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." 2 



Sugar and starch (carbohydrates) may be used in the nutri- 

 tion of the living parts of the plant ; or, by the addition of 



1 If 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 343. 



