Fruits and Seeds 



257 



Fig. 



The structure of seeds. Although 

 seeds vary as much in form as do 

 other plant organs, the different ar- 

 rangements of the three essential 

 parts may be illustrated by a castor 

 bean, a lima bean, and a grain of 

 corn. 



In the castor bean the seed coats 

 consist of a hard outer layer and a 

 thin inner membrane. These inclose 

 an endosperm, which is a mass of 

 cells containing food in the form of 

 starch, oil, and protein. Within 

 the endosperm lies the embryo, 

 ready to grow when favorable ex- 

 ternal conditions for germination 

 come. The embryo consists of the 

 hypocotyl and two very thin coty- 

 ledons, with a small bud between the cotyledons, called the plumule. 

 The cotyledons are the first leaf-like organs of the plant. The 

 hypocotyl is the first stem, and the plumule is the first bud. No 

 root is found in the embryo ; but when the seed germinates the 

 hypocotyl elongates, and from its basal tip the primary root de- 

 velops. The cotyledons at first absorb food from the endosperm, 

 expand, and when exposed to the light turn green and carry on 

 photosynthesis. The plumule grows upward and forms the stem. 

 These early stages of growth use up most of the food in the 

 endosperm. 



The lima-bean seed consists merely of the embryo, with a seed 

 coat inclosing it. The food in this seed has already been absorbed 

 into the embryo and stored in the greatly thickened cotyledons ; 

 that is, the young embryo continued its growth in the ovule and 

 absorbed all the food from the endosperm. The parts of the 

 embryo are the same as in the castor bear , but the cotyledons are 



U.S. Dept. of Agriculture 



165. Avocado, or alligator 

 pear, a salad fruit now being grown 

 in southern Florida and California. 



