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, O 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 O 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 CO a , 
' . rr / FIG. 650. Upper part of a plant of 
On photosynthesis. It IS not sat- Potamogeton attached to a glass rod and 
isfactory for quantitative deter- submersed, showing escape of gas bubbles 
. ' (mostly oxygen) from cut end of stem in 
mmations. 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 
