically more stable than metallic fibers for electrical applications.

The small size, easy to weave in wearable devices, light weight and flexibility make these fibers promising candidates to meet the demands of next-generation photovoltaic cells [START_REF] Peng | Electrochromatic carbon nanotube/polydiacetylene nanocomposite fibres[END_REF][START_REF] Pan | Efficient dye-sensitized photovoltaic wires based on an organic redox electrolyte[END_REF][START_REF] Fu | A fiber-shaped solar cell showing a record power conversion efficiency of 10%[END_REF],

smart textiles [START_REF] Di | Carbon-Nanotube Fibers for Wearable Devices and Smart Textiles[END_REF], actuators [START_REF] Foroughi | Torsional carbon nanotube artificial muscles[END_REF][START_REF] Cheng | Graphene fibers with predetermined deformation as Moisture-triggered actuators and robots[END_REF], batteries [START_REF] Ren | Twisting carbon nanotube fibers for both wire-shaped micro-supercapacitor and micro-battery[END_REF][START_REF] Zhang | Super-stretchy lithium-ion battery based on carbon nanotube fiber[END_REF][START_REF] Liu | Conductive carbon nanofiber interpenetrated graphene architecture for ultra-stable sodium ion battery[END_REF], electrochemical sensors [START_REF] Zhao | Carbon Nanotube Thread Electrochemical Cell: Detection of Heavy Metals[END_REF], biofuel cells electrodes [START_REF] Delord | Carbon nanotube fiber mats for microbial fuel cell electrodes[END_REF][START_REF] Gao | Engineering hybrid nanotube wires for high-power biofuel cells[END_REF], and supercapacitors [START_REF] Yu | Scalable synthesis of hierarchically structured carbon nanotube-graphene fibres for capacitive energy storage[END_REF][START_REF] Sun | A Bamboo-Inspired Nanostructure Design for Flexible, Foldable, and Twistable Energy Storage Devices[END_REF][START_REF] Wang | Conductive Carbon Microfibers Derived from Wet-Spun Lignin/Nanocellulose Hydrogels[END_REF][START_REF] Yu | Transforming Pristine Carbon Fiber Tows into High Performance Solid-State Fiber Supercapacitors[END_REF][START_REF] Yu | Emergence of fiber supercapacitors[END_REF]. CFs can be produced from different precursors, such as polyacrylonitrile (PAN), pitch, cellulose (rayon), lignin, carbon nanotubes and graphene [START_REF] Frank | Carbon Fibers : Precursor Systems, Processing, Structure, and Properties[END_REF][START_REF] Liu | Unconventional supercapacitors from nanocarbon-based electrode materials to device configurations[END_REF][START_REF] Meng | Graphene-Based Fibers: A Review[END_REF]. To date, commercial CFs are essentially based on PAN, which is derived from petroleum. The precursor represents half the cost of manufacturing the final product of CFs [START_REF] Frank | Carbon Fibers : Precursor Systems, Processing, Structure, and Properties[END_REF][START_REF] Ogale | Recent advances in carbon fibers derived from biobased precursors[END_REF]. In addition to its high cost, PAN carbonization involves the production of hazardous chemicals. Thus, bio-based and low-cost precursors, such as lignin, are alternatives of fast growing interest. Industrial lignins are currently obtained as co-products from the manufacture of cellulose pulp for paper, as well as from other biomass-based industries. There are various types of lignin depending on their extraction process and purity [START_REF] Olsson | Carbon fibres from precursors produced by dry-jet wet-spinning of kraft lignin blended with kraft pulps[END_REF][START_REF] Vishtal | Challenges in industrial applications of technical lignins[END_REF][START_REF] Fang | Manufacture and application of lignin-based carbon fibers (LCFs) and lignin-based carbon nanofibers (LCNFs)[END_REF]. The main advantages of this precursor are its high carbon content (>60%) and potentially high carbon efficiency after the carbonization steps, the absence of toxic disposal products, such as nitrous gases, during carbonization, as well as its availability [START_REF] Baker | Recent Advances in Low-Cost Carbon Fiber Manufacture from Lignin[END_REF]. Compared to PAN, lignin-based CFs tend to have a larger fiber diameter, and relatively low conductivity, tensile strength and modulus [START_REF] Kadla | Lignin-based carbon fibers for composite fiber applications[END_REF]. The poor properties generally found for lignin-based CFs are the results of a poor graphitic structure. Indeed, lignin is an amorphous and branched polymer in its natural state [START_REF] Ogale | Recent advances in carbon fibers derived from biobased precursors[END_REF]. Therefore, improving the structure of lignin CFs is a critical challenge to make these fibers industrially viable. This is why control of the structure and its relationship to properties during graphitization are the focus of extensive studies [START_REF] Frank | Carbon Fibers : Precursor Systems, Processing, Structure, and Properties[END_REF][START_REF] Baker | Recent Advances in Low-Cost Carbon Fiber Manufacture from Lignin[END_REF]. High-temperature treatments (up to 3000 °C) are generally necessary to obtain better defined graphitic structures [START_REF] Kurban | Turbostratic graphite nanofibres from electrospun solutions of PAN in dimethylsulphoxide[END_REF].

However, graphitization at high temperature is very expensive. This cost has a significant economic impact and should, therefore, be controlled and minimized [START_REF] Faruk | Lignin in polymer composites[END_REF]. Besides, the structure of the CFs depends not only on the precursors but also on the manufacturing conditions (spinning, carbonization, graphitization) [START_REF] Frank | Carbon Fibers : Precursor Systems, Processing, Structure, and Properties[END_REF][START_REF] Ogale | Recent advances in carbon fibers derived from biobased precursors[END_REF][START_REF] Kadla | Lignin-based carbon fibers for composite fiber applications[END_REF][START_REF] Faruk | Lignin in polymer composites[END_REF].

Graphene oxide (GO) is the oxygenated form of a monolayer graphene platelet with strong mechanical properties, chemical functionalization capability, and large surface area [START_REF] Kim | Graphene oxide liquid crystals[END_REF]. GO can be solubilized in water and polar organic solvents, and can form nematic liquid crystalline phases [START_REF] Kim | Graphene oxide liquid crystals[END_REF][START_REF] Xu | Aqueous liquid crystals of graphene oxide[END_REF]. The formation of Graphene Oxide Liquid Crystal (GOLC) offers a unique opportunity for the production of macroscopic, periodic self-assembled materials from oriented graphene sheets [START_REF] Naficy | Graphene oxide dispersions: Tuning rheology to enable fabrication[END_REF]. For example, GO can play a dual role as dopant and soft template for the preparation of polyaniline nanotubes and new layered structures [START_REF] Yang | Layered nanostructures of polyaniline with graphene oxide as the dopant and template[END_REF][START_REF] Rana | Graphene oxide/polyaniline nanostructures: Transformation of 2D sheet to 1D nanotube and in situ reduction[END_REF].

Papkov et al. reports that the graphitic structure of PAN-based nanofibers is improved after carbonization with an addition of a small amount of GO into PAN prior to processing [START_REF] Papkov | Improved graphitic structure of continuous carbon nanofibers via graphene oxide templating[END_REF].

Similarly, Li et al. have prepared highly conductive microfibers based on GO and nanofibrillated cellulose, where GO serves as a template for cellulose carbonization [START_REF] Li | Highly conductive microfi ber of graphene oxide templated carbonization of nanofi brillated cellulose[END_REF].

Huang et al. have found that adding a small amount of GO to glucose or cellulose can significantly alter the morphology of its hydrothermal carbonization product (from spheres to sheets), resulting in more conductive carbon materials with higher degree of carbonization [START_REF] Krishnan | Graphene oxide assisted hydrothermal carbonization of carbon hydrates[END_REF]. Recently, Tran et al. have prepared lignin/GO films for energy-storage systems [START_REF] Tran | Sustainable Energy-Storage Materials from Lignin-Graphene Nanocomposite-Derived Porous Carbon Film[END_REF] and Fӧllmer et al. have shown larger graphitic domains in carbonized films of lignin with the addition of GO [START_REF] Föllmer | Structuration of lignin-graphene oxide based carbon materials through liquid crystallinity[END_REF].

In this paper, we present an original approach that uses GOLC as a templating agent not only to direct the ordering of lignin molecules during the wet-spinning process, but also to accelerate the formation of graphitic structures in fibers at low carbonization temperature.

Furthermore, a complete study of the structural properties of lignin and hybrid lignin/GOLCbased CFs as a function of the carbonization/graphitization temperature is presented and discussed.

Experimental section 2.1 Preparation of lignin and lignin/GOLC fibers:

2.1.1 Lignin dope: Kraft Lignin (KL), without any chemical modification, was dissolved in DMSO at 35 wt. % under magnetic stirring at room temperature. A polyvinyl alcohol (PVA-195 k g. mol -1 ) solution was prepared at 10 wt. % in DMSO at 90 °C under strong stirring.

After cooling to room temperature, both solutions were mixed (14 wt. % KL + 6 wt. % PVA in solution) until they form a homogeneous dope solution.

Lignin/GOLC dope:

Firstly, an aqueous GO solution (Graphenea) at 0.4 wt. % was concentrated to 3-5 wt. % by means of two centrifugation steps (1400 g, 20' + 50440 g, 60') and a solvent exchange was performed from water to DMSO by means of two more centrifugation steps (50440 g, 120') until the desired concentration was obtained (1.6 wt. %).

Afterwards, Kraft Lignin powder was added to the GO solution stirred with a magnetic bar at room temperature for 24 h. Finally, a PVA solution, previously mentioned, was added to the mixture and stirred at room temperature for 30'. The final concentration of solid content inside the fibers, in dried state, was 66 % Lignin, 29 % PVA, 5 % GO.

In this work, a conventional wet-spinning process [START_REF] Mercader | Scalable Process for the Spinning of PVA-Carbon Nanotube Composite Fibers[END_REF][START_REF] Fourne | Synthetic Fibers[END_REF] was chosen for the preparation of the lignin and lignin/GOLC fibers. The prepared spinning solutions were checked for homogeneity by optical microscopy and were continuously injected into a coagulation bath containing isopropanol through a 1-hole spinneret (diameter 300 μm), in which the solutions formed fibers at sufficiently high concentration and PVA content. The obtained fibers were directly washed in isopropanol and dried in-line in an infrared oven at 90 °C.

Fiber characterization

Thermo-stabilization and carbonization process:

During the stabilization process, the precursor fibers were attached to graphite frames and heated to 300 °C in an oven (Nabertherm) with a heating rate of 2 °C.min -1 under an oxidizing atmosphere. Subsequently, a first carbonization process was carried out for all fibers in the same conditions in a homemade oven up to 1000 °C with a heating rate of °C.min -1 until 550 °C, followed by 5 °C.min -1 until 1000 °C in Argon atmosphere. At the end of the step, a plateau of 1h or 10h was employed before return to room temperature. A second carbonization process was accomplished from 1000 °C to 2700 °C in a homemade oven under Argon with a heating rate of 10 °C.min -1 .

X-ray diffraction (XRD):

X-ray measurements were conducted using a Xeuss 2.0 set-up (Xenocs, Sassenage, France) equipped with a GeniX Low Divergence Cu-Kα source. The Xray wavelength was 0.154 nm. A Pilatus 300K detector (Dectris Ltd, Switzerland) collected SAXS patterns and the integration time was 1200 seconds. The radial integrations of the 2D SAXS patterns were undertaken using Foxtrot (SOLEIL, France). Calibrated with silver behenate powder, the sample-to-detector distance was 145.44 mm. All data were recorded using only 1 fiber in a horizontal position.

Raman spectroscopy: Raman spectroscopy characterization was performed on an

Xplora spectrometer from Horiba-Jobin-Yvon with an excitation energy of 2.33 eV (532 nm laser wavelength) using a macro-sample holder. The peak positions were calibrated using the G-band of highly ordered pyrolytic graphite (HOPG-1582 cm -1 ). All Raman experiments were carried out on the fiber surface in 3 different areas.

Electron Microscopy:

The surface and the cross-section of the fibers were observed with scanning electron microscopy (SEM) using a field emission gun (SEM-FEG) Jeol-6700F. Transmission electron microscopy (TEM) images were acquired using a TEM-FEG HR (JEOL 2200FS) microscope operating a 220kV. TEM grids have been prepared by ultramicrotomic technique.

Thermo-gravimetric Analysis (TGA):

Thermo-gravimetric characterization was carried out on a Texas Instruments TGA5500. About 10 mg of fibers were weighed in a Pt crucible.

The measurements were performed under a nitrogen atmosphere from 30 °C with a heating rate of 2 °C.min -1 until 550 °C, followed by 5 °C.min -1 until 1000 °C, ending with a plateau of 1h.

Electrical measurements:

2-points measurements were carried out using a KEITHLEY 2000 multimeter. Contacts were made with silver ink. The length between the contacts and surface area were measured using optical microscopy.

Mechanical properties:

Tensile tests of the fibers were carried out by using a tensile machine Zwick 2.5/TN1S at room temperature. Each fiber is fixed to a 20 mm long paper frame, and at least 3 measurements were performed for each type of fiber. The diameter was determined by optical microscopy.

Results and discussion

Preparation and characterization of lignin and lignin/GOLC-based CFs

Lignin and lignin/GOLC solutions have been used as a dope for the preparation of fibers. As shown in figure 1, the solution with a concentration of 0.8 wt. % GO, 10.6 wt. % KL and 4.6 wt. % PVA displays birefringence, and no phase separation. The system forms a homogeneous liquid crystalline phase. These results are in agreement with the recent phase diagram shown by Follmer et al. [START_REF] Föllmer | Structuration of lignin-graphene oxide based carbon materials through liquid crystallinity[END_REF], even with PVA addition. Dopes containing lignin and PVA with and without GO have been used in a wet-spinning process for fiber preparation.

PVA was added to the lignin spinning dopes as a plasticizer to increase the ease of spinnability, as pure lignin and pure lignin/GOLC fibers exhibit high brittleness. Liquid crystallinity provided by the addition of GO is expected to promote flow-induced ordering.

Figure 1b

presents a sketch of the mechanism, as well as the continuous lignin/GOLC fiber collected in a pilot line (figure 1c). The surface and the cross-section of the spun fibers were observed by SEM (figure 1d). A slightly crumpled structure is observed for lignin/GOLC fibers that differs from the smooth surface and dense cross-section of neat lignin fibers (figure SI1). The differences in texture are attributable to the organization of the GO during the spinning and drying process, which has also been observed in previous works [START_REF] Xu | Ultrastrong fibers assembled from giant graphene oxide sheets[END_REF][START_REF] Li | Ultrastrong Graphene-Based Fibers with Increased Elongation[END_REF][START_REF] Xu | Graphene fiber: A new trend in carbon fibers[END_REF][START_REF] Li | Hydrothermally Activated Graphene Fiber Fabrics for Textile Electrodes of Supercapacitors[END_REF]. The obtained fibers can be carbonized by a thermal treatment. 2D SAXS patterns of carbonized fibers were recorded as a function of the carbonization temperature for lignin and lignin/GOLC fibers (figure 2a). The observed arcs reflect a polycrystalline carbon structure.

However, there is a preferential orientation of the crystalline planes revealed by variations in the intensity of the arc as a function of the azimuthal angle [START_REF] Shukla | Advanced Carbon Materials and Technology[END_REF]. Reflection 002, which corresponds to the diffraction by graphene planes, is composed of pronounced arc reflection. This indicates that the graphitic crystals have a preferential orientation along the fiber axis. It can be observed that the carbon structure of the lignin-based CFs remains rather disordered below 2700 °C. A clear anisotropy becomes visible only at 2700 °C. By contrast, carbonized fibers with GO already show an orientation at 1000 °C, and more distinctly at high temperatures. In figure 2b, we show the azimuthal integration of the 002 peaks for fibers carbonized at 2700 °C. Both spectra have been fitted with a single Gaussian function. From these data, we quantify the orientation of carbon structures by defining the order parameter ( ) from equation 1. The presently used order parameter varies between 0 and 1. =0 means absence of preferential orientation, while =1 means a completed alignment along the fiber axis [START_REF] Landois | Growth of aligned multi-walled basic solid state physics carbon nanotubes: First in situ and time-resolved X-ray diffraction analysis[END_REF]. [START_REF] Peng | Electrochromatic carbon nanotube/polydiacetylene nanocomposite fibres[END_REF] where I max is the maximum intensity of the peak and I min is the minimum intensity in the valley.

The results are presented in figure 2c. We find that the order parameter is low (η=0.06) for lignin fibers carbonized at 1000 °C. The value increases up to 0.2 for carbonization at 2700 °C. On the other hand, lignin/GOLC-based CFs have already an order parameter of 0.26 at 1000 °C. This parameter gradually increases to ~0.7 for a treatment at 2700 °C. The increase in temperature leads to a higher alignment of carbon structures on the fiber axis. It should be noted that an increased alignment is already achieved at rather low carbonization temperatures for lignin/GOLC fibers compared to lignin fibers.

Similarly, the SAXS spectra for both fibers carbonized at 2700 °C are reported in figure 2e. The scattered intensity at small angles reflects the appearance of porosity presumably formed during the carbonization process [START_REF] Upton | Strategies for the Conversion of Lignin to High-Value Polymeric Materials : Review and Perspective[END_REF]48]. This phenomenon is confirmed by SEM images for lignin and lignin/GOLC CFs at all carbonization temperatures (figure SI2). The 002 peak is broad and spread over the area 2θ=22-27° for lignin fibers. This behavior indicates the existence of small graphitic domains present among a poorly ordered carbon structure. This result is consistent with previous reports [START_REF] Frank | Carbon Fibers : Precursor Systems, Processing, Structure, and Properties[END_REF][START_REF] Kurban | Turbostratic graphite nanofibres from electrospun solutions of PAN in dimethylsulphoxide[END_REF]. By contrast, the 002 peak of lignin/GOLC-based CFs is sharper and centered at 2θ~26°. From these results, we calculate the spacing between the sheets (d 002 ) using the Bragg's equation, and the crystallite thickness (L c ) from the Scherrer's equation:

(2) (3)
where θ is the diffraction angle, λ is the wavelength of the X-rays, is the full width at half maximum (FWHM), and the form factor K is 0.89. The structure of both sets of CFs carbonized at different temperatures was also examined by Raman spectroscopy [START_REF] Matthews | Origin of dispersive effects of the Raman D band in carbon materials[END_REF][START_REF] Pimenta | Studying disorder in graphite-based systems by Raman spectroscopy[END_REF]. All Raman spectra were recorded on the surface of the fibers and were normalized by the G-band. The results are shown in Figures 3a-b for lignin and lignin/GOLC fibers as a function of the temperature. All spectra exhibit the typical three bands for carbon materials [START_REF] Matthews | Origin of dispersive effects of the Raman D band in carbon materials[END_REF][START_REF] Pimenta | Studying disorder in graphite-based systems by Raman spectroscopy[END_REF]. A defect induced D-band, which is located around ~1350 cm -1 , a G-band, situated around ~1600 cm -1 and a third characteristic 2D-band at 2700 cm -1 originating from the stacking of graphite planes [START_REF] Pimenta | Studying disorder in graphite-based systems by Raman spectroscopy[END_REF]. The D-band increases and gets sharper with increasing carbonization temperature. It reaches a maximum at 2000 °C for lignin fibers and at 1200 °C for lignin/GOLC fibers. According to the three-stages model proposed by Ferrari et al. [START_REF] Ferrari | Interpretation of Raman spectra of disordered and amorphous carbon[END_REF], in the low-temperature range, the beginning of clustering leads to an increase of the D-band intensity. With increasing graphitization at higher temperature, the growth of graphene sheets and graphite crystallites and the reduction of defects lead to a decrease of the D-band intensity. Moreover, the G-band is shifted to higher frequencies until 1800 °C and 1200 °C for lignin and lignin/GOLC-based CFs, respectively. Above these temperatures, the G-band is shifted to low frequencies. According to Ferrari et al., the threestages model results from the increasing content of sp 2 hybridized carbon, as well as from the beginning of clustering of sp 2 hybridized carbon, with the conversion of amorphous carbon into nanocrystals. The shift of the G-band to lower frequencies at 2000 °C was attributed to an increasing graphitization in agreement with the model [START_REF] Ferrari | Interpretation of Raman spectra of disordered and amorphous carbon[END_REF]. The 2D band is observed from 1500 °C in both CFs and is also shifted to higher frequencies as the carbonization temperature was raised. This behavior was interpreted as an increase in the degree of graphitization. The analysis of these data thus further confirms that GO acts as a templating agent, leading to a significant improvement in the graphitic structure of CFs. It should be mentioned that no improvement of the graphitic structure of the fibers was achieved by changing the duration of the temperature plateau (10 h instead of 1 h) in the carbonization process (figure SI3).

Furthermore, the degree of structural disorder can be characterized by the intensity ratio between the two peaks (I D /I G ) and by the FWHM of the G-band [START_REF] Ferrari | Interpretation of Raman spectra of disordered and amorphous carbon[END_REF]. The D and Gbands of the carbonized samples were fitted using Lorentzian functions and the integrated intensities, I D , I G , and the FWHM of the G-band were calculated. As shown in figure 3c, the I D /I G ratio increases due to defects in the graphite structure until 2000 °C and 1200 °C for lignin and lignin/GOLC-based CFs, respectively. Above these temperatures, I D /I G decreases significantly for both CFs, indicating a crystalline graphite formation. Similarly, the FWHM for both fibers decreases gradually as the temperature increases (figure SI4). We observed that the FWHM for lignin/GOLC-based CFs decreases faster than for lignin before reaching a constant value at above 2000 °C. This value is much lower than the value obtained at 2700 °C for lignin-based CFs. In Raman spectroscopy of carbon materials, I D /I G ratio is often correlated to the crystal size using the Tuinstra-Koening model for crystal sizes greater than 2 nm [START_REF] Tuinstra | Raman Spectrum of Graphite[END_REF] and the Ferrari-Robertson model for L a below 2 nm [START_REF] Ferrari | Interpretation of Raman spectra of disordered and amorphous carbon[END_REF]. The results are shown in figure 3d. In both sets of fibers, L a increases with increasing I D /I G ratio until it reaches an expected maximum of 2 nm (or less). From this value, L a increases exponentially while I D /I G decreases, due to the growth of graphene sheets, graphite crystallites and the reduction of defects in the fibers. It should be noted that higher values of L a are achieved for lignin/GOLC fibers. This result confirms the fact that neat lignin is a precursor difficult to graphitize even at 2700 °C. However, there are clear structural improvements due to the presence of GO, even at low temperatures, starting from 1200 °C. To the best of our knowledge, these L a values are the largest values obtained in comparison to other lignin-based CFs reported in literature [START_REF] Baker | Recent Advances in Low-Cost Carbon Fiber Manufacture from Lignin[END_REF].

These results confirm that GO is a particularly effective structuring agent. In particular, it allows a preferential orientation to be obtained from low temperatures. Similar results have been reported incorporating GO flakes into PAN nanofibers prepared by Electrospinning [START_REF] Papkov | Improved graphitic structure of continuous carbon nanofibers via graphene oxide templating[END_REF] and GO flakes into cellulose [START_REF] Li | Highly conductive microfi ber of graphene oxide templated carbonization of nanofi brillated cellulose[END_REF]. These results are also consistent with the SAXS data presented above. The structure becomes more graphitic with increasing temperature, while remaining ordered. The Fast Fourier Transform (FFT) images of these samples do not show a preferred orientation of graphene planes, even at 2700 °C (inserts in figure 4a). It should be noted that regardless of the temperature, the poor graphite structure is oriented randomly. The irregular, complex and amorphous structure of lignin likely inhibits rearrangements and ordering during carbonization [START_REF] Sagues | Are lignin-derived carbon fibers graphitic enough?[END_REF]. An even less pronounced graphitic structure for these fibers and an absence of orientation are also found in the core of the fibers (figure SI5). However, in the case of lignin/GOLC-based CFs, the cross-sectional HRTEM shows a clearer graphitic structure at early stages of carbonization (1000 °C). This structure becomes more defined at elevated temperatures (figure 4b). In addition, FFT images show ordering of the graphene planes. Here, GO layers provide multiple nucleation sites for graphite growth. They act as a template to form orientated layers parallel to the GO flakes. At higher temperatures, these layers develop further to form extended graphitic planes parallel not only to the GO layer surface but also along the fiber axis due to the alignment of GO layers during the wet-spinning process. A similar effect can also be observed in the core of the fiber, especially at 2000 °C and 2700 °C, where several GO layers allow for the growth of parallel graphitic planes in the same direction (white arrows in figure SI5). Nevertheless, regardless the presence of GO, the core of the carbonized fibers seems always less graphitized than the surface. This behavior is likely due to the fact that the thermal treatment acts firstly at the fiber's surface before developing towards the center. Nevertheless, this scenario remains speculative and would need in-situ characterization during carbonization for validation.

Figure SI6

presents typical stress-strain curves of lignin and lignin/LCGO-based fibers (a) before carbonization and (b) after carbonization at 1000 °C, 1200 °C and 1500 °C. The precursor fibers before carbonization display similar mechanical performances, meaning that GO does not act as a reinforcement agent at the used concentration. The fiber strength is only of about 96 MPa with a Young´s modulus of 4.9 GPa. However, the tensile strength and Young's modulus of lignin/LCGO-based fibers are 87 MPa and 7.4 GPa, respectively. CFs are substantially improved after carbonization at 1000 °C. Their mechanical properties respectively reach values of 763 MPa and 52 GPa. CFs in absence of GO are also improved but to a lower extent, with a tensile strength of 300 MPa and a Young's modulus of 44 GPa.

From the above results, we conclude that the greater improvement in the tensile strength for lignin/GOLC-based fibers is basically due to the effect of GO acting as templating agent, rather than direct reinforcement. Nevertheless, this effect cannot be easily exploited at higher temperatures of carbonization. Indeed, it is observed that the mechanical properties are decreased, in spite of the higher graphitic structure achieved at higher temperatures. The fibers become more brittle. Actually, mechanical tests for fibers carbonized at temperatures above 1500 °C were too difficult because of the brittleness of the fibers. This behavior is likely due to the presence of pores formed by the carbonization process (figure SI2). The process is associated to the release of gaseous compounds, such as CO 2 , O 2 , CH 4 , etc. [START_REF] Frank | Carbon Fibers : Precursor Systems, Processing, Structure, and Properties[END_REF]. The electrical conductivity of the fibers as a function of the carbonization temperature is presented in figure 5. For lignin fibers, the values are in the order of magnitude of ~2x10 3 S.m -1 , with a slight increase at 2700 °C. However, the electrical conductivity of the lignin/GOLC fibers increases more strongly with temperature, obtaining a maximum value of ~0.7x10 5 S.m -1 . Furthermore, the behavior upon thermal treatment up to 1000 °C for both fibers was characterized by thermogravimetric analysis (TGA). A carbon yield of around 43 % was found both for lignin and lignin/GOLC fibers (Figure SI7), showing that the high yield of neat lignin can be maintained in the presence of GO.

Conclusion

In conclusion, the addition of GOLC to the lignin-based CFs allows not only to orient complex organic polymers along the fiber axis during the wet-spinning process, but also to improve the formation of additional graphite layers adjacent and parallel to the present graphite layers at low carbonization temperature, which is clear evidence for templating. By further increasing the heat treatment temperature, longer layers and a preferred orientation along the fiber axis were achieved, resulting in highly conductive CFs. This finding provides a new, highly economical approach to the production of low-cost CFs at low carbonization temperatures for wearable and electrical applications. Nevertheless, the obtained fibers display some porosity at higher temperatures which is detrimental for mechanical properties. This point should be improved in future work to extend the uses of the present fibers beyond electrical or electrochemical applications.

Notes

The authors declare no competitive financial interest.

Figure 1 .

 1 Figure 1. a, Sketch of Lignin/GOLC dope solution and its optical micrograph containing 0.8 wt. % GO, 10.6 wt. % KL and 4.6 wt. % PVA in DMSO under normal light (left) and crosspolarized light (right). Scale bars 200 μm. b, Sketch of the wet-spinning process of the dope solution where GOLC serves as a templating agent for better organization of the lignin structure. c, As-spun Lignin/GOLC fiber and d, lateral and cross-section views of the lignin/GOLC fiber. Scale bars 50 μm.

  As a schematic representation of the parameters, d 002 , L c , and L a , are shown in figure 2d. L a represents the lateral extension of the crystallites. The results for both fibers are presented in figure 2f for different carbonization temperatures, showing a decrease of d 002 when the temperature increases. Lower values are always observed for lignin/GOLC fibers with respect to those for lignin fibers, especially for carbonization temperatures above 1500 °C. d 002 for lignin/GOLC-based CFs can reach a value of 0.338 nm at 2700 °C, which is a value close to the spacing in HOPG (0.335 nm-dotted line). The higher d 002 values obtained for lignin fibers demonstrate that neat lignin is very difficult to graphitize. It is thus shown that the presence of GO flakes greatly promotes graphitization of the carbon structure based on lignin. The dependence of the crystallite thickness (L c ) as a function of the carbonization temperature is shown in figure 2f. L c is, on average, higher for lignin/GOLC-based CFs compared to lignin-based CFs. Values of ~22 nm are obtained for lignin/GOLC fibers in comparison to ~1.8 nm obtained for lignin fibers treated at 2700 °C. To the best of our knowledge, both d 002 and L c , have improved values in comparison with other bio-sourced and PAN-based CFs already reported in literature [25,49-51].

Figure 2 .

 2 Figure 2. a, 2D SAXS patterns for lignin fibers (upper part) and lignin/GOLC fibers (lower part) carbonized at different temperatures. b, Azimuthal integration of scattered intensity for the reflection 002 for lignin and lignin/GOLC fibers carbonized at 2700 °C. c, Order parameter determined by equation 1. d, Schematic representation of the carbon crystallites. e, SAXS spectra for both lignin and lignin/GOLC fibers graphitized at 2700 °C. f, Crystalline interlayer spacing (d 002 ) and crystallite size (L c ) of lignin and lignin/GOLC fibers as a function of carbonization temperature. The dotted line represents the carbon-carbon interlayer spacing for HOPG.

Figure 3 .

 3 Figure 3. Raman spectra for a, Lignin and b, Lignin/GOLC fibers treated at different carbonization/graphitization temperatures. All spectra were normalized by the G-band. Variation of the I D /I G ratio as a function of c, Different carbonization/graphitization temperatures, and d, crystalline size, L a . The dotted lines are guides for the eyes, and the vertical dotted line represents the boundary between the two models.

Figure 4 .

 4 Figure 4. High-Resolution Transmission Electron Microscopy (HRTEM) images and the corresponding FFT images for lignin-based CFs (left part) and lignin/GOLC-based CFs (right part) at different carbonization/graphitization temperatures. All images were taken at the edge of the fiber. The scale bars are 5 nm and 10 nm -1 for the HRTEM and FFT images, respectively.

Figure 5 .

 5 Figure 5. Conductivity as a function of carbonization/graphitization temperatures for lignin and lignin/GOLC fibers with a plateau of 1 h. The open red point represents the lignin/GOLC fibers carbonized at 1000 °C with a plateau of 10 h.

Acknowledgements

This work was supported by Low cost Carbon Fibers (FORCE) project. We thank the staff of the CANOE platform for their help in preparing the fibers. The authors thank to Marion Gayot (PLACAMAT-UMS3626) for HRTEM images and the Bordeaux Imaging Center (BIC) for the use of the cryo-ultramicrotome.

Appendix A. Supplementary data.

Supplementary data to this article can be found online at


Borja Cascales-Minana

Philippe Steemans

Thomas Servais

Philippe Gerrienne

An alternative model for the earliest evolution of vascular plants

Keywords: Rhynie chert, Horneophyton, Tracheophyte, Lower Devonian, Cladistics

Land plants comprise the bryophytes and the polysporangiophytes. All extant polysporangiophytes are vascular plants (tracheophytes), but to date, some basalmost polysporangiophytes (also called protracheophytes) are considered non-vascular. Protracheophytes include the Horneophytopsida and Aglaophyton/Teruelia. They are most generally considered phylogenetically intermediate between bryophytes and vascular plants, and are therefore essential to elucidate the origins of current vascular floras. Here, we propose an alternative evolutionary framework for the earliest tracheophytes. The supporting evidence comes from the study of the Rhynie chert historical slides from the Natural History Museum of Lille (France). From this, we emphasize that Horneophyton has a particular type of tracheid characterized by narrow, irregular, annular and/or, possibly spiral wall thickenings of putative secondary origin, and hence that it cannot be considered non-vascular anymore. Accordingly, our phylogenetic analysis resolves Horneophyton and allies (i.e., Horneophytopsida) within tracheophytes, but as sister to eutracheophytes (i.e., extant vascular plants). Together, horneophytes and eutracheophytes form a new clade called herein supereutracheophytes. The thin, irregular, annular to helical thickenings of Horneophyton clearly point to a sequential acquisition of the characters of water-conducting cells.

Because of their simple conducting cells and morphology, the horneophytophytes may be seen as the precursors of all extant vascular plant biodiversity.

Borja Cascales-Miñana [borja.cascales-minana@univ-lille.fr], CNRS, University of Lille, France. Philippe Steemans [p.steemans@uliege.be], EDDy lab/Palaeopalynology, UR Geology, University of Liège. Quartier Agora, Allée du 6 Août,14,Belgium. Thomas Servais [thomas.servais@univ-lille.fr], CNRS, University of Lille, , Université de Lille, CNRS, Université Littoral Côte d'Opale, UMR 8187 Laboratoire d'Océanologie et de Géosciences, 59000 Lille, France. Philippe Gerrienne [P.Gerrienne@uliege.be], EDDy lab/Palaeobotany, UR Geology, University of Liège. Quartier Agora, Allée du 6 Août, 14, Bât. B18. B-4000 Liège 1, Belgium Documenting the greening of the earliest land ecosystems requires a full understanding of the major steps in plant evolution, most particularly the origin of vascular land plants. Land plants (embryophytes) comprise the bryophytes (i.e., plant with parasitic unbranched sporophyte and a single sporangium) and polysporangiophytes (i.e. plants with independent branched sporophyte and multiple sporangia) [START_REF] Kenrick | The Origin and Early Diversification of Land Plants[END_REF][START_REF] Kenrick | The relationships of vascular plants[END_REF]. All living polysporangiophytes are vascular plants (tracheophytes), but the basalmost representatives of the clade are considered non-vascular [START_REF] Kenrick | The relationships of vascular plants[END_REF] and structurally intermediate between bryophytes and vascular plants. Those non-vascular polysporangiophytes, also called protracheophytes, include the Horneophytopsida (Caia, Horneophyton and Tortilicaulis) and Aglaophyton/Teruelia (Cascales-Miñana & Gerrienne 2017, and references therein). Most of them are known in great detail because they come from the Early Devonian (mid-Pragian-?earliest Emsian, Wellman 2006) Rhynie Lagerstätte, Aberdeenshire, in Scotland (see [START_REF] Kidston | On Old Red Sandstone plants showing structure, from the Rhynie Chert Bed, Aberdeenshire. Part I. Rhynia gwynne-vaughani Kidston and Lang[END_REF], 1920a, 1920b;Edwards et al. 2018), where the preservation of the fossils is exquisite.

The earliest preserved terrestrial ecosystem of Rhynie was indeed formed in a hot-spring environment [START_REF] Channing | A review of active hot-spring analogues of Rhynie: environments, habitats and ecosystems[END_REF]Edwards et al. 2018), which results in a unique and exceptional anatomical preservation of the early land plants, among other organisms. Consequently, four Rhynie chert plants (Aglaophyton, Horneophyton, Rhynia and Asteroxylon), showing different grades of complexity of water-conducting cells, occupy key steps in current plant phylogeny [START_REF] Kenrick | The relationships of vascular plants[END_REF][START_REF] Ligrone | Major transitions in the evolution of early land plants: a bryological perspective[END_REF].

Those plants are essential to elucidate the origin of vascular plant biodiversity, especially Horneophyton lignieri (Kidston & Lang) Barghoorn & Darrah, currently considered, together with Caia and Tortilicaulis, the earliest lineage of polysporangiophytes.

Horneophyton is one of the historical plants firstly described by Kidson and Lang from the Rhynie chert (Kidston & Lang 1920a;[START_REF] Barghoorn | Horneophyton, a necessary change of name for Hornea[END_REF]. Morphologically, Horneophyton is a small leafless free-sporing plant characterized by bulbous rhizomes, erect dichotomous axes and terminal sporangia with a central columella (Kidston & Lang 1920;[START_REF] Kenrick | The Origin and Early Diversification of Land Plants[END_REF]. After Kidson and Lang's original diagnosis, several researchers have focused attention on this plant, generating a wide literature. For instance, there is well documented evidence of the anatomy of aerial axes and rhizomes [START_REF] Edwards | Embryophytic sporophytes in the Rhynie and Windyfield cherts[END_REF][START_REF] Kerp | Organs and tissues of Rhynie chert plants[END_REF], of the structure of sporangia [START_REF] Eggert | The sporangium of Horneophyton lignieri (Rhyniophytina)[END_REF]El-Saadwy & Lacey 1979), as well as of the in situ spores [START_REF] Wellman | Spores of the Rhynie chert plant Horneophyton lignieri (Kidston & Lang) Barghoorn & Darrah[END_REF][START_REF] Wellman | Palaeoecology and palaeophytogeography of the Rhynie chert plants: further evidence from integrated analysis of in situ and dispersed spores[END_REF]. Even the gametophyte, Langiophyton mackiei, is well known [START_REF] Kerp | Organs and tissues of Rhynie chert plants[END_REF]. However, the phylogenetic position of Horneophyton is controversial. Horneophyton presents a mixture of characters typical of tracheophytes (e.g., branched sporophyte, well-developed cuticles with stomata on axes; Kidston & Lang 1920a;[START_REF] Kerp | Organs and tissues of Rhynie chert plants[END_REF], but by still retaining some bryophyte features (e.g., columellate sporangium; Kidston & Lang 1920a;[START_REF] Eggert | The sporangium of Horneophyton lignieri (Rhyniophytina)[END_REF]. Recent observations on the basal part of the upright axes further show the presence of waterconducting cells with annular to slightly helical thickenings [START_REF] Kerp | Organs and tissues of Rhynie chert plants[END_REF], which obviously calls into question the non-vascular nature of Horneophyton. This paper aims to address this issue and its evolutionary implications.

Material and methods

We studied for the first time the series of petrographic thin sections from the historical W. Hemingway slides of the Natural History Museum of Lille, which belong to the same Rhynie chert collection as those originally studied by [START_REF] Kidston | On Old Red Sandstone plants showing structure, from the Rhynie Chert Bed, Aberdeenshire. Part I. Rhynia gwynne-vaughani Kidston and Lang[END_REF], 1920a, 1920b). Concretely, we studied 30 glass slides (8x Aglaophyton, 8x Rhynia, 9x Horneophyton, and 5x Asteroxylon; see Table 1) showing typical exquisitely well preserved reproductive and vegetative structures of the Rhynie flora. Photographs were taken using a Nikon Df camera coupled to a Zeiss Stemi 2000-C stereomicroscope and with a Zeiss Axioskop microscope coupled to a Zeiss Axiocam camera.

The phylogenetic position of Horneophyton was assessed via PAUP* 4.0 (Phylogenetic Analysis Using Parsimony, and other Methods, http://paup.phylosolutions.com/). We codified a character matrix (Data S1) including 21 core taxa of earliest land plants [START_REF] Kenrick | The Origin and Early Diversification of Land Plants[END_REF][START_REF] Kenrick | The relationships of vascular plants[END_REF]. Data matrix was prepared on 32 morphological and anatomical characters (Table S1). List of characters (and character states) was adapted from [START_REF] Kenrick | The Origin and Early Diversification of Land Plants[END_REF] and Cascales-Miñana & Gerrienne (2017), with some modifications (Table S1). Bryophytes were transferred to outgroup, and treated as monophyletic according to [START_REF] Puttick | The Interrelationships of Land Plants and the Nature of the Ancestral Embryophyte[END_REF] S3). Strap analysis was performed according to Bell and Lloyd's (2014) tutorial.

Results and discussion

The observed diversity in the water-conducting cells of the four Rhynie chert plant studied here is shown in Fig. 1. In Aglaophyton (Fig. 1A,E,I,M,N), the water-conducting cells are characterized by the presence of chains and clusters of interconnected bubble-like structures in their lumen [START_REF] Edwards | Aglaophyton major, a non-vascular land-plant from the Devonian Rhynie Chert[END_REF][START_REF] Edwards | Embryophytic sporophytes in the Rhynie and Windyfield cherts[END_REF]). Whether those bubbles are of primary or secondary origin is unknown, but previous studies have shown that they were most probably unlignified [START_REF] Boyce | Chemical evidence for cell wall lignification and the evolution of tracheids in early Devonian plants[END_REF]. The conducting cells of Rhynia (Fig. 1B,F,J,O,P) are of the S-type as defined by Kenrick & Crane (1991). S-type conducting cells are elongate cells with large annular to helical thickenings including a spongy interior covered towards the lumen by a thin decay-resistant microporate layer (Kenrick & Crane 1991[START_REF] Kenrick | The Origin and Early Diversification of Land Plants[END_REF], considered unlignified by [START_REF] Boyce | Chemical evidence for cell wall lignification and the evolution of tracheids in early Devonian plants[END_REF]. The conducting cells of Horneophyton (Fig. 1C,G,K,Q,R; Fig. 2) include thin irregular, annular to helical thickenings. Those thickenings are of unknown origin and nature, but their dark colour is suggestive of the presence of a decay-resistant component that may have locally allowed the preservation of abundant organic matter. The tracheids of Asteroxylon (Fig. 1D,H,L,S,T) are of the G-type as defined by Kenrick & Crane (1991). They show annular to helical thickenings with pores distributed between the thickenings. [START_REF] Boyce | Chemical evidence for cell wall lignification and the evolution of tracheids in early Devonian plants[END_REF] have shown that Asteroxylon tracheids are most probably heavily lignified.

Two important observations are derived from the water-conducting cells diversity illustrated in Fig. 1. Firstly, the Aglaophyton and Rhynia conducting strands (Fig. 1A,E,B,F) are more closely similar to each other than to those of Horneophyton and Asteroxylon, which show thicker cell walls in cross sections (Fig. 1C,D,G,H; Fig. 2). Secondly, the complexity of the conducting cells obviously increases from Aglaophyton, where there is an apparent lack of tracheid thickenings (Fig. 1I,M,N), to Asteroxylon, which shows well-defined annular secondary thickenings (Fig. 1L,S,T). Most importantly, we note that Horneophyton presents a genuine type of conducting cells characterized by irregular, annular and/or, possibly spiral wall thickenings (Fig. 1K,R,S) of putative secondary origin. Accordingly, our phylogenetic analysis resolves Horneophyton and allies within tracheophytes, but interestingly, not as the basalmost lineage but as a sister clade to eutracheophytes (Fig. 3). The two plant groups constitute a new clade that we call herein supereutracheophytes. This clade is characterized by narrow, irregular, annular and/or, possibly spiral wall thickenings of putative secondary origin, while eutracheophytes present a clear thick, lignified wall layer. Fig. 3 further shows that Horneophyton and allies are more derived than the paratracheophytes (=Rhyniaceae), while Aglaophyton is resolved as the earliest diverging polysporangiophyte, and the single lineage between the bryophytes and the vascular plants.

In 1920, Kidston & Lang (1920a) referred to Horneophyton (formerly Hornea) as "another vascular plant from the Rhynie peat-bed" and documented cells with distinct narrow thickenings forming irregularly connected rings or a spiral (Kidston & Lang 1920a, figs. 52-53). Indeed, the Hornea original diagnosis describes the vascular strand as follows: "stele of stem with a zone of phoem surrounding the xylem composed of small central and wider peripheral tracheids" (Kidston & Lang 1920a). The same description appears in [START_REF] Barghoorn | Horneophyton, a necessary change of name for Hornea[END_REF] Horneophyton diagnosis. More recently, [START_REF] Edwards | Embryophytic sporophytes in the Rhynie and Windyfield cherts[END_REF] pointed out that the Horneophyton terete strands "show unequivocal narrow interconnected annular or spiral thickenings, with the smaller elements situated centrally", and further added "the tracheids were surrounded by elongate, thin-walled cells interpreted as phloem". Likewise, [START_REF] Kerp | Organs and tissues of Rhynie chert plants[END_REF] has recently illustrated the presence of "water-conducting cells with annular to slightly helical wall thickenings in the basal part of the upright axis" for Horneophyton. Our observations also show annular and/or spiral wall thickenings in Honeophyton conducting cells (Fig. 1Q,R). This is definitely consistent with the original diagnosis (Kidston & Lang 1920a;[START_REF] Barghoorn | Horneophyton, a necessary change of name for Hornea[END_REF], as well as with the subsequent descriptions and the current interpretations [START_REF] Edwards | Embryophytic sporophytes in the Rhynie and Windyfield cherts[END_REF][START_REF] Kerp | Organs and tissues of Rhynie chert plants[END_REF], which provides strong support for Horneophyton being a vascular plant.

Fundamental characters supporting a close relationship between Horneophyton and eutracheophytes are (i) the decay resistance of the conducting cell wall and (ii) thickenings of cell wall (characters 16 and 17 from Table S1, respectively). Decay resistance is a character linked to the presence of lignin in the secondary wall of the conducting cell, which accounts for an increased thickness of the wall [START_REF] Kenrick | The anatomy of Lower Devonian Gosslingia breconensis Heard based on pyritized axes, with some comments on the permineralization process[END_REF]. Unthickened water-conducting cells are typical of mosses [START_REF] Roberts | Moss cell walls: structure and biosynthesis[END_REF]). They can be also observed in Aglaophyton (Fig. 1E,I). The wall of the conducting cells of the paratracheophytes presents a thin decay-resistant layer facing the lumen (Kenrick & Crane 1991;Kenrick et al. 1991aKenrick et al. , 1991b), but it is unclear whether the decay-resistant layer is of the secondary nature and whether it includes lignin. Recent evidence however suggests that the development of the wall thickenings in Rhynia occurs shortly after elongation and hence that it is most probably of secondary origin [START_REF] Kearney | Whole-Plant Regeneration via Epidermal Cells in the Axis of the Early Devonian Rhynie Chert Plant Rhynia gwynne-vaughanii Kidston et Lang[END_REF]. In another Rhynie chert plant, Nothia aphylla, for which decay resistance is accepted [START_REF] Kenrick | The Origin and Early Diversification of Land Plants[END_REF], water-conducting cells have no thickenings [START_REF] Edwards | Xylem in early tracheophytes[END_REF].

However, the walls of the conducting cells of Nothia appear remarkably thicker than those of Aglaophyton and Rhynia [START_REF] Kenrick | The Origin and Early Diversification of Land Plants[END_REF]. The walls of Horneophyton water-conducting cells (Fig. 1G) are also thicker than those of Rhynia (compare our Fig. 1F with Fig. 1G), and much thicker than those of Aglaophyton (compare Fig. 1e with Fig. 1G). The thickness of the walls of Horneophyton conducting cells is indeed much closer to that of the conducting cells of Asteroxylon (Fig. 1G,H). So, as for Asteroxylon, Horneophyton decay resistance was scored as extensive in our analysis, while that of Rhynia is scored as limited (character 16, Table S1).

Early polysporangiophytes mainly present water-conducting cells with annular or helical thickenings (Kenrick & Crane 1991). These tracheid-like cells or tracheids are well known in paratracheophytes, as well as in zosterophylls or basal lycopods such as Asteroxylon or Baragwanathia (Kenrick & Crane 1991;Kenrick et al. 1991aKenrick et al. , 1991b)). The presence of this type of thickenings in Horneophyton has been described in several studies (Kidston & Lang 1920a;[START_REF] Barghoorn | Horneophyton, a necessary change of name for Hornea[END_REF][START_REF] Edwards | Embryophytic sporophytes in the Rhynie and Windyfield cherts[END_REF][START_REF] Kerp | Organs and tissues of Rhynie chert plants[END_REF] and is again illustrated here (Fig. 1G,K). The thickenings are however irregular and less welldeveloped than in Asteroxylon (Fig. 1S,T), or even in Rhynia, where we see a banded appearance (Fig. 1O,P). Nevertheless, because annular or helical thickenings, even if irregular, are obvious in Horneophyton, the character "tracheid thickenings" was scored as present in our analysis (character 17, Table S1).

The results of our cladistic analysis contrast with the currently accepted fossil-based plant phylogeny (Fig. 4A) that presents two intermediate lineages between the bryophytes and tracheophytes. These lineages correspond to the Rhynie chert plants Horneophyton and Aglaophyton. This conception mainly derives from the presence of a columella in Horneophyton sporangium and of apparently uniform cell walls in Aglaophyton conducting cells. According to this model, tracheophytes include two sister clades, represented here by the Rhynie chert plants Rhynia (paratracheophytes, Fig. 3) and Asteroxylon (eutracheophytes, Fig. 3), the latter being sister to euphyllophytes, i.e., the plants with "true leaves" or megaphylls, a lineage that includes almost all living vascular plants. Our results however suggest an alternative, and more parsimonious model for interpreting the basal relationships in land plant evolution (Fig. 4B), with Horneophyton (and allies) being sister to eutracheophytes instead of being the earliest, non-vascular polysporangiophyte.

The acquisition of more and more efficient water-conducting cells was crucial for plant evolution and decisive during their terrestrialisation process. The presence of vascular tissues is therefore a key synapomorphy for understanding polysporangiophyte evolution. Tracheids are defined in terms of form, function and biochemical composition [START_REF] Niklas | Evidence for a conducting strand in early Silurian (Llandoverian) plants: implications for the origin of land plants[END_REF], the latter character being the most difficult to assess in fossils. All the features identifying tracheid (secondary wall deposition, presence of lignin, pitting of wall) were presumably acquired sequentially; this is exemplified by the four plants illustrated here that document the initial steps of the process. In fossil plants, the presence of lignified conducting cells can only be assumed from the presence of a decay-resistant layer. This layer is evaluated from comparative morphology with extant plants. There are no direct unequivocal measures showing the presence of lignin neither for any Early Devonian tracheids nor for any early land plant.

However, the fossil record shows a great variety of conducting cells and not all of them possess a decayresistance nature. The best example is the S-type conducting cell. These cells, characteristics of paratracheophytes such as Sennicaulis, Rhynia or Huvenia and related plants, are elongate, with annular/helical thickenings, but with a very thin decay-resistant layer (Kenrick & Crane 1991) in which the presence of lignin is still elusive [START_REF] Boyce | Chemical evidence for cell wall lignification and the evolution of tracheids in early Devonian plants[END_REF][START_REF] Kearney | Whole-Plant Regeneration via Epidermal Cells in the Axis of the Early Devonian Rhynie Chert Plant Rhynia gwynne-vaughanii Kidston et Lang[END_REF]. Interestingly, Aglaophyton water-conducting cells strongly resemble the morphology found in Rhynia or Huvenia conducting tissues [START_REF] Remy | New information on gametophytes and sporophytes of Aglaophyton major and inferences about possible environmental adaptations[END_REF][START_REF] Kearney | Whole-Plant Regeneration via Epidermal Cells in the Axis of the Early Devonian Rhynie Chert Plant Rhynia gwynne-vaughanii Kidston et Lang[END_REF]. This fact suggests, together with similarity observed between Aglaophyton and Rhynia central strands (see Fig. 1E,F), and the morphology of its reproductive structures [START_REF] Kenrick | The Origin and Early Diversification of Land Plants[END_REF][START_REF] Cascales-Miñana | Teruelia diezii gen. et sp. nov.: an early polysporangiophyte from the Lower Devonian of the Iberian Peninsula[END_REF], that Aglaophyton might be included in the paratracheophytes, likely as its basalmost representative. This has already been discussed by [START_REF] Edwards | Embryophytic sporophytes in the Rhynie and Windyfield cherts[END_REF], who suggested that similarities in xylem elements between Aglaophyton and Rhynia would seem to narrow the differences between both plants.

The other key element in the currently accepted phylogenetic position of Horneophyton is the presence of columellate sporangia (Eggert 19774;[START_REF] El-Saadawy | The sporangia of Horneophyton lignieri (Kidston and Lang) Barghoorn and Darrah[END_REF]. This character is considered a bryophyte-like trait and it is the mean feature advocating for the basalmost position of Horneophyton within polysporangiophyte evolution [START_REF] Kenrick | The relationships of vascular plants[END_REF]. However, columellate sporangium is neither a character of all bryophytes, nor present in bryophytes only. This type of sporangium is absent in liverworts, not present in all hornworts, and is characteristics of mosses only [START_REF] Goffinet | Bryophyte biology[END_REF].

Importantly, the fossil record shows also collumelate sporangia in several Carboniferous lycopsids, i.e., in basal eutracheophytes, such as Oxroadia [START_REF] Alvin | A new fertile lycopod from the Lower Carboniferous of Scotland[END_REF] or the sigillarian plant Mezocarpon [START_REF] Pigg | Morphology and Reproductive Biology of the Sigillarian Cone Mazocarpon[END_REF]. This means that the character "presence of a columella in the sporangium" might by homoplasious, which reinforces the position of Horneophyton as a sister lineage to eutracheophytes (Figs. 2,3B), and therefore, as a precursor of the living vascular plant biodiversity.

There is an important "time gap" between the earliest evidence for land plants, provided by the mid-Ordovician dispersed spore fossil record [START_REF] Rubinstein | Early Middle Ordovician evidence for land plants in Argentina (eastern Gondwana)[END_REF], and the earliest vascular plant evidence, which corresponds to mid-Silurian Cooksonia macroremains [START_REF] Libertín | Sporophytes of polysporangiate land plants from the early Silurian period may have been photosynthetically autonomous[END_REF]. This gap could still be larger if we consider the recently suggested [START_REF] Morris | The timescale of early land plant evolution[END_REF] timescale. New molecular-based results estimate indeed that land and vascular plants emerged during mid-Cambrian-Early Ordovician and Late Ordovician-Silurian time intervals, respectively [START_REF] Morris | The timescale of early land plant evolution[END_REF]. No fossil evidence confirms such intervals, but a series of small fossil plant remains have been recently discovered from the uppermost Ordovician [START_REF] Salamon | Putative Late Ordovician land plants[END_REF]. No unequivocal evidence of tracheophytes is known before Silurian times. As shown in Fig. 3, Cooksonia is the oldest tracheophyte, and importantly also, is the oldest and basalmost eutracheophyte, but it is not the most primitive vascular plant. Due to its exceptional anatomical preservation, the Rhynie chert plants provide unique knowledge about the earliest steps of polysporangiophyte evolution. This is the reason why they occupy key positions in many models of plant evolution, especially tracheophytes. This fact also goes with an underrepresentation and with an identification bias of basal tracheophytes, even more so if we take into account that many Early Devonian plants are known from compression fossils only, with no information about their vascular strand.

Conclusions

We show herein the water-conducting cells of four Lower Devonian plants from the Lagerstätte of Rhynie (Aberdeenshire, Scotland) present in the slide collections of the Natural History Museum of Lille (France). Those plants, namely the protracheophytes (considered non-vascular) Aglaophyton and Horneophyton, the paratracheophyte Rhynia and the eutracheophyte Asteroxylon, are key taxa in the earliest evolution of vascular plants. This work confirms previous descriptions and interpretations of the water-conducting cells of Aglaophyton, Rhynia and Asteroxylon, but emphasizes that Horneophyton has a particular type of tracheid characterized by narrow, irregular, annular and/or, possibly spiral wall thickenings, and hence that it cannot be considered non-vascular anymore. Indeed, our new phylogeny resolves Horneophyton within tracheophytes, but as sister to eutracheophytes in a larger clade called supereutracheophytes. Results further suggest that the horneophytophytes are more derived than the paratracheophytes. The Rhynie Chert plants lived some 20 million years after the earliest Silurian records for vascular plants. Although based on a group of coeval plants, many of them displaying ancestral characters, the alternative scenario for the basalmost evolution of vascular plants proposed herein suggests a sequential acquisition of water-conducting cell characters. This results in a more parsimonious model characterized by a progressive increase of the complexity of the conducting tissues.

Figure legends

  recent maximum-likelihood and Bayesian estimations. See Text S1 for implementation. The topology of resulted consensus tree (Consistency index = 0.875; Homoplasy index = 0.125; Retention index = 0.9213, Rescaled consistency index = 0.8062; Text S2) was plotted against stratigraphy using strap (Stratigraphic Tree Analysis for Palaeontology; Bell & Lloyd 2104). The calibration of polysporangiophyte splits is based on to the temporal distribution of the considered fossil plants (Text

Fig. 1 .

 1 Fig. 1. Rhynie chert water-conducting cells observed from the historical collections of the Natural

Acknowledgments. -We thank Paul Kenrick (Natural History Museum of London, UK) for his helpful comments for discussion. We thank Jessie Cuvelier (CNRS, France) for technical assistance and Thierry Oudoire (Natural History Museum of Lille, France) for allowing the examination of specimens. We also thank Hans Kerp (Munster University, Germany) who very kindly helped us to illustrate this work and provided insightful comments on the Rhynie flora. K.L. acknowledges ANR M6fossil grant (ANR-15-CE31-0003-01). P.G. and P.S. are F.R.S-FNRS Senior Research Associates. This research was funded by the IRePSE (FED 4129, University of Lille).

gwynne-vaughanii (LS, scale bar = 20 µm; . Q-R. Detail of water-conducting cells of H. lignieri (LS, scale bar = 25 µm; MGLame-18). S-T. Detail of tracheids of A. mackiei (LS, scale bar = 20 µm; MGLame-29, MGLame-27). Tables Table 1. List of slides of the Hemingway historical collection of the Rhynie chert housed at the Natural History Museum of Lille used in this study. For each slide, it is specified both the original and given code as well as the considered plant diversity.

Supplementary data

Data S1. Character matrix used in the phylogenetic analysis.


Louise Barrière
email: louise.barriere@posteo.net

Introduction

From the early 2000s onward, the punk-inspired feminist festival concept named "Ladyfest" circulated around the world, adapting itself to local contexts and scenes. In Germany, a Ladyfest network has been in development since 2003. German Ladyfests claim a strong influence from the North-American Riot Grrrl movement. And yet, they are also anchored in local and national DIY music scenes, as the constant mention of a specific city in their names recalls it (i.e. Ladyfest Mannheim, Ladyfest Hamburg, Ladyfest Berlin, Ladyfest Dresden, and so on). In Germany, Ladyfests are generally small-scale festivals, happening at local live music or feminist venues.

As an ethnographer of the German Ladyfest scene, I have spent the last three years visiting a variety of local punk-inspired festivals, and their venues. Local venues play a crucial in shaping the atmosphere of each event I have been visiting. On another hand, the organization of a Ladyfest also (temporarily, at least) shaped the venue atmosphere, through their queer-feminist activism. Ladyfests indeed strive to transform the gender power relationships in their own local music scenes, making women, LGBTIQ, gender-non conforming people -and their bands, collectives, zines, and so on -more visible, and providing them with educational resources around music, gender and sexuality.

When the scene began to (slowly) move online this year, due to the covid pandemic, I was forced to realize the importance of venues, which had been replaced by Zoom conferences, Spotify podcasts, YouTube gigs and Facebook events, making the concreteness of the festival spaces temporarily disappear. Graffitis on the walls were no longer there. Posters from previous and forthcoming events, flyers, zines, lights, disco balls, and bodies of people dancing on or in front of the stage had also disappeared from my field observations. I suddenly realized how the material space of venues, as much as their philosophical spirit, participated in shaping each Ladyfest's atmosphere.

And yet, in a 2016 article, Andy Bennett and Ian Rogers noted that, though live music venues had become "integral understandings of popular music history", in academia "little attention [had] been focused (…) on the music venue." They pointed that, within research articles, music venues were rarely thoroughly described and remained anonymous. Meanwhile aspects such as the appearance, the regular audience and bands, or the aesthetic appeal of music venues remained absent. Such observations motivated me to take a further look at the music venues I had been visiting for my own research.

This paper draws on multi-sited ethnography. Between 2017 and 2019, I have visited 10 Ladyfests and Ladyfest-inspired festivals, as a musician, workshop facilitator or audience member. During fieldwork, I conducted interviews with 15 festivalgoers and organizers, as well as countless informal conversations. On a more historical side, I also searched for the festivals' archives (which are mostly digital), trying to trace where and when Ladyfests had happened, prior to the beginning of my research.

In a first part, I will highlight how the Ladyfest collectives generally aim to transform the atmosphere of the venues they occupy. Then, I'll go through the history of Germany's youth center movement and feminist movement -both of which originated in the 1970s. As some youth centers and women's centers are still nowadays active in Germany, they interact also with the Ladyfest scene. I'll thus showcase what studying the histories of both movements has taught me about the German Ladyfest network.

Conflictual spaces. The impact of Ladyfest collectives' on live music venues and their audiences.

At first, Ladyfest organizers do not always have a completely positive relationship with the festival venue. Organizing a festival sometimes becomes a mean to reclaim a male-dominated space. I'm going to get a little bit out of the geographical focus of this talk, and refer to a text released by a French Ladyfest, because I find that the way the Ladyfest organizers speak about the festival venue really enlightens this kind of relationship between the event and the space. As you can read on the screen, the organizers insisted on introducing the venue from a "female point of view", then describing it as "essentially, or even exclusively, occupied by men", making the atmosphere "quite masculinist and absolutely unattractive for women." Hence why they decided to host the Ladyfest at les Tanneries, hoping to claim back their part of the space and make things change.

While Benett & Rogers highlighted the role of "unofficial" live music venues in the construction of a shared sense of intimacy or belonging, studying punk-inspired feminist networks underscores that belonging is a gendered experience. Indeed, some interviewees recalled that they did not feel any sense of inclusion while casually attending their local DIY music venues, or even while preparing the Ladyfest. An interviewee told me that:

"we held the festival in a space that was used mostly by university students and not explicitly feminist. So we had to engage with cis men during the whole time who were inconsiderate and annoying. (…) we wanted to create gender neutral toilets and the people from the space were weirdly resistant to that for no discernable reason. We just did it anyway." Because the Ladyfest later brought together like-minded feminist and queer individuals in the targeted venue, it allowed its organizers to claim back a male dominated space.

That being said, it is interesting to look at which means the Ladyfest organizing collectives use to transform the gendered distribution of power within DIY/punk venues. One of these means is the implementation of a safer space policies. The research on safer space policies remains quite scarce, but has benefited of a growing interest in the recent times. And if you're willing to learn more on the topic, here's a list of references. In the German Ladyfest scene, safer space policies are a way to influence the atmosphere of the venue, by emphasizing the importance of bystander intervention or organizing Awareness teams -that is, feminist security teams.

Coming to music gigs themselves, Ladyfests organizing collectives also seek to achieve a better gender balance in the music programs and in the audience. Among 328 concerts organized by the entire network of Ladyfests in Germany, between 2003 and 2019, 216 were performed by female and queer bands, 98 by mixed gender bands, and 14 by male bands. But the festivals also bring diversity in terms of music genres. The Ladyfest Mannheim 2017 was for instance held at a local DIY venue, namely the JUZ Friedrich Dürr. In 2017, the JUZ mostly hosted punk, hardcore punk, metal, and crust shows. A few electronic music or hip hop events were also noticeable in the venue calendar -which may still be accessed online. Nonetheless, non-punk gigs remained rare. The Ladyfest Mannheim claimed punk-feminist roots, in accordance with the JUZ atmosphere, and the festival hosted a show by Personal Best, a UK indie band... but also a gig by Fartuuna, a pop singersongwriter from the local scene, thus making the JUZ music programing more diverse in terms of music genres.

Cross-fertilizing networks. The Ladyfest scene meets the Jugendzentrumsbewegung. The Ladyfest Mannheim 2017 was actually the first time I went to a "Jugendzentrum", and I think it's worth it to focus on what is a Jugendzentrum (literally youth center). When I first came to the JUZ Friedrich Dürr in Mannheim, the place really impressed me. The place looked spacious, it had several rooms, the concert room was well-equipped with a proper PA system, it had a stage and artists didn't played directly on the floor, it had proper lightnings… And yet it didn't look like a professional music venue either. The place had graffitis and stencils on the walls (inside and outside), often displaying political messages, there were fanzines and photocopied flyers here and there, and overall, a DIY spirit. I couldn't figure out what was this place, which didn't look like anything I knew. It was not a squat, not a bar, not an official music venue either. Plus, the venue was in no way looking like the youth centers I knew in France, and looked way less institutionalized.

During my stay in Mannheim, I found some documentation in the JUZ itself. As mentioned in a Ladyfest booklet, the JUZ: "full name is 'Self-managed youth center Friedrich Dürr.' Self-managed means: here, there's no social worker employed by the city, but JUZ-Activists manage and design the JUZ themselves. During the Ladyfest, the JUZ is THE main venue. During the entire week-end, you'll be able to find here the information booth, with current information about the different workshops. Childcare also happens here."

Yet, this was not enough to reconstruct the entire history of the movement the venue had been part of. After coming back home, I browsed through my corpus of festivals and identified that other Ladyfests, in different cities, had been held at venues that had a similar name (be it Jugendzentrum, or Autonomes Jugendzentrum or Autonomes Zentrum). It seemed pretty obvious that these venues were networked together, but I still couldn't find how. A few years later, I finally found David Templin's thesis which recalls the history of the Jugendzentrumsbewegung (literally youth centers movement) in the 1970s and 1980s. Templin terms the youth center movement as "a hidden part of the history of the federal republic of Germany."

In Western Germany, in the 1970s, youth leisure was supervised by the church or social workers.

Alternative options comprised bars and clubs, especially in big cities -but those required consumption of drinks. Youth groups all around the country formed collectives to claim their right to "leisure without control" (Freizeit ohne Kontrollen). These collectives were formed by youth groups from leftist organizations but also non-politically-affiliated young people. They networked together through medias, such as journals, TV and radio shows. A huge part of their demands revolve around music. Youth were craving for spaces where they could organize gigs, book bands and listen to the music of their choice (as opposed to having to pay and drink in order to listen to music they couldn't even chose in clubs). In the mid-1970s, more than 1000 activists groups advocated for the creation of a self-managed center in their city. Their mobilization wasn't always successful, but a fair part of the local collectives got heard by city councils, resulting in the development of several hundreds of self-managed youth leisure center in the following years, in the FRG. The movement declined at the end of the 1970s, but some youth centers are nowadays still active, including the JUZ Friedrich Dürr in Mannheim.

Jugendzentren appeared decades before the Ladyfest scene. And yet, the Ladyfest scene seems to meet some of the basic reasons for which the Jugendzentrumsbewegung was founded. Both networks rely on a non-hierarchical and autonomous mode of organization. The youth center movement provided leisure spaces for the youth, managed by the youth. Yet, as highlighted by Templin, the youth centers were, since the very beginning, male dominated -despite their claimed appeal for gender diversity -and remain so nowadays. Ladyfests collectives bring diversity, in terms of gender and music genres in such venues. Being events for women and queer people, organized by women and queer people, Ladyfests expand the function of self-managed youth centers to a new public.

Frauenzentren and inter-generational feminism.

Frauenzentren (Women's centers) appeared approximately at the same time as Jugendzentren, in the 1970s. They weren't link with youth movements, but rather with the so-called second wave of feminism. Of course women's centers weren't just a (Western) German phenomenon.

And neither are they the only way that second-wave feminism influenced the contemporary Ladyfest scene. Indeed, West German feminists, in the late 60s and 70s, fought for abortion rights, launched the Walpurgisnacht marches -a form of women-only night protest march, which has later influenced the development of the international Reclaim the Night movement. Walpurgisnacht and Reclaim the Night marches are still a key influence of the Ladyfest scene, more than 30-40 years later. Second wave German feminists also hosted Frauenfesten (women's festivals)… which could be, to some extent, considered as the local ancestor of Ladyfests. The influence of Walpurgisnacht and 1970s feminism may be found in the names of queer-feminist punk festivals (such as the Noc Walpurgii which happens every two years in Berlin), or in song lyrics. During Ladyfest Wiesbaden 2019, the singer of Molly Punch, a local punk band, for instance dedicated a song to former generations of feminist activists.

In the 1970s, Women's centers were places where women could organize, discuss politics, and learn. They published journals, hosted conferences, debates, workshops and sometimes concerts. Women's centers were also often networked with women shelters against domestic violence, and family planning centers. At a local scale, they were, therefore, important places for the organization of the feminist movement.

Nowadays, Women's centers that are in some way connected to the Ladyfest scene rarely host shows, but rather different activities, such as workshops, conferences or movie screenings. Thereby, they still stick to their original educational focus. For example, during the Ladyfest Freiburg in 2019, the FZ hosted a radio workshop, a dance workshop, a screenprinting workshop, but also discussions on motherhood and sexuality. The activities revolved around a DIY spirit, while theoretical discussions tackled feminist issues.

Freiburg's FZ was formerly named Women's and lesbian's center -a name that was recently changed for Feminist Center (Feministisches Zentrum), as to reflect more inclusivity. On the walls of the FZ Freiburg, the Ladyfest participants could observe the history of the place, from the 1970s to nowadays. The transmission process between second wave feminism and contemporary punkinspired feminist collectives and festivals was thereby materialized, symbolizing how the contemporary Ladyfest scene was still under the influence of second wave feminist movements. Meanwhile, we could also consider that Ladyfests and contemporary feminist movements allowed Frauenzentren to move beyond a binary understanding of gender.

Conclusion

So, why did I want my research on the German Ladyfest scene to take a more precise look at live music venues? Because… 1-If we were to consider live music as a mediation, the venue is part of that mediation process, just as other gatekeepers (booking agents) or audience members. The venue shapes the ways that the music is transmitted and received by the audience. 2-Within live music venues, including punk and DIY venues, the division of the space is gendered. This has been highlighted by previous research, and is also often mentioned by the Ladyfest collectives themselves. Research on space and its materiality allows for a better comprehension of how the Ladyfest collectives seek to transform that gendered distribution. 3-I hope my paper has also highlighted that, while the Ladyfest collectives are trying to transform gender power relationships within local scenes and venues, looking at the history of such venues also provided me with valuable information on the politics of the Ladyfest network, especially regarding self-management of leisure activities and trans-generational feminist activism.

To conclude, this research is still a work in progress, and there are plenty of other venues I could have taken as examples, such as SO36 in Berlin, but those will need further investigation.