STRUCTURE AND REQUIREMENTS OF TREES 117 



it is well to apply the covering early enough and to keep 

 it on late enough to overcome this difficulty. 



The mechanical injuries from frost are also important. 

 Snow and sleet will weigh down branches but rarely break 

 them, while frost will cause them to become brittle and to 

 break easily. Those who climb and prune trees should 

 be especially cautious on frosty days. 



(6) Influence of air: On the under side of leaves and 

 on other surfaces of a tree little pores known as stomata 

 may be found. In the bark of birch and cherry trees these 

 openings are very conspicuous and are there known as 

 lenticels. These pores are necessary for the breathing 

 of the tree (respiration), whereby carbonic acid gas is 

 taken in from the air and oxygen given out. The process 

 of assimilation depends upon this breathing process and it 

 is therefore evident that when the stomata are clogged 

 as may occur where a tree is subjected to smoke or dust, 

 the life processes of the tree will be interfered with. The 

 same injurious effect results when the stomata of the roots 

 are interfered with. Such interference may occur in cases 

 where a heavy layer of soil is piled around the base of a 

 tree, where the soil about the base of a tree is allowed to 

 become compact, where a tree is planted too deep, or where 

 the roots are submerged under water for any length of 

 time. In any case the air cannot get to the roots and the 

 tree suffers. Nature takes special cognizance of this 

 important reciuirement in the case of cypress trees, which 

 habitually grow under water. Here the trees are provided 

 with special woody protuberances known as " cypress 

 knees," which emerge above water and take the necessary 

 air. See Fig. 18. 



Conclusions: From the foregoing it will be seen that 

 trees have certain needs that nature or man must supply. 



