21 
leaf nutrition stimulated by warmth but root-absorption 
also increases with a rise in temperature, so that all nutri- 
tive processes are favoured by warmer conditions of soil 
or climate. 
A secondary but important function of leaves was re- 
ferred to in a preceding lecture, namely, the importance 
of getting rid of a certain amount of water vapour into 
the atmosphere. This process of transpiration aids the 
upward passage of sap and also causes the concentration 
of the necessary mineral salts, which are only contained 
in dilute solution in the soil. 
Let us now consider how the structure of leaves is 
adapted to perform its important functions. For both of 
them it is essential that the air should be in close contact 
with the cells of the leaf. It is esscntial to prevent too 
great a loss of water from the leaves and also for pro- 
tective purposes it is necessary that the surface of the 
leaf should be covered with a more or less resistant layer. 
Consequently the passage of air to the interior of the leaf 
is effected through microscopic pores called stomata, which 
are usually found in greatest number on the under sur- 
face of leaves. These pores have the power of opening 
and closing so that the aperture can be regulated to the 
needs of the plant. They open with the approach of day- 
light and thus enable the carbonic acid to enter the leaves 
when the conditions are favourable for leaf nutrition. At 
the same time, of course, water vapour escapes outwards 
through the pores. The inner parts of the leaf are so 
arranged that the cells containing most of the green colour- 
ing matter are near the upper surface of the leaf where 
they will receive most light | Throughout the interior of 
the leaf there are wide spaces through which the air can 
circulate readily between the cells. The veins which bring 
up the supply of water and which also have to collect 
and conduct away through their bast portion the organic 
material formed by the leaf, become very finely branched 
so as to be in contact with all parts of the leaf. 
If we consider that the larger the leaf surface the 
larger the amount of light absorbed and hence the greater 
the nutrition, we might wonder why all plants have not 
larger leaves. The limitation in the size of leaves is 
largely due to mechanical considerations. A slender stem 
cannot bear leaves beyond a certain size. Moreover, 
though a large leaf would increase the process of carbon 
