A study of the mature megagametophyte of Stipa elmeri

Abstract

In Stipa elmeri Piper & Brodie ex Scribn., the pollen tube enters at the filiform apparatus of the degenerated synergid. The degenerated synergid has electron-dense cytoplasm in which organelles are not discernible. All other cells of the mature megagametophyte have nuclei, endoplasmic reticulum, plastids, mitochondria, dictyosomes, and vacuoles. Starch is found in the persistent synergid (in minute quantities), egg, and central cell. Lipids occur in the persistent synergid, central cell, and antipodals. The filiform apparatuses of the two synergids are hypothesized to perform different functions. In the degenerated synergid, the filiform apparatus serves to increase the surface area of the plasma membrane and thereby to offer a large area for pollen-tube-growth-directing compounds to diffuse out of the synergid. In the persistent synergid, the filiform apparatus is part of a suite of features which indicate that the persistent synergid is involved in the transference of materials into the megagametophyte. Another possible function of the persistent synergid is to aid in establishing the polarity of the egg. The pollen grain and tube have distinctive polysaccharide spheres that serve to delimit the pollen tube cytoplasm after discharge into the degenerated synergid. Associated with the degenerated synergid are bodies of dense materials as seen under electron microscopy, and bodies of RNA and protein as determined histochemically. These are probably the same thing and come from the degenerating synergid. The antipodals are the most cytologically active cells of the megagametophyte. They have some features which are characteristic of transfer cells and possibly function in the transference of materials into the megagametophyte. Other studies (Brink and Cooper 1944) have indicated that grass antipodals are involved in the control of endosperm development. The active cytoplasm of the antipodals may reflect the synthesis or transference of growth-controlling substances.