68 IMPORTANT TIMBER TREES 



sure, or the cellular tissues of the wood would be ruptured. 

 Just how this ascent of sap to such great heights as we 

 know it attains is brought about we do not yet understand. 

 There are various explanations, but each one of them has a 

 weak link in its chain. 1 Notwithstanding that we can see no 

 active life in a tree in winter-time, yet it is certain that the 

 sap ascends then, for if from any cause, such as dry or deeply 

 frozen ground, moisture cannot be obtained by the roots 

 for a long time, the tree will frequently be killed, because 

 evaporation goes on through the pores in the bark and, in 

 the case of evergreens, through the stomata or breathing- 

 pores in their leaves. 



Steins and Leaves. Having now seen the functions and 

 the growth of the roots, attention will be given to the stem 

 and leaves to see what part they play in the economy of 

 tree-life. The ascent of the sap to the leaves, carrying with 

 it the mineral food, has already been noted, and a descrip- 

 tion of the leaves and the functions they perform will be 

 necessary to comprehend fully the growth of the tree, and 

 especially the character of the stem from which lumber 

 must be cut. The leaves of all trees have a framework of 

 fibrous material called veins. The largest of them are pro- 

 minent and plainly visible to the unaided eye. In broadleaf 

 trees they are quite so. Close inspection will show that in 

 some leaves there is a fine network of them. This network 

 is technically called the leaf's venation. These veins are in- 

 closed between two films known as the epidermis or leaf's 

 skin. There are small holes in the skin of the leaves, and 

 in broadleaf trees they are much more numerous on the 

 under than on the upper side, but in some of the conifers 

 there are about as many on one side as the other. They are 



1 " The principal cause of the upward movement of water is unknown. 

 The most divergent views are held, not one of which has proved capable of 

 satisfactory demonstration. ... It is hardly possible to deny the existence 

 of root pressure, capillarity, and the lifting power of evaporation and osmotic 

 pressure. The relative importance of these, the manner in which they work, 

 and the existence of other factors are points that it is impossible to settle at 

 present." Plant Physiology and Ecology, by Frederick Edward Clements, 

 PkD., page 56. 



