. Principles of modern biology. Biology. COTYLEDON SEED COATS Fig. 12-21. An opened bean seed with one of the two cotyledons removed. This reveals the body of the em- bryo, which consists of the hypocotyl and the epicotyl. and other surrounding tissues, while the seed is ripening. Such a reserve of organic matter is vitally important to the embryo when the seed begins to sprout. Until the young sporo- phyte develops a good root system and raises its stem and leaves above ground where light is available, it cannot synthesize its own or- ganic materials and grow in a sell-sufficient manner. Duri
. Principles of modern biology. Biology. COTYLEDON SEED COATS Fig. 12-21. An opened bean seed with one of the two cotyledons removed. This reveals the body of the em- bryo, which consists of the hypocotyl and the epicotyl. and other surrounding tissues, while the seed is ripening. Such a reserve of organic matter is vitally important to the embryo when the seed begins to sprout. Until the young sporo- phyte develops a good root system and raises its stem and leaves above ground where light is available, it cannot synthesize its own or- ganic materials and grow in a sell-sufficient manner. During the first rapid growth of the young sporophyte, the cotyledons gradually give up their content of organic material, passing it to the growing body of the plant. Consequently the cotyledons keep shrinking as the seedling grows. In most seedlings the withered cotyledons finally drop off, but then the young sporophyte is no longer called a seedling—it is an independent plant. The point of attachment between the cotyledons and the body of the embryo (Fig. 12-21) subdivides the body into two parts: (1) the hypocotyl (literally, below the cotyle- dons), a tapering, slightly curved, rodlike part; and (2) the epicotyl (literally, above the cotyledons), a small upper part, which must be dissected with needles before its full struc- ture can be seen. Such a dissection reveals that the epicotyl of the bean consists of a pair of delicately folded embryonic foliage leaves, the plumules, and (hidden between the plumules) a small central conical mass, the embryonic COAT COTYLEDON HYPOCOTYL Fig. 12-22. Longitudinal section of a pine seed. Seed structure varies widely among differ- ent species, but three main types are gen- erally recognized. In gymnosperm seeds, the embryo possesses 6 to 10 cotyledons, which are needlelike in form and not very conspic- uously swollen (Fig. 12-22). And among an- giosperm (p. 616) seeds the embryo possesses either two cotyledons, as in the Dicotyle- d
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