208 



Science of Plant Life 



the plumule. The cotyledons are the first leaflike 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 end the primary root de- 

 velops. The cotyledons at 

 first absorb food from the en- 

 dosperm and later expand into 

 photosynthetic organs, which 

 become green when exposed 

 to the light. The plumule 

 grows upward to form the 

 stem. All these early changes 

 are made at the expense of the 

 food in the endosperm. 



The bean seed consists 

 merely of the embryo with a 

 seed coat about it. The food 

 in this seed has already been 



FIG. 124. Development of mangrove seed- . 



lings. This small tree grows on soft mud absorbed into the embryo and 

 flats in the tropics and semi-tropics. The s t ore d in the greatly thickened 



seed (A and C) germinates while still at- . 



tached to the tree and forms an embryo a Cotyledons ; that IS, the young 



foot or more in length. The embryo embryo has Continued its 



finally drops endwise like an arrow into , _,, -, , .1 , 



the mud and starts a seedling (D). g rowth m the Sf ; ed Untl1 * 



has all the food inside itself. 



The parts of the embryo are the same as in the castor 

 bean, but the cotyledons are thick and contain a great 

 supply of food for the young plant. The bean is an ex- 

 ample of a large class of plants, including the pea, squash, 



