108 PHYSIOLOGY 



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 + 5H 2 0) = (C*H 10 0.)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 atmosphere, 

 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 pur- 

 pose 784 com. 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 mentioned. It is also increased by raising the per- 

 centage 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 

 03 per cent, carbon dioxide at 100, the assimilation in an atmosphere con- 

 taining 1 per cent, was 237, and was not increased by raising the percentage 

 of carbon dioxide to 7 per cent. Owing to the decomposition of the organic 

 matter of the soil, the percentage of carbon dioxide near the ground is always 

 greater than in the higher strata of the atmosphere a fact which is taken 

 advantage of by the low-growing plants and herbage. Other necessary con- 

 ditions of assimilation are the presence of water and the maintenance of a 

 certain external temperature. The absorption of the sun's rays by the leaf 

 raises the temperature of the latter above that of the surrounding medium, 

 and so quickens the process of assimilation. 



The assimilation of carbon dioxide, the formation of starch, and the 



