62 PLANT LIFE 



the cells. This property of respiration secures, 

 inter alia, an economic transformation of 

 energy within the organism. 



In respect, then, of the functions of respira- 

 tion and photosynthesis an oak leaf does not 

 primarily differ, in essential respects, from a 

 seaweed. But in the important matter of 

 water relations the two are on a very different 

 footing. It has already been pointed out 

 that a supply of water to the living cells is 

 essential for the exercise of their functions. 

 The alga, in its watery habitat, has no diffi- 

 culty in this respect, but the oak leaf, so far 

 from obtaining, is continually losing water 

 from its surfaces. Even in wet weather very 

 little, if any, of the rain which falls on it is 

 absorbed by the cells. This is owing to the 

 circumstance that the outer layer of the wall 

 of the external sheet of cells (epidermis) has 

 undergone a change, and no longer consists 

 of cellulose, through which water can readily 

 pass. It has become converted into cuticle, 

 which is extremely impervious to water, 

 and partially so to gases as well. This 

 cuticle is of extreme importance to terrestrial 

 plants, inasmuch as it provides one of the 

 chief means for preventing their losing water 

 by the ordinary process of evaporation. All 

 the water required by the leaf is received 

 from the root by way of the stem, and it is 

 distributed to all parts of the leaf by means 

 of the vascular bundles, which are often known 

 as the " veins " of the leaf. It is these 



