Embryological studies on Spiranthes australis Lindl.

Abstract

The flower comprises two trimerous whorls of perianth lobes, of which the labellum is supplied by four to six vascular strands and the rest by three bundles each. Some of the mesophyll cells become enlarged and contain raphides. The fertile stamen is quadrilocular. At the tetrad stage only the epidermis and the fibrous endothecium persist, while the two middle layers and tapetum disintegrate. The tapetal cells are uninucleate and the microspores remain together in tetrads. The pollen grains are bicelled at the time of shedding. The tricarpellary inferior ovary bears an indefinite number of bitegmic anatropus ovules arising from three parietal placentae. The hypodermal archesporial cell differentiates directly into the megaspore mother cell which gives rise to a linear tetrad of megaspores. The upper dyad cell rarely degenerates before the division is completed. In one instance a row of five cells was noticed. This might have arisen from two superposed megaspore mother cells or from an extra division of one of the megaspores. The chalazal megaspore functions. The mature embryo sac is usually six-nucleate, probably due to a suppression of the last division of the two chalazal nuclei. The single antipodal cell degenerates early and may not be seen in mature embryo sacs. The two polar nuclei may fuse to form the secondary nucleus or degenerate without fusion. Syngamy occurs, but triple fusion is delayed or is omitted altogether so that there is no endosperm formation. The first division of the egg is transverse. The mature embryo is ovoid in shape and undifferentiated. It has no suspensor haustoria and is enclosed within the seed coat formed by the outer integument. Various abnormalities have been recorded. Evidence is presented to indicate that parthenogenetic development of the egg is possible and does occur in some ovules. In such embryo sacs the undischarged pollen tube is usually seen beside the egg, but parthenogenetic embryos have also been observed in embryo sacs that had apparently not received a pollen tube at all. These observations have been confirmed by chromosome counts during division stages of the egg as well as in cells of the pro-embryo. Occasionally an embryo sac receives more than one pollen tube. Both the male nuclei of a pollen tube or the male nuclei of different pollen tubes may fertilize the egg, indicating the possibility of polyploid embryos.