Reproduction in Multicellular Plants - 223 



EPICOTYL 



HYPOCOTYL 



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 bud. 



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- 

 doneae; or only one cotyledon, as in the 

 Monocotyledoneae (see Fig. 12-23). 



COAT 



COTYLEDON 



EPICOTYL 



HYPOCOTYL 



STALK 



Fig. 12-23. Longitudinal section of a corn grain, a 

 one-seeded fruit. 



