16 PEACTICAL BOTANY 



of carbon dioxide and water is used in making starch and 

 sugar, but much of it is set free and may pass out into the air. 

 The oxygen thus set free by plants may be collected as shown 

 in Fig. 12 and then tested. 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 some free oxygen in some of their later f ood-making proc- 

 esses. This series of occurrences by means of which green 

 plants under the influence of sunlight make foods, such as 

 starch and sugar, from carbon dioxide and water, is known 

 as photosynthesis. The word photosynthesis means " putting 

 together by means of light." l 



1 The chemistry of photosynthesis is not completely known, but some of 

 the simpler aspects of it may prove valuable at this point. Water is usually 

 expressed by the chemist by the formula H 2 0, in which H stands for hydro- 

 gen and for oxygen, and the figure 2 indicates that two parts of hydrogen 

 are united with one part of oxygen. Similarly C0 2 indicates that one part 

 of carbon is united with two parts of oxygen to form carbon dioxide. When 

 these compounds are broken up, there is, for a very brief time at least, 

 free C, H, and O. If one unit of each compound (H 2 O and C0 2 ) is thus 

 broken up, there will be two H, one 0, one C, and two (or in all three O). 

 After photosynthesis has been going on for some time, starch is usually 

 formed. Starch consists of (C 6 H 10 5 ) "n". This means that six parts of 

 carbon, ten parts of hydrogen, and five parts of oxygen unite to form starch, 

 and the ff n" means that the unit C 6 H 10 5 does not appear singly, but that 

 an unknown number of them are united. Disregarding the fact that sev- 

 eral of the starch units are hel<5 together, and considering the single unit 

 C 6 H 10 O 5 , we may be able to see what happens in the work of photosynthesis. 

 To secure the amount of carbon necessary to form starch, six times the unit 

 C0 2 must be taken, since there are to be used six units of carbon. To secure 

 the needed amount of hydrogen, five times the unit H 2 must be used, 

 since there must be ten units of hydrogen and two are secured with each 

 unit of water. We have, therefore, 6 CO 2 and 5H 2 0. When the energy 

 of the sun has broken these things into their constituent parts there are 

 6C, 120, 10 H, and 50, or 17 O in all. But starch consists of C 6 H 10 O 5 , 

 and in making this unit of starch there has been used all of the carbon, 

 all of the hydrogen, and five units of the oxygen, thus leaving twelve 

 units of oxygen to be set free or to be used by the plant in some other 

 way. Some of this free oxygen passes into the air, though some of it is used 

 by the plant in a later process. 



The compounds thus constructed, as starch and sugar, are called carbo- 

 hydrates, the name indicating that they are compounds of carbon and 

 water. 



