204 



A YEAR IN SCIENCE 



Fig. 80. Experi- 

 ment to show that 

 oxygen is given off 

 by green plants in 

 the sunlight. 



ments then immediately unite and 

 finally form sugar and starch. The 

 exact chemical processes which this 

 involves are not completely known. 

 Carbohydrates contain twice as much 

 hydrogen as oxygen. Carbon dioxide 

 (C0 2 ) and water (H 2 0) contain only 

 two parts of hydrogen for every three 

 of oxygen. Consequently there is an 

 excess of oxygen in this process. 

 This oxygen is given off as a by- 

 product. The process of starch-mak- 

 ing may be expressed with the follow- 

 ing chemical equation : 



N (6C0 2 ) + N (5H 2 0) == N (C C H 10 5 ) + N (60 2 ) 



Carbon dioxide + water = starch + oxygen 

 On bright days starch is formed very rapidly. Dur- 

 ing the night this food is transformed into soluble 

 forms and then moves from the leaf to other parts of 

 the plant. Much of this is stored away in the form 

 of starch in such parts of plants as the seeds of cereals, 

 grains, fruits, potato tubers, etc. From these all animals 

 derive food. Upon this starch-making process of the 

 green plants all life is dependent for the ultimate 

 source of food. Carbohydrates make up a, very large 

 part of the food of all animals. Indirectly the meat 

 which we eat is derived from these carbohydrates made 

 by plants. Beef, for example, is procured directly from 



