PRIMARY-CELL WALL TEXTURE AND ITS RELATION TO SURFACE EXPANSION

Recent data have revealed the frequent occurrence of helicoidal and ch olesteric-like structures in various primary and secondary walls. The present article deals with the texture specificity and the texture cha nges in the primary wall during the rapidly growing phase. First we de fine the characteristics of the cholesteric pattern and the mesophases and discuss two possible sources of misinterpretations: one technical , the fragility of constructions, and one biological, the gradients, d ifferentiation, and short-lived organization. Both features explain wh y the liquid crystal concept has emerged recently for the growing wall . Two examples of cells showing highly oriented surface growth are con sidered and compared. (a) Mung bean seedlings are often used for the s tudy of expansion. In this model the external and growth-limiting tiss ues have primary walls with early and transient planar twisted assembl ies. The latter are progressively sheared and dispersed, and growth st ops when the helicoidal pattern is completely randomized. (b) In the d ifferentiated cell wall of collenchyma the bundles of supporting cells assemble walls in which similar planar twists are found. The helicoid s are likewise randomized during the surface growth. The difference is that they are vigorously renewed and the last arced layers are no lon ger involved in the elongation process and persist without apparent mo dification of their cholesteric arrangement. Finally, we discuss the q uestion of the intermolecular interrelations and degree of freedom of the wall components through the helicoids.