Primary Phloem Development in the Shoot Apex of Rhizophora mangle L. (Rhizophoraceae)

The primary phloem in the shoot apex of the mangrove Rhizophora mangle L. is largely confined to the comparatively condensed area between the first three leaf pairs. The main extension zone, surrounded by the stipular sheath of the third leaf pair, contains vascular bundles arranged in a procambial ring and characterized by a well‐developed primary phloem and a less advanced xylem. The phloem consists of a great number of sieve elements, an equal number of associated companion cells, and a few phloem‐parenchyma cells. The differentiation of the sieve‐element protoplast (with e.g., chromatolytic nuclear degeneration, loss of the vacuole and most organelles) proceeds largely according to a well‐known pattern. Their P‐type plastids, however, form their protein crystals rather late and therefore cannot be used as an early cell marker. Lateral sieve‐element walls are distinct from other wall parts and walls of other cells by their heavy nacreous thickenings, the formation of which is shown to be strictly correlated with the occurrence and orderly arrangement of cortical microtubules.