SPERMATOPHYTES 127 



Just what conditions result in dormancy we do not know in 

 all cases, but we do know some facts that are associated 

 with it. What these are will be described in the next section. 



76. The seed. While the embryo is being formed, 

 changes are taking place in the integument of the ovule. 

 A new kind of tissue begins to develop from the cells of the 

 integument, and continues to develop until it forms a hard 

 covering that completely invests the ovule, the only breaks 

 in it being at the micropyle (whose position is indicated by 

 what looks like a small scar) and at the point where the ovule 

 (megasporangium) was attached to the carpel (mega- 

 sporophyll). When this hard coat (testa) is complete, the 

 ovule has become a seed (Fig. 100, A). 



It is evident that the seed is a very complex structure, and 

 that it has resulted from the retention of the megaspore 

 within the megasporangium (ovule), so that within this 

 sporangium the female gametophyte develops, fertilization 

 takes place, and the young sporophyte (embryo) is formed. 

 In a seed, therefore, three generations are represented : 



(1) the old sporophyte, represented by the ovule structures ; 



(2) the female gametophyte, very commonly called the en- 

 dosperm of seeds ; and (3) the young sporophyte (embryo) . 

 It is a simple thing to observe the structure of a seed and to 

 watch its " germination," but really to know the structure 

 of a seed needs the approach to it from the lower groups of 

 plants. 



A seed is said to " germinate," but it is plain that it is not 

 germination in the sense we have been using that word, for 

 only spores and fertilized eggs (oospores) germinate. Besides, 

 germination occurs when the fertilized egg within the ovule 

 produces the embryo; therefore, when a seed is said to 

 " germinate," the real germination had occurred long before, 

 usually the preceding season, sometimes many seasons 

 before. What " seed germination " means, therefore, is not 

 real germination, but the " awakening " of the young 



