Comparative embryology of Stipa elmeri (Gramineae)

Abstract

The ovule of Stipa elmeri is bitegmetic, hemianatropous, and pseudocrassinucellate. The micropyle is formed by the inner integument. The inner integument is two cells thick except at the micropyle. Its inner layer is persistent and safraninophilic, and some of its cells develop secondary walls. A cuticle is also present outside the inner integument. The outer integument is two cells thick except for a bump four or five cells thick at the chalazal end. At later stages of development, the outer integument breaks down except at the chalazal end. As the ovule develops, it undergoes changes in orientation as a result of growth pattern changes in the nucellus. A multiple protoderm is present in some parts of the nucellus.The outer layer of the nucellus persists to later stages of development and is covered by a cuticle. Fruit wall changes during development are (1) formation of a thick, unlignified wall on the protoderm, (2) elongation of cells lining the locule, and (3) decrease in number of cells. Megagametophyte development is normal and the antipodals proliferate. Before fertilization, both synergids undergo cytological change and one decreases in size. The pollen tube appears to enter at the base of the larger synergid. The endosperm is free nuclear at first but then becomes cellular. At later stages of development, the outer layer of the endosperm is meristematic. On the basis of embryological data, the following phenetic series can be constructed: S. elmeri → S. lemmonii – S. hendersonii → S. tortilis → Oryzopsis miliacea. Available embryological data does not indicate a close relationship between the Gramineae and the Juncaceae or Cyperaceae.