142 OUTLINES OF FORESTRY. 



out or remove from the blood these dead and 

 decaying parts, and so change the dark, venous 

 blood to bright red arterial blood. 



The oxygen brings about this change mainly by 

 combining with and slowly burning the waste 

 products so as to form water vapor and carbonic 

 acid gas. 



If there was nothing to oppose this action of 

 animal life all the oxygen would, in the end, be 

 removed from the air and changed into carbonic 

 acid gas, and no further animal life would be pos- 

 sible on the earth. Plants, however, during their 

 growth, in the presence of sunshine, take in or 

 absorb carbonic acid gas. In the delicate structure 

 of the leaf this gas is broken up into carbon, 

 which is retained by the plant to form its woody 

 fibre, and into oxygen, which is given off and 

 passes into the atmosphere. 



Plants, therefore, during active growth take in 

 carbonic acid gas and give out oxygen. 



A wonderful balance is thus maintained in na- 

 ture, and the composition of the atmosphere is 

 kept practically constant. 



What animals reject, plants need for their ex- 

 istence. What plants reject, animals need for their 

 existence. It is like the case of the renowned 



