Chapter 6 212 rapid growth and seasonal fluctuations. On the other hand, (semi-)woody plants invest heavily in the development of secondary tissues, such as wood and bark, to support their height and withstand environmental stressors. Consequently, (semi-)woody plants often have reduced or absent pith, as their primary structural support comes from the surrounding tissues (Esau, 1965). The linkage of pith elongation and radial division with SAS is still unknown, but from our current data, we did observe that pith cell elongation is conserved in shade-responsive species’ stem elongation, thus suggesting a potential connection between pith cell behavior and shade adaptation strategies in these plants. Unlike the woody layers responsible for structural support, pith cells may have lower lignin content and they exhibit larger, rounder shapes (Gunning and Steer, 1996). Their differentiation and development from meristematic cells during growth set them apart, as they become specialized for nutrient and water storage, whereas other cell layers differentiate into cells dedicated to transport and structural support. Furthermore, in woody plants, secondary growth primarily occurs in the vascular cambium and cork cambium, distinct from the pith region, which generally does not directly contribute to secondary growth processes (Venning, 1949; Lovell et al., 1971). We hypothesize that their elongation in SAS is facilitated by relatively thinner cell walls and lower lignin content, allowing them to expand and store larger volumes of materials (Esau, 1953). Given the inherent capacity of pith cells for elongation, they are poised to play a role as key drivers in the elongation of differentiated vascular tissues.
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