Microsporogenesis in Pinus sylvestris L. VIII. Tapetal and late pollen grain development

This last portion of our developmental study of Pinus sylvestris L. pollen grains extends from just prior to the first microspore mitosis to the micro sporangial dehiscence preparatory to pollen shedding. In nine years of coll ecting each day the duration of the above period was 7 to 11 days. Tapetal cells extended into the loculus and embraced microspores during the initial part of the above period. Thereafter tapetal cells receded, became paralle l to parietal cells and so imbricated that there appeared to be two or thre e layers of tapetal cells. Tapetal cells were present up to the day before pollen shedding, but only rER and some mitochondria appeared to be in good condition at that time. A callosic layer (outer intine) was initiated under the endexine before microspore mitosis. After the first mitosis the first prothallial cell migrated to the proximal wall and was covered on the side next to the pollen cytoplasm by a thin "wall" joining the thick outer intin e. There are plasmodesmata between pollen cytoplasm and the prothallial cel l. After the second mitosis the second prothallial cell became enveloped by the outer intine. The inner intine appears after formation of the two prot hallial cells but before the third mitosis. During this two-prothallial cel l period before the third mitosis, plastids had large and complex fibrillar assemblies shown to be modified starch grains. After the third mitosis pla stids of the pollen cytoplasm contained starch and the generative cell (ant heridial initial), the product of that mitosis, is enveloped by the inner i ntine. On the day of pollen shedding cells are removed from the microsporan gial wall by what appears to be focal autolysis. The tapetal and endothecia l cells for 10-15 mum on each side of the dehiscence slit are completely re moved. One or more epidermal cells are lysed, but both a thin cuticle and t he very thin sporopollenin-containing peritapetal membrane remain attached to the undamaged epidermal cells bordering the dehiscence slit. Our study t erminates on the day of pollen shedding with mature pollen still within the open microsporangium. At that time there is no longer a clear morphologica l distinction between the outer and inner intine but, judging by stain reac tions, there is a chemical difference. The exine of shed pollen grains was found to be covered by small spinules on the inner surface of alveoli. Thes e had the same spacing as the Sporopollenin Acceptor Particles (SAPs) assoc iated with exine initiation and growth.