122 PHYSIOLOGY 



phyll corpuscles, and regarded these as the first products of assimila- 

 tion. The organs of carbon dioxide assimilation are the chloroplasts. 

 These, which are responsible for the green colour of plants, are generally 

 small oval bodies embedded in the cytoplasm, but sometimes, as in 

 spirogyra, may have the form of spiral bands. In a plant which has 

 been .kept for some time in the dark, or in an atmosphere free from 

 carbon dioxide, they present no enclosed granules. Within three to 

 five minutes after exposure to light in the presence of carbon dioxide, 

 starch granules make their appearance within them, and grow rapidly, 

 assuming the typical laminated structure. Engelmann has pointed out 

 a means by which it can be proved that the chloroplasts carry out this 

 process without the co-operation of the rest of the cytoplasm. Certain 

 bacteria have a great avidity for oxygen and present movements only 

 in the presence of this gas. If a filament of spirogyra be placed in a 

 suspension of these bacteria and be examined under a microscope, the 

 bacteria will be seen to congregate in the immediate neighbourhood of 

 the chlorophyll bands. The same phenomenon is observed in the case 

 of chlorophyll corpuscles isolated by breaking up the cells in which 

 they were contained. These corpuscles therefore take up carbon 

 dioxide and water, and form carbohydrate and oxygen, as follows : 



n(6C0 2 4- 5H 2 0) = (C 6 H 10 5 ) n +n(60 2 ) 



The whole structure of the green leaf is directed to the furthering of 

 this process. Its cells contain chlorophyll corpuscles, which change 

 their position according to the intensity of the illumination. A free 

 supply of air to all the cells is provided by means of the stomata 

 on the under surface of the leaf. Horace Brown has shown that the 

 rate at which carbon dioxide diffuses through such fine openings is as 

 great as if the whole leaf were an absorbing surface. We get, therefore, 

 optimum absorption of carbon dioxide by the leaf, with the maximum 

 protection of the absorbing tissue and the necessary limitation of loss 

 of water by transpiration. 



In view of the very small amount of carbon dioxide in the atmo- 

 sphere, the extent of the assimilatory process is remarkable. One 

 square metre of leaf of the catalpa can lay on 1 grm. of solid per 

 hour, using up for this purpose 784 ccm. carbon dioxide. The rapidity 

 of assimilation is increased within limits by increasing the intensity 

 of the light falling on the plant, though an over-stimulation of the 

 process is prevented by the movements of the chloroplasts just men- 

 tioned. It is also increased by raising the percentage of carbon dioxide 

 in the atmosphere supplied to the leaf. The optimum percentage of 

 carbon dioxide will of course vary with the other conditions of the leaf. 

 In certain experiments Kreusler found the optimum to be about 

 1 per cent. Taking the amount of assimilation in normal air with 



