ONTOGENETIC STUDIES OF FLORETS IN POA (GRAMINEAE): ALLOMETRY AND HETEROCHRONY.

Species distinctions in grasses are based on differences in sizes of parts, particularly floral parts, and on differences in distribution of hairs; both sets of characters are apparently subject to repeated parallel evolution. Developmental studies of florets in the genus Poa reveal that there are only two developmental trajectories for lemma length relative to anther length (a measure of time to maturity): either lemma growth is continuous with anther growth, or the lemma reaches its adult size well before the anthers do. The tuft of long, cobweb-like hairs on the callus of many species forms early in lemma development, as might be expected for a character that marks a large group of species. The five possible patterns of hair distribution on the lemma itself represent stages of a single developmental pathway; differences between species presumably reflect differences in timing of onset or truncation of the developmental program (heterochrony).