DEVELOPMENTAL MORPHOLOGY OF THE EMBRYO AND SEEDLING OF RHIZOPHORA MANGLE L. (RHIZOPHORACEAE)

The embryo of Rhizophora mangle L. is initially attached to the integument by a long multiseriate suspensor. Its basal cells lyse, and intrusive growth of the endosperm envelops the embryo, forces the micropyle open, and often carries the embryo out of the integument. Thus, “germination” is effected by growth of the endosperm rather than of the embryo. The surface of the endosperm differentiates into a layer of peculiar transfer cells. The cotyledonary body initiates as a toroidal primordium, which later becomes lobed; most of the free portions ultimately fuse. After “germination,” the axis of the viviparous seedling grows by a diffuse intercalary meristem below the cotyledonary node. Before seedling abscission, the shoot apex produces three pairs of leaves, the first of which aborts, leaving the rest of the plumule protected by their stipules. The (immersed) radicle apex is nearly inactive, but lateral roots arise early in seedling development; these are usually the first or only roots to grow during establishment. Ten provascular strands “differentiate” in the cotyledons; a hollow provascular cylinder develops in the hypocotyl. Initial vascular differentiation in the latter is of many alternate poles of xylem and phloem; later, de novo differentiation of metaxylem opposite the protophloem poles, and vice versa, produces collateral bundles. Xylem maturation is endarch over most of the length of the hypocotyl, but tangential and random series of metaxylem vessels occur in the radicle end.