io THE LIFE HISTORY OF A TREE 



of two elements, carbon and oxygen. The leaves keep the 

 carbon and return most of the oxygen to the air, and, as 

 oxygen is the gas we require when we breathe, this process 

 purifies the air from our point of view. 



How the tree breathes. All plants, like animals, breathe in 

 air, keeping the oxygen and exhaling the carbon dioxide gas. 

 This process, which is called respiration^ is the exact opposite 

 to that of photosynthesis. The difference between the two 

 processes is as follows : 



Photosynthesis. Respiration. 



1. Is a feeding process. I. 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 



4. Purifies the air. to 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. 



Curious as it may seem, these two processes go on together, 

 but whereas respiration goes on always in all the living parts 

 of the tree, photosynthesis only takes place in green parts and 

 in the day time. During the day photosynthesis is the more 

 active process, and more oxygen is returned to the air than 

 is retained. At night respiration alone is going on. Plants 

 therefore, on the balance, purify the air in the day, and foul it 

 at night, but on the whole the purifying action is much greater 

 than the other. 



Under a microscope little openings can be seen through the 

 skin of a leaf, especially on the under surface. These are 

 called stomata, and it is through them that the leaves take in 

 and give out gases. On the bark of the hazel and cherry long 

 slit-like openings are seen ; these are also used for breathing 

 purposes, and are called lenticels. They exist on all trees, 

 but are not easily seen on rough barked ones. 



Transpiration. Very small quantities of mineral matters 

 are contained in the water coming up from the roots, and 



