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 
