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 ?: 
The mill: Palisade-cells and underlying 
cells of the leaf. 
Raw material used : Carbon dioxide, water. 
Milling apparatus: Chlorophyll grains. 
Energy by which the mill 
is run: Sunlight. 
Manufactured product: Starch. 
Waste product: Oxygen. 
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 be 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. 
