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. 1 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. 



