The ontogeny of Aeschynanthus seeds – A comparative study using scanning electron microscopy

Seed morphology in the genus Aeschynanthus (Gesneriaceae) is important in sectional classification. The number and type of appendages at the hilar end of the seed, papilla structure and testa cell orientation are all significant. SEM studies of seed and appendage development, at seven-day intervals from pre-pollination to maturity, were carried out in order to investigate possible sectional and seed type relationships. Fifteen species, representing six sections, were examined. Results show that there are no significant differences pre-pollination but two patterns were evident post-pollination, for both seed and hilar appendage development. Pattern I occurred in sects Microtrichium and Haplotrichium s.s. Pattern II was seen in sects. Aeschynanthus, Diplotrichium and Polytrichium. Patterns were less clear in species belonging to a group not yet circumscribed and here referred to as sect. X, but most closely resembled Pattern II. This study shows that the orientation of the testa cells is a developmental feature, defining the two major clades in the genus. It also shows that, in the two sections possessing more than one hilar appendage, there is initial development of one appendage relative to the coma in sect. Polytrichium and of one appendage relative to the second in sect. Diplotrichium. The results of this study are discussed with respect to molecular and morphological findings, and the evolutionary significance considered.