Floral morphogenesis and proliferation in Poa labillardieri (Poaceae)

Inflorescence and spikelet development in Poa labillardieri Steud. were investigated by scanning electron microscopy. Thirteen developmental stages were described in detail, starting with the vegetative shoot apex which was shown to be of the short type (stage zero), followed by a conversion from vegetative to. oral meristem at Stage 1 and ending at Stage 12, with a mature panicle consisting of a variable number of florets at anthesis within each spikelet. The occurrence of short-type vegetative apices in this perennial grass adds further support to the view that there is no correlation between life span and the apex type. The branches of the P. labillardieri panicle are formed in acropetal succession; however, it is the upper branches that first bear rudiments of the spikelets, starting at the tip of the branches. In contrast differentiation of florets within each spikelet occurs in acropetal succession, so that the basal floret is farthest advanced and each successively upper floret less advanced. P. labillardieri occasionally produces inflorescences containing spikelets in which some or all of the florets are replaced by a plantlet that is structurally similar to a vegetative tiller. Proliferous development ranged from a situation where all florets were converted to vegetative propagules that can be detached and rooted readily in soil, to cases where proliferation extended only as far as an enlargement of the lemma, with either functional or non-functional sexual organs in its axil. Under greenhouse conditions, there was a shift from occasional cases of partially proliferating spikelets that occur in the wild to complete vigorous proliferation stimulated by unknown factor(s). Departure from the normal sexual pattern took place from early Stage 5 (initiation of spikelet primordial) to late Stage 6 (differentiation of florets).