232 THE PHYSIOLOGY OF PLANTS 



should have a picture of what actually occurs in the cocoa-nut. 

 The hollow centre of the cocoa-nut, in which the milk is con- 

 tained, is nothing else than the central cavity of the embryo 

 sac, which has grown to so enormous a size, and becomes only 

 partially filled with endosperm cells, forming the edible coat 

 of the nut. 



But all seeds do not possess a large mass of albumen at the 

 side of or around the embryo ; many seeds are exalbumiuous. 

 In such cases the development of the embryo greatly exceeds 

 the growth of any of the other parts of the ovule; the cells 

 of the endosperm become absorbed again, and are made use of 

 by the developing embryo, which grows so large that it fills 

 up the entire cavity within the two integuments. We then 

 get a seed of the same kind as that of the bean. In this case 

 the former integuments of the ovule have formed the hard 

 seed-coat (testa), and the latter encloses the embryo, which was 

 formerly represented by a single cell at the apex of the embryo 

 sac. The dark oval patch on the glistening testa is the point 

 at which the stalk of the ovule (funiele) was attached, fastening 

 the young seed to the placenta. This mark is termed the 

 hilum of the seed. 



The main portion of the seed consists of the two large seed- 

 leaves or cotyledons, which will be found attached one to the 

 other at the base, the attachment being continued into a small 

 papilla (the radicle) at one end, while between the cotyledons 

 is enclosed the embryonic shoot (plumule) with its rudimentary 

 leaves. That these parts are really the young organs of the 

 plant can best be seen on the germination of the seed. The 

 seedling then bursts through the testa, the young root apex 

 growing down into the soil. Then the two cotyledons expand as 

 succulent leaves, and between them grows up the young shoot. 

 As the latter increases in size the cotyledons become poorer 

 in contents, wrinkled, and ultimately shrivel up and fall off; 

 they have performed their duty in supplying the young root 

 and shoot with the food material with which they were filled. 

 The cotyledons, therefore, have the same function in the exal- 

 bumiuous seeds which the endosperm performs in the albu- 

 minous seeds, namely, to nourish the seedling during the early 

 stages of its growth. 



