28 Science of Plant Life 



like glucose, which are the first abundant products of photo- 

 synthesis, contain six atoms of carbon, twelve atoms of hydro- 

 gen, and six atoms of oxygen in each molecule. For every 

 molecule of glucose manufactured, therefore, it would require 

 six molecules of carbon dioxid to furnish the carbon and six 

 molecules of water to provide the hydrogen. These amounts 

 of water and carbon dioxid, however, contain eighteen atoms of 

 oxygen, twelve more than are needed for the making of glucose, 



6CO 2 +6H 2 O = C 6 Hi 2 O 6 +i2O. 



We should therefore expect oxygen to be given off from 

 leaves during photosynthesis. That this actually happens 

 may easily be shown by inverting under water a bundle of 

 the branches of some water plant, like Elodea, with the cut 

 ends placed under the mouth of a test tube that is filled with 

 water (Fig. 18). When exposed to the light for a day, the 

 tube will be partly filled with gas. By testing with a glowing 

 match or splinter, the gas may be shown to be mostly oxygen. 1 



maltose (malt sugar) may be built up by combining two simple sugars, 



glucose fructose cane sugar water 



one molecule of water being lost in the process ; and they may be split up into 

 two of the simple sugars by the addition of a molecule of water. The starches 

 and celluloses are formed by combining many molecules of sugar and removing 

 as many molecules of water as there are molecules of sugar used. Conse- 

 quently their formulas are (C 6 HioO 5 )n, in which n represents a rather large 

 number. The starches and celluloses may also be split up into simple sugars 

 by adding the required number of molecules of water. This last process is 

 the one by which corn sirup (glucose) is made from corn starch. The process 

 may be represented by the equation, 



(C 6 Hi 5 ) n + (H 2 0) ->(C 6 Hi 2 6 ). 



starch water glucose 



1 Water containing a considerable amount of dissolved carbon dioxid should 

 be used in this experiment, so that photosynthesis may go on rapidly. For 

 this reason pond water is better than tap water. 



