. Practical botany. Botany. 116 PEACTICAL BOTAKY Each mature grain contains a generative nucleus and a tvhe nucleus {g and t, Fig. 107, A). After the pollen grain lodges on the stigma the inner coat of the grain becomes slightly distended by osmosis, pro- duced by contact with the moist stigmatic surface. The distention of the inner coat causes it to protrude through the outer coat and it at length develops into the wall of a pollen tube (Fig. 107). This tube has the nucleus (f) at its tip and a generative cell (^) somewhere within the tube. Finally the genera- tive cell divides in- to two mal
. Practical botany. Botany. 116 PEACTICAL BOTAKY Each mature grain contains a generative nucleus and a tvhe nucleus {g and t, Fig. 107, A). After the pollen grain lodges on the stigma the inner coat of the grain becomes slightly distended by osmosis, pro- duced by contact with the moist stigmatic surface. The distention of the inner coat causes it to protrude through the outer coat and it at length develops into the wall of a pollen tube (Fig. 107). This tube has the nucleus (f) at its tip and a generative cell (^) somewhere within the tube. Finally the genera- tive cell divides in- to two male nuclei, these develop into male cells (Fig. 107, B'), and the tube nu- cleus disappears. 108. Course of the pollen tube: fertilization. The pollen tube makes its way from the stigma to the ovary either through a canal or passage (Fig. 108), or by directly trav- ersing the cellular tis- sue of the style, upon which it acts so as to eat its way along by means of ferments (Sect. 36) secreted by the tube. The tube may require a day or more to reach the ovule. Food materials from the style are dissolved by the enzymes and used in promoting the. Fig. 107. Germination of tiie pollen grain of a dicotyledon A, an early stage in tlie germination; B, later stage, with the tube rather fully developed; g, generative cell; t, tube nucleus; Sj, S2, male cells formed from the generative cell. It is ap- parent that when the growth of the tube is far advanced thetubenucleus (<) almost disappears. Much magnified. After Bonnier and Sablon. 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 Bergen, Joseph Y. (Joseph Young), 1851-1917; Caldwell, Otis William, 1869- joint author. Boston, New York [etc. ] Ginn and company
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