212 



BOTANY. 



As the cotyledons become exposed to the light they assume a 

 green color. Some of the stored food in them goes to nourish 

 the embryo during germination, and they therefore become 

 smaller, shrivel somewhat, and at 

 last fall off. 



350. The castor-oil bean. — This 

 is not a true bean since it belongs 

 to a very different family of plants . 

 (euphorbiaceas). In the germina- 

 tion of this seed a very interesting 

 comparison can be made with that 

 of the garden bean. As the ' ' bean 

 swells the very hard outer coat 

 generally breaks open at the free 

 end and slips off at the 

 stem end. The next 

 coat within, which is also 

 hard and shining black, 

 splits open 

 at the oppo- 

 site end, that 



Fig. 187. 

 How the garden bean comes out of the ground. First the looped hypocotyl, then the 

 cotyiedons pulled out. next casting off the seed coat, last the plant erect, bearing thick 

 cotyledons, the expanding leaves, and the plumule between them. 



is at the stem end. It usually splits open in the form of 

 three ribs. Next within the inner coat is a very thin, whitish 

 film (the remains of the nucellus, and corresponding to the 

 perisperm) which shrivels up and loosens from the white mass, 

 the endosperm, within. In the castor-oil bean, then, the 

 endosperm is not all absorbed by the embryo during the forma- 

 tion of the seed. As the plant becomes older we should note 

 that the fleshy endosperm becomes thinner and thinner, and at 



