THE IJREATHINl^ OF A TREE. 



15 



HOW THE TREE ERP^A'I'HES. 



Besides giviug oat oxygen iu assimilation, trees also 

 take ill oxygen from the air through tlieir leaves, and 

 through the minute openings in the bark called lenticels, 

 such as the obh)ug raised spots or marks on the young 



Fig. 12.— Wood and bark 

 of the Western Yellow 

 Pine. Tlie cut is a 

 cross section ami 

 would have been hor- 

 izontal as tlie speci- 

 men stood in the tree. 

 Besides tlie division 

 of the bark into scales 



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 

 this picture shows i^j.^athc out carbouic acid gas. This 



twoor thedeepcracks ^ 



in the bark, at the bot proccss of respiratiou or the breathing 



tomof which lenticels ^f ^^^ ^^.^^ o-^^^ ^^^ y^^^^ ^^ ^^^j UJght, 

 are placed. ^ ^ *^ & ' 



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. 



