Elementary botany . oes into rest about the stage shown infig. 358. The conical portion ofthe nucellus which lies above is thenucellar cap. 621. Male prothallia.—By thetime the pollen is mature the maleprothallium is already partlyformed. In fig. 343 we can seetwo well-formed cells. Two othercells are formed earlier, but theybecome so flattened that it is diffi-cult to make them out when thepollen grain is mature. These areshown in fig. 357, pl and p2, andthey are the only sterile cells of themale prothallium in the pines. Thelarge cell is the antheridium wall,its nucleus in fig. 357. The
Elementary botany . oes into rest about the stage shown infig. 358. The conical portion ofthe nucellus which lies above is thenucellar cap. 621. Male prothallia.—By thetime the pollen is mature the maleprothallium is already partlyformed. In fig. 343 we can seetwo well-formed cells. Two othercells are formed earlier, but theybecome so flattened that it is diffi-cult to make them out when thepollen grain is mature. These areshown in fig. 357, pl and p2, andthey are the only sterile cells of themale prothallium in the pines. Thelarge cell is the antheridium wall,its nucleus in fig. 357. Thesmaller cell, , is the central cellof the antheridium. During thesummer and autumn the maleprothallium makes some farthergrowth, but this is slow. TheFig 358. larger cell, called the vegetative Section of ovule of white pine, int, in- cell or tube cell, which is in realitytegument, pc, pollen chamber; pt, pollen a.„.j,«„„, ol«„ tube; n, nucleus; m, macrospore cavity. the wall of the antheridium, elon-. 306 MORPHOLOGY. gates by the formation of a tube, forming a sac, known as the pollen is either simple or branched. It grows down into the tissue of the nu-cellus, and at a stage represented in fig. 358, winter overtakes it and itrests. At this time the central cell has divided into two cells, and thevegetative nucleus is in the pollen tube. 622. The endosperm.—In the following spring growth of all these parts
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