THE MANUFACTURE OF FOOD BY LEAVES 253 
and second, the various factors which modify the rate of photo- 
synthesis. 
As the student already knows, carbon dioxide and water 
furnish the elements from which sugar is synthesized. The 
carbon dioxide is obtained from the air through the stomata, 
while the water is brought up from the roots through the vascular 
system, which through its numerous fine divisions in the meso- 
phyll supplies either directly or indirectly all of the chlorenchyma 
cells. The carbon dioxide is dissolved in the water with which 
it passes into the cells and comes in contact with the chloroplasts 
where the photosynthetic process takes place. The details of 
the process involved in forming sugar from carbon dioxide and 
water are not well known; but, leaving out the intermediate steps, 
the equation 6CO, + 6H,O = CsH1.05 + 602 represents the 
nature of the process. From the equation it is seen that there 
are as many molecules of oxygen liberated as molecules of carbon 
dioxide used. Whether all or only a part of the 60. liberated 
for each molecule of sugar formed comes from the carbon dioxide 
is not known. It is possible that only the carbon of the carbon 
dioxide is used, in which case all of the oxygen liberated comes 
from the carbon dioxide, or it may be that both water and carbon 
dioxide have their constituents dissociated and some oxygen from 
each is included in the 6QOx. 
Since photosynthesis removes carbon dioxide from the air to 
which it returns an equal amount of oxygen, it is obvious that 
it purifies the air and makes it more wholesome for animal life; 
for animals in their respiration use oxygen and liberate carbon 
dioxide, which, if allowed to accumulate, becomes injurious to 
animals. Not only ordinary respiration of both plants and 
animals but also fermentation, ordinary combustion, and all 
other processes which use oxygen and liberate carbon dioxide 
have their effects on the air counteracted by photosynthesis. On 
the other hand, the oxidation processes maintain the supply of 
earbon dioxide for photosynthesis. Thus photosynthesis and 
the oxidation processes tend to support each other. 
Photosynthesis takes place in the chloroplast, but the exact 
function of either the body or the chlorophyll of the chloroplast 
is not known. It is generally believed that the chief function of 
the chlorophyll is to provide energy for the process; and this it 
does by transforming the sun’s rays into available forms of 
