138 ESSENTIALS OF BOTANY 



the plant. Starch is composed of three elements : hydro- 

 gen (a colorless, inflammable gas, the lightest of known sub- 

 stances), carbon, and oxygen. Water is composed largely 

 of hydrogen, and therefore carbon dioxide and water con- 

 tain all the elements necessary for making starch. The 

 chemist cannot put these elements together to form starch, 

 but at suitable temperatures photosynthesis usually ending 

 in 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 atmos- 

 phere containing only carbon dioxide, even in the light. 



A very good comparison of the leaf to a mill has been 

 made as follows^: 



164. Plants Destitute of Chlorophyll not Starch-Makers. 



— Aside from the fact that newly formed starch grains 

 are first found in the chlorophyll bodies of the leaf and 



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 he found there by ordinary tests is starch. 

 See Pfeffer's Physiology of Plants, translated by Ewart, Vol. I, pp. 317, 318. 



2 By Professor George L. Goodale. 



