178 THE STORY OF THE PLANTS. 



acid. Gradually, natural selection 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 opposing 

 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 caTjcrTTihe 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 

 remember that it has three main functions. 

 The first is, to rajise the foliage, with the flowers 

 and fruits as well, visibly above the surface of 

 the ground on which they grow, so that the 

 leaves may gain the freest possible access to 

 rays of sunlight and to carbonic acid, while the 

 flowers and fruit may receive the attentions of 

 insects and birds, or other fertilising and dis- 

 tributing agents. The second is, to conduct 

 from the root to the foliage and other growing 

 parts w r hat is commonly called the raw sap 

 that is to say, the body of water absorbed by 

 the rootlets, together with the nitrogenous 

 matter and food-salts dissolved in it, all of 

 which are needed for the ultimate manufacture 



