174 THE STORY OF THE PLANTS. 



Such leaves are therefore usually thick and fleshy, 

 and possess a very small allowance of pores. The 

 forms of several leaves, again, are largely de- 

 pendent upon the necessity for keeping the pores 

 free from wetting, and promoting evaporation 

 whenever it is needful for the plant's health and 

 growth ; and this is particularly the case with 

 what are called " rolled leaves," such as one sees 

 in the heaths and the common rock-roses. Many 

 such additional principles have always to be taken 

 into consideration in attempting to account for 

 the various shapes of foliage: indeed, we can 

 only rightly understand the form of any given, 

 leaf when we know all about its habits and its 

 native situation. 



The stem, then, besides raising the leaves and 

 flowers, for which purpose it is often strengthened 

 by means of mechanical woody tissue, also acts 

 as a conductor of raw sap from the tips of the 

 roots to the leaves and growing points, for which 

 purpose it is further provided with an elaborate 

 system of canals and vessels, running direct from 

 the absorbent root to all parts of the compound 

 plant community. 



The third function of the stem and branches 

 is to convey and distribute the elaborated prod- 

 ucts of plant-chemistry and plant-manufacture 

 from the places where they are made to the 

 places where they are needed for practical pur- 

 poses. 



We saw^ long since that starches, sugars, pro- 

 toplasms, and chlorophyll are manufactured in 

 the leaves under the influence of sunlight; and 

 from the materials so manufactured every part of 

 the plant must ultimately be constructed. But 



