936 



ECOLOGY 



peas, and also in the cereals, where the scutellum represents the coty- 

 ledon (figs. 703, 706) ; such germination is termed hypogaean. In other 

 cases the cotyledons emerge from the seed with the plumule and come 

 into the light, usually turning green, as in the beech (fig. 1231), pump- 

 kin, and mustard (fig. 700); such germination is termed epigaean. 

 In h3^ogaean species the cotyledons die, as soon as the foods are re- 

 moved, or the seedling is well established as an autophyte. The same 

 is true in some epigaean species, as in various beans, but in many other 

 epigaean forms the cotyledons enlarge and doubt- 

 less manufacture considerable food. Cotyledons 

 are much more uniform in shape than are ordi- 

 nary leaves, perhaps because of the relatively 

 uniform conditions in which they are developed. 

 Usually they are undivided, though occasionally 

 divided, as in Tilia. In the monocotyls the en- 

 closure of the delicate part of the plumule within 

 its older sheathing leaves prevents injury in 

 breaking through the soil. In many epigaean 

 dicotyls the cotyledons adhere at the tips, pro- 

 tecting the plumule until it emerges from the 

 ground. In other cases (as in the pumpkin) the 

 hypocotyl elongates considerably, while the coty- 

 ledons remain within the seed, resulting in an 

 arching of the young stem and in the pulling 

 of the delicate tip from the seed and through the 

 ground instead of pushing. Where seeds germi- 

 nate too near the surface, the contraction or other 

 movement of the growing root exerts a pull on 

 the shoot, so that the position proper to the species 

 is eventually acquired. 

 The germinative processes of buds. — The structural features of buds. — 

 Buds commonly are divided into two classes, active and resting. Active 

 buds are associated with all seasons in uniform climates and with the 

 vegetative seasons of periodic climates, while resting buds are associated 

 with the unfavorable seasons of periodic climates. Germinative processes 

 are conspicuous as resting buds develop into active buds. The resting 

 buds of shrubs and trees, commonly called winter buds in temperate 

 and in cold climates, are protected by tough and impermeable bud 

 scales, whose structure and role have been considered elsewhere (p. 643). 



Fig. 123 1. — A seed- 

 ling of a dicotyl, the 

 beech (Fagus grandi- 

 folta), illustrating epi- 

 gaean germination; c, 

 cotyledons ; /, first foli- 

 age leaves. 



