100 Field, Forest, and Wayside Flowers 
But green plants make their own food. The 
chlorophyll which they contain is a lure to catch 
the sunbeams, which, when caught, are set to 
work to help the protoplasm in the work of food- 
making and tissue-building. 
This work can prosper only under certain con- 
ditions. Sunshine must fall upon the plant, car- 
bon dioxide gas must be mingled with the air 
which surrounds it, the temperature must not be 
too low, and water must come up from the roots 
into the leaves and green stems. Under these 
circumstances food-making goes merrily on. 
The first evident product of the plant’s industry 
is starch. This is a much less complex substance 
than the proteids, for it contains but three ele- 
ments, carbon, hydrogen, and oxygen, and they 
are mingled in accurately known and unvarying 
proportions. 
The carbon comes out of the carbon dioxide 
which the leaves breath in; the hydrogen is a 
chemical constituent of the water which the roots 
suck up, and the oxygen comes in as the other 
element of the water, or is inhaled from the at- 
mosphere by the green stems and the foliage. 
Some surplus oxygen is left after the starch- 
making, and this is exhaled by the leaves. When 
