The Trees of Texas a 
removal of the waste oxygen formed during the process. This is 
made possible by numerous very small openings in the outer coat 
of the leaf tissue, known as breathing pores or stomates. It is 
thru these minute pores that all the gaseous exchanges take place, 
which make photosynthesis and respiration possible. These 
openings are therefore necessary for both of these processes. 
But it is also through them that the water escapes to the outside 
in the form of vapor, where it evaporates and is lost from the 
tree. It appears, then, that the functions of the leaf are some- 
what antagonistic. It must be aeriated in order to carry on 
photosynthesis and respiration, but the provision for aeriation 
is the source of the evil of excessive water loss. It should not be 
understood, however, that all water loss is a disadvantage. 
This is true only when it becomes excessive. Doubtless the cooling 
of the leaf on hot summer days, due to this evaporation of water, 
keeps the heat from destroying the delicate living substances 
which compose it. 
It is evident that the condition of the soil and air in which 
the tree grows must determine, to a great extent, the amount of 
transpiration, since evaporation of water depends directly upon 
the temperature and the amount of water vapor in the at- 
mosphere. 
But the plant itself is not without devices to reduce the loss 
of water. The breathing pores are principally on the lower side 
of the leaf, while the upper surface presents, usually, an un- 
broken covering, which is coated with a waxy substance that 
does not readily permit the escape of water. Such waxy cover- 
ings of the leaf characterize trees which grow in dry situations 
where the evaporation rate is high, and doubtless have played 
an important part in fitting these trees to their environment. 
The lower side of many leaves is covered with a more or less 
dense growth of hair-like outgrowths, which also serves to retard 
the evaporation of water. Such hairy coverings are almost al- 
Ways present on both surfaces of leaves when they first expand 
from the bud. They frequently remain throughout life or they 
may disappear with the development of the wax-like, water-proof 
coat which characterizes the mature leaf. 
The size and number of leaves also bears a definite relationship 
to the environmental conditions. Trees growing in very dry, hot 
