6 THE SEED AND ITS GERMINATION 



The cotyledons are usually borne on a little stem, called the 

 hypocotyl (meaning beneath the cotyledon) (Fig. 1, c ; Fig. 2, c). 

 Often a little seed bud, or plumule (Fig. 3), is easily recogni- 

 zable in the embryo, more or 

 less inclosed by the cotyledons, 

 v , y /^---f if there are two of these. 



/m ia K$ffi" co 3 * The seed coats * Tlie em ~ 



bryo (and sometimes other seed 



contents) is inclosed by one or 

 more seed coats, which in many 

 cases preserve the embryo from 

 injuries of various kinds, and 

 also serve other purposes. The 

 principal seed coat is called the 

 testa; it varies greatly in thick- 

 ness, hardness, color, and mark- 

 ings, and also in other respects, 

 as is evident when one recalls 

 the varied appearance of such 

 familiar seeds as those of the 

 mustard, squash, bean, pea, locust, 

 apple, poppy, and Brazil nut. 



4. Topics for investigation. 

 The student should learn at first 

 hand (that is, from the seeds and 

 the young seedlings themselves), 



in connection with the present chapter, something about the 



following topics : 



1. The anatomy of a few typical seeds. 



2. Some of the conditions for germination. 



3. Some of the chemical changes produced in germinating 

 seeds, and their effect upon the surrounding air. 



4. The early steps in the development of seeds into plants. 

 The brief outline of the structure of the seed just given should 



be much enlarged by the details learned in the laboratory. 



FIG. 2. The castor bean and its 

 germination 



A, lengthwise section of ripe seed: t, 

 testa; co, cotyledon; c, hypocotyl. 

 B, sprouting seed covered with en- 

 dosperm. C, same, with half of en- 

 dosperm removed. D, seedling :T, 

 primary root ; r', secondary roots ; 

 c, arch of hypocotyl 



