358 



The Living Plant 



sometimes seed coats so constituted as to take days or weeks for 

 water to penetrate them, sometimes a delay in the development 

 of the enzymes needed to soften the endosperm, sometimes no 

 doubt in yet other ways that are still undetermined. 



Thus is the new plant developed in the seed prior to its birth. 



2. The Growth Cycle; Germination. This is a very distinct 

 though brief stage. When the resting period is completed, the 

 seed germinates on the first access of water in conjunction with 

 warmth. The water is absorbed and passed on to the embryo, 



. 



FIG. 136. A generalized drawing of a typical case of germination, from the dry seed to 

 the fully grown embryo. The controlling factors are discussed in the text. 



which swells powerfully, and thus bursts open the seed coats. 

 Immediately the root grows rapidly out, and, no matter in what 

 position the seed or embryo may happen to he, invariably turns 

 downward under the stimulus of gravitation, and, develop- 

 ing a zone of anchoring and absorbing hairs, proceeds to grow 

 straight into the earth (figure 136). This is an obvious adapta- 

 tion to the new plant's first needs, a firm anchorage in the soil 

 and a supply of water therefrom. When the root is thus firmly 

 anchored, the embryonic stem, under the stimulus of gravitation, 

 begins to turn upward, and, guided by other stimuli, works the 

 cotyledons out of the seed, and carries them upward, where, 



