LEAVES— FUNCTION OR WORK 97 



course in the most intimate contact with the soluble carbo- 

 hydrates and products of photosynthesis. In the build- 

 ing up or reconstructive and other processes it is therefore 

 available. We may properly conceive of certain of the 

 simpler organic molecules as passing through a series of 

 changes, gradually increasing in complexity. There will 

 be formed substances containing nitrogen in addition to 

 carbon, hydrogen, and oxygen. Others will contain also 

 sulfur and phosphorus, and the various processes may 

 be thought of as culminating in protoplasm. Protoplasm 

 is the living matter in plants. It is in the cells, and is 

 usually semifluid. Starch is not living matter. The 

 complex process of building up the protoplasm is called 

 assimilation. 



Respiration. — Plants need oxygen for respiration, as 

 animals do. We have seen that plants need the carbon 

 dioxid of the air. To most plants the nitrogen of the air 

 is inert, and serves only to dilute the other elements ; but 

 the oxyge7t is necessary for all life. We know that all 

 animals need this oxygen in order to breathe or respire. 

 In fact, they have become accustomed to it in just the 

 proportions found in the air; and this is now best for 

 them. When animals breathe the air once, they make it 

 foul, because they use some of the oxygen and give off 

 carbon dioxid. Likewise, all living parts of the plant must 

 have a constant supply of oxygen. Roots also need it, for 

 they respire. Air goes in and out of the soil by diffusion, 

 and as the soil is heated and cooled, causing the air to 

 expand and contract. 



The oxygen passes into the air-spaces and is absorbed 

 by the moist cell membranes. In the living cells it makes 

 possible the formation of simpler compounds by which 

 energy is released. This energy enables the plant to 



