LEAVES 19 
This explains why leaves are adjusted in so many ways to 
obtain light, as described in § 12. It also gives name to 
the process, photosynthesis, the name indicating that the 
work is done in the presence of light. 
The process demands that carbohydrates shall be made 
from raw materials common in nature and easily obtained 
by plants, and in photosynthesis two such substances are 
used. One of these is water, which in the plants com- 
monly thought of is absorbed by the roots from the soil, 
passes up through the stem, and reaches the green working 
cells of the leaves through the veins. The other substance 
is carbon dioxide, a gas present in small proportion in the air 
(really in the form of carbonic acid gas), but one which is 
being constantly renewed as it is used, so that it is always 
available. Water is made up of one part of oxygen and two 
parts of hydrogen; while carbon dioxide consists of two 
parts of oxygen and one part of carbon. These are just the 
elements that enter into the structure of a carbohydrate. 
In photosynthesis the elements of water and carbon 
dioxide are separated and recombined to form a carbo- 
hydrate, and when this has been accomplished it is found 
that some oxygen has been left over. That is, in the 
process oxygen is a waste product and is given off by the 
working cells. Therefore, in the sunlight a leaf is absorbing 
carbon dioxide and giving off oxygen; and this gas exchange 
is the superficial indication that photosynthesis is going on. 
It is very easy to discover that oxygen is being given 
off by a leaf exposed to light, and that the amount given 
off (and hence the amount of work) depends upon the 
intensity of the light. If an actively growing water-plant, 
submerged in water in a glass vessel, be exposed to bright 
light, bubbles may be seen coming from the plant and 
rising through the water (Figs. 13 and 14). Shading the 
vessel diminishes the number of bubbles. That the gas 
being given off is mainly oxygen may be proved by invert- 
