. Elementary botany. Botany. nucellar cap. Only one end of the endosperm 271. upper end of the endcipcrm (prothallium) are aid us in determining what portion is the female prothallium. The nucellus is of course formed before the prothallium. The latter arises from a cell (macro- spore) near the center of the nucellus. This cell is larger, and has a larger nucleus than its fellows (see fig. 268). The prothallium here is formed much in the same way as in selaginella, where we recollect it begins to de- velop before the macrospore has. Fig. 269. Fig. 270. Section of ovule of white pine, int, inte
. Elementary botany. Botany. nucellar cap. Only one end of the endosperm 271. upper end of the endcipcrm (prothallium) are aid us in determining what portion is the female prothallium. The nucellus is of course formed before the prothallium. The latter arises from a cell (macro- spore) near the center of the nucellus. This cell is larger, and has a larger nucleus than its fellows (see fig. 268). The prothallium here is formed much in the same way as in selaginella, where we recollect it begins to de- velop before the macrospore has. Fig. 269. Fig. 270. Section of ovule of white pine, int, integ- Upper portion of nucellus of wliite pine, ument; pc. pollen chamber; pt, pollen tube; fg, pollen-grain remains; spc, sperm cells; n, nucellus; /«, macrospore cavity. vn, vegetative nucleus; pt^ pollen tube. reached its full size, and where the archegonia begin to form before it leaves the macrosporangium. 424. Male prcthallia.—By the time the pollen is mature the male pro- thallum is already partly formed. In fig. 255 we can see two well-formed cells. Other cells are said to be formed earlier, but the)-become so flattened that it is difficult to make them out when the pollen grain is mature. At this stage of development the pollen grain is lodged at the mouth of the ovule, and is drawn up into the pollen chamber. 425. Farther growth of the male prothallium.—During the summer and autumn the male prothallium makes some farther growth, but this is slow. The larger cell, called the vegetative cell, elongates by the formation of a tube, forming a sac, known as the pollen tube. It is either simple or branched. Inside of this sac the cells of the prothallium are protected, and farther. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Atkinson, George Francis, 1854-1918. New York, H. Holt
Size: 1640px × 1524px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookpublisher, booksubjectbotany
