Cell differentiation in microsporangia of Pinus sylvestris. V. Diakinesis to tetrad formation

Following the diffuse stage, the progression of meiosis in Pinus sylvestris unlike that earlier in meiosis, was conspicuously asynchronous. During the diffuse stage of meiosis tapetal cells dedifferentiated. Plasmodesmata were formed, the cells developed a uniform, meristematic appearance and the nuclei underwent mitosis. Throughout the stages covered by this report tapetal cells redifferentiated, again becoming hypersecretory cells, and Ubisch bodies (orbicules) formed. In angio-sperms Ubisch bodies apparently form only once whereas in Pinus they are produced several or many times with a different and characteristic form each time. The future Ubisch bodies are filled from connections with cisternae of the endoplasmic reticu-lum, then coated by plasma membrane and its glycocalyx. The plasma membrane and glycocalyx coating are likely to be responsible for the specific exine form of Ubisch bodies. Cytokinesis after meiosis was typical of plant cells, but no cell wall formed. Thus deep invasions of callose between microspores give an appearance of furrowing, as was often suggested in classical literature.