. Botany; principles and problems. Botany. THE SPERM A TOPII VTA 347 structure (the stigma) which is nearby. Since the egg cells are buried in the nuccllar tissue, it is evident that the male gametes cannot approach them directly, as in the lower plants, and a new structure has accordingly been developed which conveys them to the egg. This is the pollen-tube. It arises from the pollen-grain as a slender, thin-walled projection and into it the contents of the gi'ain passes, led by the Fig. 218.—Development of the male gametophyte in a gymnosperm (Pine). A, longitudinal section (d
. Botany; principles and problems. Botany. THE SPERM A TOPII VTA 347 structure (the stigma) which is nearby. Since the egg cells are buried in the nuccllar tissue, it is evident that the male gametes cannot approach them directly, as in the lower plants, and a new structure has accordingly been developed which conveys them to the egg. This is the pollen-tube. It arises from the pollen-grain as a slender, thin-walled projection and into it the contents of the gi'ain passes, led by the Fig. 218.—Development of the male gametophyte in a gymnosperm (Pine). A, longitudinal section (diagrammatic) of a staminate or "male" cone. B, one of the sporophylls from A, much enlarged. The sporangium is filled with micro- spores or pollen, in various stages of development. C, section across the sporo- phyll at right angles to that in B, showing the two sporangia which are borne on each sporophyll. D, a mature pollen grain. The outer wall (in the case of pine pollen) is p^iffed out somewhat at two points, forming balloon-like "wings" which add to the buoyancy of the grain. The single nucleus of the megaspore has now divided to form the generative cell, lying next the wall, and the tube nucleus, in the center of the cell. E, a germinating pollen grain. The tube nucleus follows close behind the end of the growing tube. The generative cell has divided into a stalk cell (lighter) and a body cell (darker). F, the end of the pollen-tube just before fertilization. The body cell has developed into the two male cells or gametes, which have now come down the tube and are ready to effect fertilization. Fertilization and Seed-production (Figs. 217 and 229).—The poll(Mi tube grows rapidly and penetrates the tissues, much as a fungus filament penetrates the tissues of its host, until it reaches the embryo-sac and the egg-cells. A large nucleus in the tube, often the only one at this point except the tube nucleus, now divides into two male gametes, and as the
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectbotany, bookyear1923
