NUTRITION 



377 



The by-product. —The by-] >rod- 

 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 theme into the air. The final 

 step in its exit i an 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 fat tors, intensity of light, 

 temperature, and supply of C0 2 , 

 on photosynthesis. It is not sat- 

 isfactory for quantitative deter- 

 minations. 



Fir.. 650. — Upper part of a plant of 



ton attached to a glass rod and 



submersed, showing escape of gas bubbles 



(mostly oxygen) from cut end of stun in 

 sunlight. 



3. THE SYNTHESIS OF PROTEINS 



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

 mean- the only process of food making. Indeed it may be looked upor 

 a> merely the firsl stage in the construction of proteins, of which carbo- 

 hydrates are important components. As the living protoplasm appears 



