• Premise of the study: Reaction wood (RW) in seed plants can induce late and usually secondary changes in organ orientation. Conifers produce compression wood (CW), generated by compression tracheids, which generate a push force. Angiosperms produce tension wood (TW), generated by tension wood fibers (TWF) often described as “gelatinous fibers,” which exert a pull force. Usually RW is produced eccentrically, but it can occur concentrically, as in aerial roots of Ficus. However, gymnosperms can produce gelatinous fibers (tension fibers, TF), as in cortical and secondary phloem tissues (Gnetum). TFs are therefore limited neither to wood, xylem, nor angiosperms. Here we demonstrate that TFs in secondary phloem are involved in contraction of roots of cycads and compare them with TFs of Ficus.
• Methods: We sectioned root material of cycads at various stages of seedling development using simple staining and histochemical procedures to follow the course of secondary phloem development. Aerial roots of Ficus were compared with the cycad root material.
• Key results: Tension fibers (gelatinous fibers) occur extensively and continuously in the secondary phloem in roots that undergo contraction. Older tissues, but notably the xylem, become distorted by contraction. TFs in cycads correspond in cell wall features to TFs that occur in Ficus, but do not occur in secondary xylem. The individual fibers visibly contract.
• Conclusions: Tissue contraction in Cycas and Zamia corresponds to that found in angiosperms and Gnetum and further broadens the scope of the activity of tension tissues. This finding possibly indicates that gelatinous fibers originated at a very early period of seed plant evolution.
Via Pierre-Marc Delaux