CONSTKTJCTIVE METABOLISM. 123 



and thin sections of them are examined under the micros- 

 cope, there will be seen minute starch grains in each 

 chlorophyll-corpuscle. 



If green leaves are cut off from the stem during the 

 night and are examined in the same way, there will be 

 found no starch grains in chlorophyll-corpuscles. 



From the first experiment it will be seen that green 

 leaves absorb carbonic acid, decompose it by the help of 

 light, and evolve oxygen. 



From the second experiment it will be understood that 

 the bubbles of oxygen gas can not be given off without a 

 fresh supply of carbonic acid. 



From the third and fourth experiments it will be known 

 that starch grains are formed only during the day in chloro- 

 phyll-corpuscles. 



Now, from the preceding experiments it is clear that 

 chlorophyll-corpuscles absorb carbonic acid, decompose it 

 by the help of light, give off part of its oxygen, and produce 

 starch grains by combining the residue with the elements 

 of water. ' Starch grains are indeed the first organic com- 

 pounds that can be detected under the microscope. This 

 process of the formation of starch grains in chlorophyll- 

 corpuscles is known as Assimilation. 



8. CONSTRUCTIVE METABOLISM. 



Starch grains produced in chlorophyll-corpuscles are 

 constantly converted into sugar and removed from the 

 corpuscles ; the sugar thus formed may be used up for the 

 formation of cellulose, or albuminous matter combining 

 with other substances, or may be reconverted into starch 

 grains which are stored in different organs for future use. 



In short, starch grains formed in chlorophyll-corpuscles 



