ABSTRACT
PRIMARY GROWTH The aerial stem typically bears the green photosynthetic leaves and the reproductive organs (26, 336, 337), whereas underground stems are frequently perennating and food storage organs (338). Most unthickened stems are cylindrical, but ridged and rectangular forms (339, 340) are common, and in a few species stems are flattened, leaf-like structures called phylloclades (341-343). In many plants of very dry habitats, leaves are reduced to scales and the stem’s persistent, chlorophyllous cortex is the main site of photosynthesis (344, 345, 347, 362). Succulent stems store water as well as carrying out photo synthesis, and are often protected by spines that are modified leaves in cacti (362-364), and modified axillary branches in Euphorbia (347, 368). Starch is commonly stored in the parenchymatous ground tissue of the stem and is particularly abundant in the swollen stems of succulents and the underground stems of corms, tubers, and rhizomes (338). On the condensed shoots of rosette species (226) the leaves are crowded and the internodes short, but at flowering the internodes commonly become much more widely spaced as is dramatically shown in Agave (337).
