. Botany of the living plant. Botany. 250 BOTANY OF THE LIVING PLANT process is essentially the same. But the pollen-mother-cell, after the first nuclear division, is itself partitioned by a wall into two cells, in each of which the second nuclear division follows. The final result is thus the same, except that the arrangement of the tetrad is not tetrahedral, as it usually is in Dicoty- ledons. But the grains lie in pairs, the longer axis of one pair being at right. Fig. 197. A later stage of development of a pollen-sac, showing the young fibrous layer, containing starch. The tapetum (shaded)
. Botany of the living plant. Botany. 250 BOTANY OF THE LIVING PLANT process is essentially the same. But the pollen-mother-cell, after the first nuclear division, is itself partitioned by a wall into two cells, in each of which the second nuclear division follows. The final result is thus the same, except that the arrangement of the tetrad is not tetrahedral, as it usually is in Dicoty- ledons. But the grains lie in pairs, the longer axis of one pair being at right. Fig. 197. A later stage of development of a pollen-sac, showing the young fibrous layer, containing starch. The tapetum (shaded) surrounds the sac in which the tetrads float freely. ( x 100.) F. O. B. angles to that of the other. These are minor points ; in all essentials the tetrad-division which produces the pollen is the same throughout Flowering Plants. For the present this brief description must suffice. But later (ChapterXXXI.) the details of behaviour of the nuclei in this important process of tetrad-division, and chromosome-reduction will be described and discussed at greater length (p. 467). The pollen-grains, as their development shows, are produced from internal tissues of the plant, and are set free by rupture of the superficial tissues. Such bodies are called spores, and the pollen-grains being of relatively small size are called micro-spores. A step in their production is the tetrad-division. The tetrad breaks up later into its four constituent spores. Tetrad-division, is a constant feature in the production of spores in all spore-bearing Plants, such as Mosses, Ferns, and Seed-Plants. When a marked feature such as this recurs with constancy in a large group of organisms, even though they differ in many other respects, it may be assumed that it. 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 Bower, F. O. (Frederick Orp
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1919
