THE WALLFLOWER 101 



they appear thinner, after secondary growth in thick- 

 ness has begun, than they did before. This is because 

 the root at first loses more by the death of its 

 external tissues than it gains by secondary growth. 

 The loss, however, is soon made good. 



Now let us consider what purpose all this secondary 

 formation of new tissues serves. So long as a plant 

 like the Wallflower lives, it keeps putting out new 

 branches ; every new branch bears a number of leaves, 

 and so the older the plant grows the greater is the 

 total surface of its foliage, in spite of the loss of old 

 leaves. Now the leaves perform two main functions : 

 they give off watery vapour through their stomata, 

 and thus keep up the ascending current of sap from 

 the roots ; and also they take up food from the air, 

 namely, the carbon which they obtain by the de- 

 composition of carbon dioxide. Therefore the greater 

 the leaf-surface, the greater will be the amount of 

 water which has to pass up the stem from the roots, 

 and the greater also will be the amount of assimilated 

 food which has to be conveyed away from the leaves 

 to the growing parts. Consequently, as the leaf -surface 

 increases, it is necessary that the amount of conducting 

 tissue should increase also, and this applies both to 

 the wood, which conducts the sap from the roots to 

 the leaves, and to the bast, which conducts the 

 assimilated foods from the leaves to the seats of 

 growth. This increase is provided by the activity of 

 the cambium, and while this is going on the absorptive 

 surface of the root is also increasing. The root 

 branches again and again, and the number of young 



