THE STEM AND BRANCHES. 



163 



FIG. 49. First steps in the evolu- 

 tion of the stem. 



we should get the beginning of a compound plant, 

 many-leaved, and with a sort of early or nascent 

 stem, formed by the part that was common to 

 many of the leaves, like 

 a midrib. The accom- 

 panying diagram (Fig. 

 49) will make this clear- 

 er than any amount of 

 description could pos- 

 sibly make it. Start- 

 ing from such a point, 

 certain plants would 

 soon find they were 

 thus enabled to over- 

 top others, and to ob- 

 tain freer access to 

 light and carbonic acid. 

 Gradually, natural se- 

 lection would ensure that the common central part 

 of the growing plant, the developing stem, should 

 become harder and more resisting than the rest, so 

 as to stand up against the wind and other oppos- 

 ing forces. At last there would thus arise a 

 clearly-marked trunk, simple at first, but later on 

 branching, which would lift the leaves and flowers 

 to a considerable height, and hang them out in 

 such a way as to catch the sunlight and air to the 

 best advantage, or to attract the fertilising insects 

 or court the wind under the fairest conditions. I 

 leave you to think out for yourself the various 

 stages of the process by which natural selection 

 must in the end secure these desirable objects. 



In order to understand the nature of the stem, 

 in its fully developed form, however, we must re- 

 member that it has three main functions. The 



