. Principles of modern biology. Biology. ZYGOTE DEVELOPING Fig. 12-34. Development of the embryo of a dicoty- ledonous plant. Germination. When a seed is ripe, devel- opment stops temporarily. Under natural conditions most seeds remain in the dormant state throughout the winter or dry season; and some seeds cannot be forced to germinate without the intervention of a definite resting period. At the proper time, with the advent of warmth and moisture, the seed absorbs water, swells powerfully to burst the seed coats, and now the embryo resumes devel- opment. The hypocotyl elongates rapidly, form
. Principles of modern biology. Biology. ZYGOTE DEVELOPING Fig. 12-34. Development of the embryo of a dicoty- ledonous plant. Germination. When a seed is ripe, devel- opment stops temporarily. Under natural conditions most seeds remain in the dormant state throughout the winter or dry season; and some seeds cannot be forced to germinate without the intervention of a definite resting period. At the proper time, with the advent of warmth and moisture, the seed absorbs water, swells powerfully to burst the seed coats, and now the embryo resumes devel- opment. The hypocotyl elongates rapidly, forming the primary root, which soon develops a zone of root hairs just above the tip (Fig. 12-20). The primary root is positively geotropic; that is, it grows downward, toward the earth, re- gardless of the position in which the seed may happen to lie (Fig. 12-35A). At the same time, or shortly later, the epicotyl, being negatively geotropic (Fig. 12-35B), begins to grow upward, forming the primary stem and the leaves. During these early stages of growth the young sporophyte draws upon the re- serve organic nutrients stored in the cotyle- dons and endosperm (if present). This or- ganic material is partly oxidized for energy and partly transformed into essential organic components in the growing cells. Despite the fact that the total amount of organic material is decreasing during this period, the actual size of the young plant increases greatly,. HYPOCOTYL. 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 Marsland, Douglas, 1899-. New York, Holt, Rinehart and Winston
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