MINUTE STRUCTURE OF LEAVES. 119 
passed very slowly over the foliage of a plant covered with a bell glass 
and placed in full sunlight, will, if tested chemically, on coming out of 
the bell glass be found to have lost a little of its carbonic acid. The pot 
in which the plant grows must be covered with a lid, closely sealed on, to 
prevent air charged with carbonic acid gas (as the air of the soil is apt to 
be) from rising into the bell glass. 
*150. Disposition made of the Absorbed Carbonic Acid Gas. 
—It would lead the student too far into the chemistry of 
botany to ask him to follow out in detail the changes by 
which carbonic acid gas 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: Hydrogen 
(a colorless, inflammable gas, the lightest of known sub- 
stances), carbon, and oxygen. Water is composed largely of 
hydrogen, and, therefore, carbonic acid gas 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 starch-making goes on constantly in the 
green parts of plants when exposed to sunlight and supplied 
with water and carbonic acid gas. The seat of the manufac- 
ture is in the chlorophyll bodies, and protoplasm is without 
doubt the manufacturer, but the process is difficult to under- 
stand. No carbonic acid can be taken up and used by plants 
growing in the dark. 
A very good comparison of the leaf to a mill has been made 
as follows :? 
The mill: Parenchyma cells of the leaf. 
Raw material used : Carbonic acid gas, water. 
Milling apparatus : Chlorophyll grains. 
Energy by which the mill 
is run: Sunlight. 
Manufactured product : Starch. 
Waste product : Oxygen. 
1 By Professor Geo. L. Goodale. 
