The formation of integuments, megasporogenesis and megagametogenesis in
Dendrobium catenatum, an economically important orchid, are observed. After
pollination, mitotic cell divisions of the placental epidermis result in the
formation of a branching system of outgrowths. The tip of each branch
consists of an archesporial cell derived from the differentiation of the
terminal subepidermal nucellar cell. It differentiates directly into a
megasporocyte. The first division of the meiosis of the megasporocyte
produces a dyad approximately equal in size, in which the micropylar cell
promptly degenerates. The second meiotic division of the remaining dyad cell
results in the formation of two megaspores of unequal size. The larger
chalazal cell becomes functional and eventually develops into a mature
megagametophyte. The development of the megagametophyte conforms to the
Monosporic Polygonum type. The final arrangement of the mature embryo sac
conforms to a sevencelled/ eight-nucleate structure. The mature ovule is
bitegmic, tenuinucellate and has an anatropous orientation. In the present
study, we also discuss the differences between three main types of embryo
sac development and the improvement of section techniques.