13 



How a Tree Breathes. — Plants breathe, although not quite in the same way 

 as animals. Both inhale the air, keeping the oxygen and expelling the carbon 

 dioxide gas. The difference is that a tree has no lungs, but breaths over its whole 

 surface, leaves, branches, trunk and roots. Even though the roots are covered 

 with soil, they breathe tihe air which is present in the tiny air-spaces between the 

 soil particles. A plant will be just as surely drowned if its roots are kept under 

 water for long periods as a man will be if his head is kept under water for a 

 certain time. A few plants have special adaptations to enable them to grow in 

 swanrps and wet places, but they are exceptional. This process, which is called 

 "respiration," is the exact opposite of photosynthesis. The difference between the 

 two processes may be set out as follows : — 



Photosynthesis. . Respiration. 



1. Is a feeding process. 1. Is a breathing process. 



2. Carbon dioxide gas is retained. 2. Oxygen is retained. 



3. Oxygen is returned to the air. 3. Carbon dioxide gas is returned to 



4. Purifies the air. the air. 



5. Only takes place in green parts. 4. Fouls the air. 



6. Only takes place in sunlight. 5. Takes place in all parts. 



6. Goes on day and night. 



These two processes — photosynthesis .and respiration — go on together, but, 

 whereas respiration goes on always in all the livings parts of the tree, photosyn- 

 thesis only takes place in the green parts and in the day time. During the day 

 the photosynthesis is the more active process and more oxygen is returned to the 

 air than is retained. At night respiration alone is going on. Trees therefore have 

 the effect, through the giving off of oxygen, of purifying the air in the day time 

 and of vitiating it at night, but, upon the whole, the purifying action is much 

 greater than the other. 



Transpiration. — It has already been explained that the leaves of trees breathe 

 or respire. They have another function: they transpire; that is, certain matter 

 passes out through them into the atmosphere. We know already that trees suck 

 up water and certain other substances through their roots, but, under normal con- 

 ditions, the quantity taken up by the roots and sent up the trunk and through 

 every part of the tree is in excess of its requirements. A tree requires certain min- 

 eral substances for its nutrition, and, as these are present only in very small quan- 

 tity in the water, the roots have to suck up a large quantity of water in order to 

 get sufficient of these mineral substances for the use of the tree. The tree does not 

 need all the water its roots supply, and the excess is evaporated through all parts 

 of the tree above ground, but principally by the leaves. This process is called 

 "transpiration." It can be readily perceived that a good deal of power must be 

 exerted to drive the water from the roots to all parts of great trees. What that 

 power is, is still a question for final decision. It is known, however, that the roots 

 exercise some kind of pumping function and that there is above ground a sucking 

 action due to transpiration from the leaves and other parts. These two forces, 

 the latter in particular, may be the main factors in producing a steady flow of 

 water from lowest root to topmost leaf. 



The Growth of a Tree. — The source of growth of a tree lies in the additions 

 to it made by the food which it has drawn from the ground and absorbed from the 

 atmosphere. But the food does not increase the bulk of the whole tree in the same 

 way. Except in the case of leaves, buds, fruit and twigs of less than a year's age, 

 the digested food material is deposited in the form of a thin layer over the whole 

 tree between the wood and the bark. This layer is made up of wood cells, or wood 

 fibre. Year by year these successive layers of wood cells are deposited and the tree 

 increases in size. The new twigs grow in length by a kind of stretching, but only 



