Embryology of Agrostis interrupta (Gramineae)

Because of the characteristic morphology of the grass ovule, it is proposed that the end of the ovule opposite the micropyle be referred to as the basal end rather than the chalazal end. The ovule is hemianatropous, bitegmetic, and tenuinucellate. Both integuments are mostly two cells thick and exhibit some stretching of individual cells along the side of the ovule. This stretching is concomitant with increase in ovule length. The inner integument forms the micropyle and persists longer than the outer integument. Elimination of the outer integument is a result of stretching. The fruit wall is unlignified as a result of the transfer of the function of protection to the lemma. As evidenced by patterns of cell elongation, fruit elongation occurs first along the sides. There are few periclinal divisions in the nucellar protoderm, these occurring near the point of attachment of the ovule. Megasporogenesis and megagametogenesis are normal. Callose is associated with megaspore formation. The mature orientation of the megagametophyte, relative to the placenta, is obtained at approximately the two-nucleate state of development. Antipodal proliferation occurs before fertilization. After fertilization, large multinucleolate nuclei and multinucleate cells are seen in the antipodals. The cytological abnormalities seen in the antipodals may be associated with hormonal imbalance. The synergid, which will be penetrated by the pollen tube, develops a filiform apparatus before the other synergid. The pollen tube deposits sperm cells in a degenerated synergid. Endosperm is free nuclear and walls form first around the embryo. Starch appears at later developmental stages in the endosperm. When cellular, the endosperm grows by cell division at its periphery. There are two morphological axes within the grass ovule, that between the egg apparatus and the antipodals and that between the micropyle and basal end. In Agrostis interrupta, these two axes form a wide angle. Embryologically, A. interrupta is more similar to Avena. There are also embryological features that seem to characterize the Aveneae and Stipeae. Three general types of embryological similarities and differences can be discerned in the grasses, as well as in other plants: (1) similarities and differences that reflect relationships; (2) similarities of rare occurrence with no obvious taxonomic significance; (3) rare differences of no obvious taxonomic significance.