Pollen cones of Afrocarpus and Podocarpus (Podocarpaceae, Coniferales) in the light of conifer evolution and Zimmermann's telome theory

Abstract

Pollen cones from various species of Podocarpaceae were investigated and a new evolutionary model of coniferous male reproductive structures is proposed. The inflorescence-like groups of pollen cones are identified as dense pollen cone clusters. These clusters result from a significant reduction in the length of the internodes of the fertile shoot axis, coupled with a shift of the cones towards the distal end. There is strong evidence that the simple structure of each pollen cone originates from a markedly reduced pseudanthial structure. The lateral hyposporangiate microsporangiophores are dorsiventral synangia comprising numerous fused, stalked microsporangia. It is highly likely that the hyposporangiate structure evolved from an ancestral radial (= perisporangiate) condition. The distal, leaf-like scutellum is formed by one or more sterile microsporangia that are fused and compressed. Contrary to the widely held view, the scutellum is not the tip of a microsporophyll. Based on Zimmermann's telome theory it is demonstrated how microsporangia can be produced in the absence of a microsporophyll.