

PARTS OF SEEDS: THE CASTOR BEAN 9 



THE CASTOR BEAN. 



11. The castor-oil bean. The seed of the castor-oil plant is 

 often called a bean, castor-oil bean, or castor bean, because it 

 resembles a bean somewhat in shape. It is, however, not a 

 true bean, but belongs to the Spurge family. At the smaller end 

 of the seed there is a mass of spongy tissue which covers the hilum. 



Fig. 10. 

 Castor bean seeds sprouting. 



This is called the caruncle. Its func- 

 tion is to absorb moisture and thus 

 provide for the passage of water 

 through the hilum end of the seed, 

 where it passes more readily than 

 through the outer parts of the hard 

 seed coats. When the seed is cut 

 open there is seen a mass of white 

 mealy substance, the endosperm,* 

 which is covered by a thin papery 

 material, f The embryo lies within 

 the endosperm; its radicle at the end 

 next the caruncle. The two cotyle- 

 dons are thin and covered by the endosperm. 



12. Germination of the castor bean. The spongy sub- 

 stance of the caruncle becomes much swollen by absorption of 



* The endosperm is food stored in the seed outside of the embryo, 

 f This papery lining is the dead remnant of the nucellus which was used 

 up in the growth of the endosperm. See Chapter XXXVII. 



Fig. ii. 



Castor bean germinating. At 

 right, section showing the two thin 

 cotyledons lying between the en- 

 dosperm. 



