Embryogeny of the Podocarpaceae

Abstract

Although the embryogenies of 22 spp. of Podocarpaceae are considered, this investigation is concerned chiefly with Podo-carpus dacrydioides, P. urbanii, P. macrophyllns maki, P. totarra, P. gracilior, P. nankoensis and Phyllocladus alpinus, which are illustrated. A dozen other spp. of Podocarpus observed in one or several stages each could be related to known types. For the latter, satisfactory results were obtained from dried herbarium material, from which endosperms were removed and placed in water for a day or two before dissection. The embryonic cells are borne on prosuspensors of 3-22 or more elongated cells. Various combinations of few or many cells in the prosuspensor, with 1-12 embryonic cells arranged in a single tier or in 2, 3 or 4 tiers, give a wide range of embryonic types. All Podocarpaceae have binucleate embryonic cells, which represent the long-delayed telophase of a mitosis occurring in the proembryo. The cell plate is not formed while the prosuspensor elongates greatly. Finally when the pair of nuclei have undergone another mitosis, walls form between all 4 nuclei, so that the binucleate condition passes directly into the 4-celled condition in each embryonic unit. The summary, drawing upon all previous investigations, shows a wide range of embryonic types included within this family, with the genus Podocarpus having the greatest variation; its subgenera and some of its sections may be the equivalent of separate genera. The number of cells in the prosuspensor, as well as the number of embryonic cells, is greatest in the spp.: P. nagi, P. nankoensis, P. usambarensis, P. gracilior and P. amarus, all of which have cleavage polyembryony. The number of free-nuclear divisions in the proembryo (32 nuclei before walls are formed) and the early embryonic organization resemble more closely than any other conifer the Araucariaceae. These podocarps are considered most primitive.