2o0 BUI.I.KTIN UNIVKKSITV OF MONTANA 



2. Small quantities are used in the manufacture of foods, such as 

 starch, sugar, and other carbohydrates. 



3. Large quantities are used to supply the loss by evaporation that 

 is continually going on. 



Analysis of the last process will show what role it plays in the distri- 

 bution of trees. 



The evaporation of water from plants is Ivnown as transpiration. The 

 water taken in through the roots is carried through the stem to the leaves 

 and other superficial parts of the plant, whence it passes into the atmos- 

 phere as vapor just as water in any moist object may. Other things 

 being equal, the greater the surface a plant exposes to the atmosphere, 

 the greater the amount of water given off. The tree, because it has a 

 greater surface exposed than other forms of vegetation will give off more 

 water. A birch tree with about 200,000 leaves has been estimated to 

 give off from 350 to 470 pints of water on a single hot day. The amount 

 of transpiration is regulated in a gi-eat measure by the conditions of the 

 atmosphere. If the atmosphere is damp there is less transpiration than 

 when the atmosphere is dry. Again, on a windy day the tree will give off 

 more water than on a still day. Other things being equal, more water 

 will be transpired on a warm day than on a cold day. Indeed, any condi- 

 tion that will affect the evaporation from wet objects, like clothes on a 

 line, will affect the transpiration of a tree. In the growing season a tree 

 like the birch will give off more water than a pine or fir tree of equal 

 size, for the birch exposes to the air more leaf surface than the pine or 

 fir; also its leaves are more delicate in structure than their hard needle- 

 like leaves. Therefore the pine, other things being equal, can live in 

 situations, which are drier during the summer months than can the birch. 

 However, during the winter season the birch having shed its leaves ex- 

 poses nothing but bare limbs to the atmosphere, so transpiration is much 

 reduced. On the other hand the pine holds its leaves and has as much 

 surface exposed during the winter as during the summer, and it is there- 

 fore in more danger then of losing water than a tree without any leaves. 

 For even though the loss be checked by cold weather, absorption is 

 checked also, because the colder the ground, the less the absorption; and 

 if the ground be frozen around the roots, little or no water can pass into 

 the tree. 



It will be seen from the above that there is a relation between the 

 intake and the outgo of water from the tree. If the tree is to survive, 

 absorption must be greater than transpiration. As soon as it is equal 

 to or less than the transpiration, the tree is in danger of drying out. In 

 those climates where the rainfall is not sufficient to keep the soil wet 

 enough to maintain this inequality between absorption and transpiration, 

 trees cannot exist. But even in prairie regons there are places where 

 there is sufficient water in the soil, as is the case near bodies of water 

 where the underground water level is near the surface. This, no doubt, 

 accounts for the presence of trees along streams in climates where the 

 rainfall alone is insufficient to maintain trees. 



Trees, being green plants, manufacture starch, sugar, and other 

 carbohydrates. They do this in all green parts and therefore mostly in 



