Development of foliage and scale leaf in Thalassia is fundamentally identical, despite their pronounced morphological differences. The leaf primordium is at first only three layered, with the two outer layers continuous with, and derived from, the outermost tunica layer of the
shoot apex. The middle layer is derived from the layer of cells next beneath this surface layer. Outer layers give rise to the very simple epidermis, and inner tissues are derived entirely from the single mesophyll layer. The very precise method of segmentation involved in the differentiation
of this layer is described; the marked preponderance of transverse divisions in late stages produces elongation growth of the leaf from an indistinct intercalary meristem, so that distal regions mature first. Segmentation is identical in both types of leaf; the pronounced differences in shape
between them is largely the result of quantitative differences in cellular segmentation only. In scale leaves the blade aborts very early, whereas elongation and thickening of the basal sheath occurs relatively earlier and is completed sooner than in the foliage leaf, in which the blade is
well developed while the sheath, although it elongates considerably, does so only very late. Differences in the distribution of the mechanical tissues in blade and sheath of the foliage leaf are noted; a mechanically weak transition region between them accounts for the tendency for the blade
to break from the sheath.
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