GROWTH AND WORK OF PLANTS 



4. In the scarlet runner bean, and in the pea, the hypocotyl 

 remains short so that the cotyledons are left in the ground. The 

 portion of the stem above the cotyledons elongates and forms the 

 loop as the seedling emerges from the soil so that the tender leaves 

 and plumule are not injured, as they would be if pushed up while 

 they are standing erect. 



THE CASTOR BEAN. 



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

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



Fig. 7. 

 Castor bean seeds sprouting. 



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. 

 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 



Fig. 8. 

 Castor bean germinating. 



At 



through the outer parts of the hard gUSTWStS-B.I? 

 seed coats, which are smooth as if 



varnished, and not so absorbent as the dull more porous part 

 of the coat which lies under the caruncle. When the seed is 

 cut open there is seen a mass of white mealy substance, the 



