NUTRITION 



377 



The by-product. —The by-prod- 

 uct, oxygen, is used to some extent 

 in respiration (p. 406) ; the excess 

 diffuses to the surface, whence it 

 escapes into the aerating system 

 and thence into the air. The final 

 step in its exit can be observed in 

 water plants readily, because the 

 constant accumulation in the air 

 chambers leads to its escape as 

 bubbles when the passages are 

 opened by a cut or break (fig. 650). 

 If the canals are intact, 2 may 

 become abundant enough in bright 

 light to form bubbles on the sur- 

 face, which rise as they become 

 larger. The rising gases can be 

 conducted by an inverted funnel 

 into a test tube and analyzed ; 

 they are about 85 per cent oxygen, 

 the remainder being other gases 

 produced in other processes. So 

 uniform is the evolution of 2 by 

 water plants that with precautions 

 the number of bubbles given off 

 in unit time can be used to exhibit 

 the general effect of the three ex- 

 ternal factors, intensity of light, 

 temperature, and supply of C0 2 , 

 on photosynthesis. It is not sat- 

 isfactory for quantitative deter- 

 minations. 



Fig. 650. — Upper part of a plant of 

 Potamogcton attached to a glass rod and 

 submersed, showing escape of gas bubbles 

 (mostly oxygen) from cut end of stem in 

 sunlight. 



3. THE SYNTHESIS OF PROTEINS 



Proteins the end-product. — The formation of carbohydrates is by no 

 means the only process of food making. Indeed it may be looked upon 

 as merely the first stage in the construction of proteins, of which carbo- 

 hydrates are important components. As the living protoplasm appears 



