Tin Trees of Texas 5 



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 



