286 



STRUCTURE OF THE SEED. 



time, yielding to the young plant the nourishment they contain. 

 By the time this is exhausted, the true leaves and roots are suffi- 

 ciently developed, for the support of the structure; and the coty- 

 ledons, being then no longer required, decay away. Thus it is seen 

 that, in all the essential points, the history of the young Phanero- 

 gamic plant corresponds exactly with that of the young Fern ; 

 the chief difference consisting in this ; that the development of 

 the former, up to the time when its cotyledon or primary frond 

 ceased to support it, is assisted by the nourishment prepared for 

 it by the parent ; whilst the latter has no such assistance, but 

 obtains its nourishment from the surrounding air and moisture. 



441. The adjoining repre- 

 sentation of the seed of the 

 Marvel of Peru, affords an 

 example of a dicotyledonous 

 seed, possessing leafy cotyle- 

 dons, and a separate albumen ; 

 in these, the process of germi- 

 nation is the same, except that 

 the cotyledons only perform 

 the functions of temporary 

 leaves, the nutritious part of the seed being retained withm its 

 coats, until it is exhausted by the young plant. 



442. In the seeds of the 

 Monocotyledons, the structure 

 of which is illustrated by the 

 accompanying figure of that 

 of the Onion or Lily, the 

 albumen is always separate ; 

 and the embryo, which occu- 

 pies but a small proportion of 

 the whole mass, cannot always 

 be readily distinguished in the 



midst of it, until germination commences. The cotyledon at 

 first completely sheathes the plumula, which afterwards pierces 

 it, and unrolls its first true leaf. 



443. Now it is an interesting fact, that the division of the 



FIG. 89.-SKED OK THE MARVEL OF PERU. 



FIG. 90. SECTIONS OF SEED OF ONION. 

 a, a, albumen ; b, b, embryo. 



