SYNTHESIS, STORAGE AND BREAKDOWN 



119 



quite certain that the fact that the carbon dioxide has to diffuse 

 through the stomata in no way restricts photosynthesis, provided 

 the stomata are appreciably open. The gas then diffuses through the 

 continuous system of air-channels that permeates the leaf. According 

 to the general view, the carbon dioxide next becomes dissolved in 

 water present in the cell-walls bounding the air-spaces, and subse- 

 quently diffuses in solution into the cytoplasm of the cells, where the 

 chloroplasts are situated (Fig. 79). So long as the gas continues 



Fig. 79. 



Part of leaf of Narcissus in transverse section. Two of the cells of the mesophyll 

 are drawn in in detail : that to the right as seen in surface view from without ; that 

 to the left in optical section. The chloroplasts are shown black. ( x 300.) F. O. B. 



to be used up in photosynthesis by the chloroplasts, there will be a 

 constant inward flow of further carbon dioxide from the external 

 atmosphere, because of the tendency of a gas to diffuse from a region 

 of higher concentration to one of lower. In the case of submerged 

 water-plants the carbon dioxide present in the water round about 

 the plant is that utilised in photosynthesis. The carbon dioxide 

 enters these plants in solution rather than in the gaseous form : 

 it should be remembered, however, that in land-plants also entrance 

 into the actual cells can only be effected in solution. 



There is no doubt that the rate of photosynthesis is under natural conditions 

 frequently restricted by the low carbon dioxide-content of the atmosphere, 

 which can be termed the limiting factor at such times. Experiment shows 

 that the rate of photosynthesis can be accelerated by increasing the supply 

 of carbon dioxide. The growth of the plant is consequently benefited, and 

 experiments in greenhouses have proved that by enrichment of the air with 

 additional carbon dioxide the yield of tomato and other crops can be very 



