Embryology of Festuca microstachys (Gramineae)

Abstract

As the ovary wall develops, some similar events are seen in the dorsal and upper portions. These are as follows: (1) the deposition of many starch grains in most of the ground meristem cells; (2) considerable cell growth in most of the ground meristem cells; (3) the development of a thick outer wall on the outer protoderm of the ovary; and (4) elongation in the protoderm cells lining the inner part of the ovary. Unlike the upper ovary wall, starch grains appear in the outer protoderm cells of the dorsal ovary wall. A cytoplasmically dense layer of cells subjacent to the inner protoderm of the dorsal ovary wall continues to undergo cell division when other ground meristem cells are enlarging. This layer of cytoplasmically dense cells is the only recognizable layer in the ovary wall when the seed is mature. The ovule is bitegmetic, hemianatropous, and tenuinucellate. Integuments are protodermal in origin and both are mostly two cells thick. Cell division is more prolonged in the inner than in the outer integument and the inner layer of the inner integument develops safraninophilic granules in the cytoplasm of its cells. The outer integument is stretched and obliterated before the inner. As a result of faster growth in the basal end of the nucellus, the ovule undergoes a change in orientation from orthotropous to hemianatropous. Periclinal divisions are rare in the nucellar protoderm. The nucellus is eliminated by the time that the seed is mature. Megagametophyte development is normal and callose is associated with megaspores. Unlike most grasses, only three antipodals form but each antipodal is large and contains what appear to be many atypical nuclei. The endosperm initially lacks cell walls but they appear later in development. When the endosperm is cellular, its surface layer is meristematic. Starch grains are numerous in the endosperm cells and the cells have thick walls. The thick walls are a possible source of stored energy. A cuticle on the surface of the endosperm may serve a protective function. Different types of grass ovules are the result of different degrees of the same developmental phenomenon at comparable sites. Relationships as determined by cluster analysis of a similarity matrix based on detailed embryological data show that four members of the tribe Stipeae cluster together and this cluster is separate from a cluster consisting of F. microstachys and Agrostis interrupta. Furthermore, the degree of similarity between the four stipeae was higher than that between F. microstachys and A. interrupta. This correlates well with the modem opinion that Festuca and Agrostis are in separate tribes.