MINUTE STEUCTTJKE 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 hell glass he 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 ajr charged with carbonic acid gas (as the air of the soil is apt to 

 be) from rising into the bell glass. 



160, 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. 



Eaw material lised : 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. 



