The structure and vasculature in pollen cones of Taxus (Taxaceae, Gymnospermae) and its evolutionary significance

Abstract

The pollen cone structure and vasculature of Taxus baccata and T. wallichiana were investigated in the context of comparative studies in other conifers. The results indicate that the simple, flower-like pollen cone of Taxus could be derived from an ancestral compound inflorescence as in Cephalotaxus and Pseudotaxus. In Cephalotaxus, each lateral flower-like structure represents an entire pollen cone, compared with a single perisporangiate microsporangiophore in Pseudotaxus. The microsporangiophores are exclusively perisporangiate in both Pseudotaxus and Taxus, but in Pseudotaxus they are inserted in the axil of a pherophyll. By a simple reduction of these pherophylls, the Taxus pollen cone is formed—a strongly reduced inflorescence with flowers reduced to perisporangiate microsporangiophores, which are inserted directly at the cone axis, without pherophylls. Thus, the perisporangiate Taxus microsporangiophores correspond to the axillary flowers in Cephalotaxus and Pseudotaxus. The Taxus flower results from a strong reduction of an ancestral Cephalotaxus-like flower type (lateral pollen cones), where all remaining hyposporangiate microsporangiophores have fused to a radial structure during evolution. Thus, perisporangiate Taxus microsporangiophores represent radial synangia and not peltate microsporophylls, suggesting that hyposporangiate microsporangiophores are the ancestral condition in Taxaceae s.l.