Morphology, ontogeny and functional anatomy of the seeds of Colophospermum mopane

Abstract

Only one seed is carried inside the indehiscent fruit of Colophospermum mopane . Both the seed and fruit constitute the dispersal unit and the seed germinates while still inside the fruit. The mature seed of C. mopane has a thin seed coat that shows little differentiation, similar to the seed coats of typical Leguminous overgrown seeds. The malpighian cells that usually form a mechanical layer of protection are absent and this may be attributed to the seed being enclosed in an indehiscent fruit where the pericarp forms the protective layer. The thin, undifferentiated seed coat may also be an adaptation to low rainfall as it allows for the rapid uptake of water where rainfall occurs in small quantities or is of short duration. The seed originates from a bitegmic ovule. The inner layer of the inner integument persists throughout most of seed development and differentiates into an endothelium. The remainder of the inner integument degenerates completely at the time of fertilisation but the entire outer integument persists and differentiates into the seed coat. The integumentary cells display high mitotic activity in both the anticlinal and periclinal planes throughout seed ontogeny another feature reminiscent of overgrown or neotenous seeds. Up to the 5mm stage of the young seed, the older cell layers towards the outside of the integument are obliterated and resorbed as they are continually being replaced by new cell layers that originate from the inner layers. After the 5mm stage, the cells of the subepidermal layer begin to divide periclinally and anticlinally to keep pace with the inner layers. The senescent cells of the middle layer are then obliterated between the dividing cells of the inner integumentary layers and the subepidermal layers. The number of cell layers remains more or less constant throughout seed ontogeny because the obliteration of the older layers prevents the seed coat from becoming thick and multiplicative. The anticlinal divisions bring about the lateral extension of the seed coat to give rise to an aril that circumvents the seed. Another feature reminiscent of overgrown seeds is the convoluted appearance of the cotyledons which suggests that extensive seed growth is limited by the size of the fruit locule. As a consequence of the convoluted cotyledons the seeds may be regarded as ruminate. Rumination is, however, not only an outcome of seed growth inside the limited space of the fruit locule but also of uneven anticlinal divisions in the endothelium during early stages of embryogenesis. The entire vascular supply of the seed is limited to the aril and no secondary ramifications of vascular traces occur in the remainder of the seed coat. Due to extensive anticlinal divisions of the seed coat the primary vascular bundle is stretched in a lateral plane, causing the xylem strands to separate from the surrounding parenchyma cells, as well as from one another. This causes an air canal to develop in the primary vascular bundle that originates from the funiculus. The lateral extension of the aril also causes the position of cambiai-like meristematic areas that initially surround the primary vascular bundle to shift so that the area occupied by these cells becomes V-shaped or U-shaped. The cambial-like meristematic areas give rise to secondary vascular bundles. The micropyle is small and occluded with a dark staining substance that makes it impermeable to water. The integuments as well as the mature seed coat contain high concentrations of tanniniferous substances. The large seed size of the xerophytic C. mopane may be explained by its common origin from the ancestors of Guibourtia and Copaifera that occurred in high rainfall zones.