168 FOUNDATIONS OF BOTANY 
Ordinary air, containing a known per cent of carbon dioxide, 
if passed very slowly over the foliage of a plant covered with a bell- 
glass and placed in full sunlight, will, if tested chemically, on com- 
ing out of the bell-glass be found to have lost a little of its carbon 
dioxide. The pot in which the plant grows must be covered with a 
lid, closely sealed on, to prevent air charged with carbon dioxide (as 
the air of the soil is apt to be) from rising into the bell-glass. 
179. Disposition made of the Absorbed Carbon Dioxide. 
— It would lead the student too far into the chemistry of 
botany to ask him to follow out in detail the changes by 
which carbon dioxide lets go part at least of its oxygen 
and gives its remaining portions, namely, the carbon, and 
perhaps part of its oxygen, to build up the substance of 
the plant. Starch is composed of three elements: hydro- 
gen (a colorless, inflammable gas, the lightest of known 
substances), carbon, and oxygen. Water is composed 
largely of hydrogen, and, therefore, carbon dioxide and 
water contain all the elements necessary for making starch. 
The chemist cannot put these elements together to form 
starch, but the plant can do it, and at suitable temperatures 
starch-making goes on constantly in the green parts of 
plants when exposed to sunlight and supplied with water 
and carbon dioxide.! ‘The seat of the manufacture is in 
the chlorophyll bodies, and protoplasm is without doubt the 
manufacturer, but the process is not understood by chemists 
or botanists. No carbon dioxide can be taken up and used 
by plants growing in the dark, nor in an atmosphere con- 
taining only carbon dioxide, even in the light. 
1 Very likely the plant makes sugar first of all and then rapidly changes 
this into starch. However that may be, the first kind of food made in the 
leaf and retained long enough to be found there by ordinary tests is starch. 
See Pfeffer’s Physiology of Plants, translated by Ewart, Vol. I, pp.:317, 318. 
