22 PLANT LIFE 
of the sun, for its energy, reaching us through 
the mediation of the plant, is the. fons et 
origo of our existence. How, then, does the 
Chlamydomonas proceed by means of its 
chlorophyll to make these more complex 
food substances ? 
Although we are not as yet fully acquainted 
with all the steps of the process, we already 
know enough to enable us to sketch in out- 
line the main sequence of events. Putting 
the story in its simplest form, we may say that 
the carbonic acid, which is formed when 
carbon dioxide dissolves in water, is con- 
tinuously broken up as the result of the action 
of sunlight of a suitable intensity upon the 
chlorophyll of the living plant. Oxygen 
is liberated, and organic compounds, usually 
sugars, are produced inside the cell. When 
the reaction is sufficiently rapid, so that the 
concentration of sugar reaches a_ certain 
strength, starch often makes its appearance, 
but it is merely a secondary product, depending 
on the prior formation and accumulation 
within the cell of sugar in sufficient quantity. 
The earlier stages of sugar formation are still 
obscure, but there is little doubt that form- 
aldehyde (the formalin of commerce) is pro- 
duced during the process. This substance has 
been used as the starting-point for the synthesis 
of sugars in the laboratory, and although it 
is difficult to detect it with certainty in the 
plant there are strong reasons for considering 
that it really is formed as an intermediate 
