14 MORPHOLOGY OF THE SEEDLING 



seedling visibly beginning to germinate, the sprout, as it is com- 

 monly called, which projects from the seed might be either root 

 or hypocotyl, or might consist of both together, so far as its 

 appearance is concerned. A microscopic study of the cross sec- 

 tion of a root, compared with one of the hypocotyl, would show 

 decided differences of structure between the two. Their mode 

 of growth is also different, as the pupil may infer from his own 

 observations. 



18. Final position of the cotyledons. As soon as the young 

 plants of squash, bean, and pea have reached a height of three 

 or four inches above the ground, it is easy to recognize important 

 differences in the way in which they set out in life. 



The cotyledons of the squash increase greatly in surface, 

 acquire a green color and a generally leaf-like appearance, and, 

 in fact, do the work of ordinary leaves. In such a case as this 

 the appropriateness of the name seed leaf is evident enough, 

 one recognizes at sight the fact that the cotyledons are actually 

 the plant's first leaves. In the bean the leaf-like nature of the 

 cotyledons is not so clear. They rise out of the ground like the 

 squash cotyledons, but then gradually shrivel away, though they 

 may first turn green and somewhat leaf-like. for a time. 



In the pea, as in the acorn, the horse-chestnut, and many 

 other seeds, we have quite another plan, the underground type 

 of germination. Here the thick cotyledons no longer rise above 

 ground at all, because they are so gorged with food that they 

 could never become leaves ; but the young stem pushes rapidly 

 up from the surface of the soil. 



19. Development of the plumule. The development of the 

 plumule seems to depend somewhat on that of the cotyledons. 

 The squash seed has cotyledons which are not too thick to 

 become useful leaves, and so the plant is in no special haste to 

 get ready any other leaves. The plumule, therefore, cannot be 

 found with the magnifying glass in the unsprouted seed, and is 

 almost microscopic in size at the time when the hypocotyl 

 begins to show outside of the seed coats. 



