Gelatinous fibres (g-fibres) differ from most fibres in that the innermost layer of their secondary cell wall is rich in cellulose and poor in lignin. G-fibres are often produced in response to gravitational and mechanical stresses in the roots, stems, and leaves of angiosperms, with their main function being the reorientation or contraction of these organs. G-fibres also occur in the three genera (Ephedra, Gnetum, and Welwitschia) of the Gnetales, making them the only known gymnosperms with g-fibres in their shoots. The shrubby species E. aspera and E. viridis were studied to determine the function and cues for production of g-fibres in the genus. It was hypothesized that E. aspera and E. viridis would produce g-fibres as a response to gravitational and internal stresses due to downward displacement (bending). Total number of g-fibres and number of g-fibres per area did not differ between displaced and untreated (control) stems of E. aspera. For the younger stems of E. viridis, control stems had more g-fibres than displaced stems, indicating that the production of additional g-fibres in control stems may be a response to wind or other perturbations. For both species, the oldest stems studied had the lowest g-fibre frequency, suggesting that little to no new g-fibres were produced as the stems aged, regardless of treatment. Furthermore, there were no other indications of reaction anatomy (asymmetry of phloem, compression wood, etc.) for E. aspera or E. viridis. These results and the cell wall composition of the fibres, especially those in the cortex, call into question whether the fibres of shrubby Ephedra are typical g-fibres.
Secondary cell wall composition and candidate gene expression in developing willow (Salix purpurea) stems
