Abstract
An essentially unidirectional lateral meristem differentiates in the outer cortex at the hypocotyl-primary root juncture. This meristem, termed the secondary thickening meristem (STM), then differentiates acropetally towards the shoot apex. It produces secondary tissue (conjunctive tissue) centripetally. Desmogen strands (secondary amphivasal vascular bundles) differentiate from prodesmogen strands in the conjunctive tissue. Root primordia also differentiate from derivatives of this meristem. A primary thickening meristem (PTM) which appears to be continuous with the shoot apex differentiates basipetally. The PTM gives rise to the large primary body of the cortical region of the shoot, and its activity results in the elevation of the leaf bases. In the seedling stages (i.e., for about 2 years) the PTM and STM are longitudinally continuous as a result of the basipetal differentiation of the former and acropetal differentiation of the latter. In adult shoots, however, the two are discontinuous.
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