THE BEAN 153 



elongation of the radicle, which pushes out through the 

 micropyle, cracking the seed coat as it does so. If the 

 seed germinates below the surface of the ground, the proc- 

 ess is very similar to the germination of the squash seed 

 (see Chapter I, § 2, and Fig. 2), except that there is nothing 

 lilce the " peg " of the squash seed to help in pushing 

 off the seed coat. The radicle bends if necessar^', so that 

 its tip grows downward and forms the primary root. The 

 upper part of the radicle now grows in such a way that 

 the seed leaves and plumule are pushed upward. The seed 

 leaves, and for a time the seed coat, protect the small sec- 

 ondary leaves and the tip of the plumule from injury by 

 hard objects. After the plumule and seed leaves have 

 reached the surface of the soil, the radicle, which has re- 

 mained bent until now, straightens, the seed leaves sep- 

 arate, and the small secondary leaves grow and become 

 green ; they are the first foliage leaves of the plant. The 

 large seed leaves become greenish but soon begin to shrivel 

 and finally fall off. The food that they contain is used in 

 the growth of the radicle and plumule. 



In some varieties of bean and in the pea the seed leaves 

 remain within the seed coat below the surface of the soil, 

 and merely separate so that the plumule may push out and 

 up. In these cases, the plumule in its upward growth re- 

 mains bent and so protects its delicate tip as well as the 

 tender secondary leaves. 



175. Life Cycle of the Bean. — Although their life his- 

 tories are very much alike, the bean shows an advance over 

 the pine in several respects. The chief points of difference 

 between pine and bean, which are the same points that dis- 

 tinguish gymnosperms and angiosperms in general, are the 

 following : 



a. The spore leaves of the pine cone are borne at dif- 

 ferent levels upon a comparatively long axis ; in the 

 bean flower this axis is much shortened, so that all the 



