THE BREATHING OF A TREE. 



15 



HOW THE TREE BREATHES. 



Besides giving out oxygen in assimilation, trees also 

 take in oxygen from the air through, their leaves, and 

 through the minute openings in the bark called lenticels, 

 such as the oblong raised spots or marks on the young 



Fig. 12.— Wood and bark 

 of the Western Yellow 

 Pine. The cut is a 

 cross section and 

 would have been hor- 

 izontal as the speci- 

 men stood in the tree. 

 Besides the division 

 of the bark into scales 

 this picture shows 

 t wo o f the deep cracks 

 in the bark, at the bot- 

 tom of which lenticels 

 are placed. 



Fig. 13. — Bark of the "Western Yellow 

 Pine. Outer surface, showing the 

 scales made by the successive layers 

 of cork cambium. 



branches of Birch and Cherry and 

 many other trees. All plants, like 

 all animals, breathe; and plants, like 

 animals, breathe in oxygen and 

 breathe out carbonic acid gas. This 

 process of respiration or the breathing 

 of the tree goes on both day and night, 

 but it is far less active than assimila- 

 tion, which takes place only in the light. Consequently 

 more carbonic acid gas is taken into the tree than is 

 given out, and the surplus carbon remains to be used 

 in growing. 



