Flowers

Abstract

Arabidopsis thaliana is a rosette annual with separate vegetative and reproductive growth phases. The apical meristem that produced leaves in the vegetative phase switches to producing flowers in the reproductive phase. Thus, the vegetative meristem is apparently converted directly into an inflorescence meristem, which produces the primary inflorescence shoot. There appears to be no fundamental difference in organization between the two types of meristems and the distinction between them may merely be a slight change in shape and the difference in identity of lateral meristems they produce (leaf versus flower). Following germination, the apical meristem produces leaves with little elongation between successive leaves forming a rosette. The number of leaves formed is dependent upon genotype and growth conditions. When the plant becomes florally induced, the apical meristem switches to producing flowers. Subsequent to the production of the first few flower primordia, the plant bolts due to increased internode elongation between the uppermost leaves and between flowers. Thus, the basal positions on the primary inflorescence shoot are occupied by a small number of cauline (stem) leaves and the apical positions by a potentially indeterminate number of flowers. Secondary inflorescence meristems that reiterate the pattern of development of the primary inflorescence shoot develop in the axil of each of the cauline leaves of the primary inflorescence shoot and tertiary inflorescence shoots arise in the axils of the cauline leaves on the secondary inflorescence shoots. In addition, further inflorescence meristems develop in a basipetal manner in the axils of the rosette leaves. The distinction between rosette and cauline leaves may be rather arbitrary as there appears to be a continuum in shape from the first produced rosette leaves through the last produced cauline leaves. The leaves (rosette and cauline) and flowers arise from the flanks of the apical meristem in the same phyllotactic spiral. The inflorescence is an open raceme with the development of individual flowers proceeding acropetally (from basal to apical positions) (Müller, 1961). Flowers are perfect, bractless, and naturally self-fertilize with a low incidence of outcrossing under laboratory conditions (see e.g. Snape and Lawrence, 1971; Relichová, 1978).