LEAVES AND THEIR USES 249 



farther within the tissues. In respiration, the substances 

 already in the cells, including carbohydrates, fats, proteins, 

 and possibly some of the living matter itself, are broken 

 down into simpler substances, and the energy which was 

 stored in them is set free. Part of the energy so set free 

 shows itself in the form of heat ; the temperature of an active 

 part of the plant is. therefore always a little above that of the 

 surrounding air. Other portions of energy set free in respira- 

 tion are used in building up complex food substances and 

 living matter, in growth, in the movement of materials within 

 the plant (such as sap that is pumped up from the roots to 

 the leaves), and in the movements of leaves and other 

 parts of the plant. 



Of the simple substances formed in the breaking-down 

 processes of respiration, the most abundant are water and 

 carbon dioxid. The water remains within the cell where it is 

 formed or passes to a neighboring cell. The carbon dioxid 

 passes off through the cell wall into the air, unless it can be 

 immediately used again in the building up of new carbohy- 

 drates. Therefore it is that the effect of respiration upon the 

 atmosphere about the plant is just the opposite of that of 

 carbohydrate manufacture. Respiration takes oxygen from 

 the air and gives carbon dioxid to the air ; carbohydrate 

 manufacture takes carbon dioxid from the air and gives 

 oxygen to the air. So long as a plant is growing and increas- 

 ing in weight, it is making carbohydrates on the whole more 

 rapidly than it is respiring ; the net result of its activities 

 upon the atmosphere is to decrease the amount of carbon 

 dioxid and to increase the amount of oxygen. 



The net result of the activities of colorless plants and of 

 animals, which respire but do not make carbohydrates, 

 is to decrease the amount of oxygen in the air and to increase 

 the amount of carbon dioxid. Thus these two groups of 

 living beings, green plants on the one hand and colorless 

 plants and animals on the other, neutralize each other's 



