Studies on the relationships and evolution of supraspecific taxa utilizing developmental data. 1. Stipa lemmonii (Gramineae)

Abstract

This is a study of floret development and embryology of Stipa lemmonii (Vasey) Scribn. as a means of assessing its relationships and evolution. There is an advantage to using developmental data for it allows one to break complex structures down into developmental units. These developmental units can be compared and thus more characters can be analyzed. The lemma, palea, ventral lodicule, and carpel of S. lemmonii are leaf-like (sensu Sattler 1966, 1967) in initiation and early development. The stamens are stem-like and the ovule develops from the floret apical meristem. The ovule is interpreted as being cauline. The ovule is hemianatropous and pseudocrassinucellate. The inner integument delimits the micropyle. The integuments are ephemeral except for the inner layer of the inner integument. Embryo sac development is Polygonum type. Both synergids undergo change before entry of the pollen tube and the pollen tube appears to empty into the larger synergid. The antipodals are proliferated. The endosperm is at first free nuclear. Developmental features of S. lemmonii were compared with developmental features of S. tortilis, Oryzopsis miliacea, and S. hendersonii. The last is a species which hybridizes with S. lemmonii. Thirty-five characters were abstracted from the developmental data and were analyzed statistically. The results indicated that S. lemmonii and S. hendersonii are most closely related, and that these two plus S. tortilis form a group distinct from O. miliacea. The individual differences between taxa show gradation in the degree of difference. This is taken to indicate that the evolution of differences, and the evolution of taxa, is the result of the accumulation of many minor differences. Developmentally, plants are very simple, especially as compared with higher animals. Because of this, it is difficult to make generalizations about the "basic nature" of a plant structure using developmental data. Flowers seem best interpreted as functional units. This allows one to accept the concept that flowers in general are polyphyletic, a concept that is indicated by objective analysis of the data.