38 INTRODUCTION TO BOTANY 
ordinarily in the ratio of about 3 parts in 10,000 parts of air. 
Inside the leaf, therefore, is a supply of the so-called raw 
materials for food —water, carbon dioxide, and substances 
that were in solution in soil water. 
37. The manufacture of food. Carbon dioxide and water 
must undergo changes before they can be used in nourishing 
and building up the plant. The sun shines upon the leaf and 
the chlorophyll absorbs some of the energy from the sun’s 
rays. In some way, as yet unknown, this energy serves to 
break up the compounds water and carbon dioxide into the 
carbon, hydrogen, and oxygen of which they are made. The 
carbon, hydrogen, and some of the oxygen immediately unite 
again — not, however, into the compounds carbon dioxide and 
water, but into new compounds. These rapidly pass through 
several changes and may finally become sugar and starch. At 
present the changes that take place before starch and sugar 
are formed are not all known, but enough is known of them 
to show that they are quite intricate. Some of the oxygen 
resulting from the breaking up of water and carbon dioxide 
is set free and may pass out into the air. The oxygen thus 
set free by plants may be collected as shown in figure 27, and 
then tested.1 This process that is carried on by green plants is 
a principal factor in maintaining the oxygen supply that is so 
necessary to the life of animals. Plants also use free oxygen 
in some of their later food-making processes. This series of 
occurrences, by means of which green plants under the influ- 
ence of sunlight make foods such as starch and sugar from car- 
bon dioxide and water, is known as photosynthesis. The word 
photosynthesis means “ putting together by means of light.” ? 
Sugar and starch (carbohydrates) may be used in the nutri- 
tion of the living parts of the plant; or, by the addition of 
1 Tf a test for oxygen is made, it is best to precede the ordinary test by an 
experiment with oxygen that has been prepared by the electrolysis of water. 
Test the oxygen so prepared with a lighted splinter, as more meaning will 
then attach to the test of oxygen set free by the plant. 
2 See Appendix, page 3438. 
