PATTERNS OF PROCAMBIAL DEVELOPMENT IN RACHILLA AND FLORET OF MUHLENBERGIA TORREYANA (GRAMINEAE, CHLORIDOIDEAE)'

Procambial development has been followed in serial transections of spikelet primordia of Muhlenbergia torreyana. An isolated procambial strand differentiates in the node subjacent to the site of the median region of the lemma primordium. Distal to the lemma median procambial strand, two pairs of procambial lemma laterals differentiate to join it. Acropetal differentiation of the lemma lateral procambia is distracted from the lemma toward other floret organs by the latters' procambial strands. The actual connection of the lemma marginal lateral traces with the margins of the lemma is achieved by capturing bundles originating near the lemma margins. The procambial lemma intermediate lateral traces never reach the lemma. Therefore, a developmental explanation is furnished for the three-nerved lemma characteristic of many chloridoid and arundinoid genera. Comparisons of procambial development among genera of the subfamilies of grasses may help reveal phylogenetic relationships.