284 



ELEMENTARY BIOLOGY 



organs : some organs present in infancy disappear, others not 

 present make their appearance later, and others are present 

 at first in a rudimentary stage and gradually reach maturity. 



331. Development of plants. The simpler stages in plant develop- 

 ment are not so familiar to us, so that we do not have the same 

 opportunity to observe the similarities. In our study of the embryo 



of the seed, we saw 

 that the young plant 

 had all the main parts 

 of a plant body, al- 

 though the embrv'O did 

 not at all resemble the 

 full-grown plant. The 

 embryos of related 

 plants look much more 

 alike than the adult 

 individuals, just as the 

 tadpoles of the newt 

 and the frog look more 

 alike than the adults. 

 Thus, the embryo and 

 even the seedling of 

 the squash and of the 

 pumpkin are so much 

 alike that it would take 

 a verv experienced person to see any differences between them, 

 aside from size. The different kinds of beans appear very distinct in 

 the full-grown plant and in the seeds, but the seedlings with the first 

 pair of leaves look very much alike. 



If we examine the stages in the development of the embr}'o, before 

 the seed is ripe, we shall find still greater similarities in the earlier 

 stages, before the stem, leaf, and root of the embryo are distinguish- 

 able, among plants that are not so closely related as are the squash 

 and pumpkin, for example (see Fig. 120). 



Fig. 120. Development of a plant embryo 



Beginning, like an animal, as a single cell, a, the plant 



passes, by a series of cell divisions, b, c, d, e, into a mass 



without any definite form, and gradually assumes distinct 



structure and organs, f, g 



