86 PHOTOSYNTHESIS AND RESPIRATION. 



the epidermis and 140 on the lower; a large leaflet, with 

 area about 20 sq. cms., therefore has nearly half a million 

 stomates. Still, the total area of the air-openings is only 

 about O'Ol of the total leaf -area. 



It has, however, been found that the rate of diffusion of 

 gases through a plate with small openings is much greater 

 than through a single opening of the same total area, and 

 that when the small openings are placed about ten times their 

 diameter apart the rate of diffusion is as rapid as when no 

 partition is present. These requirements are met by the size 

 and the distribution of the stomates on a leaf, and a given 

 area of a leaf has been found by experiment to absorb two- 

 thirds the amount of carbon dioxide taken up in equal time 

 by area of potash solution exposed to the air. 



121. Carbon Assimilation ; Photosynthesis. In the 



green leaf exposed to light and air a somewhat complicated 

 series of processes goes on, leading usually (but with some 

 exceptions) to the appearance of starch at one stage. What 

 we have learnt from our experiments on plants, together with 

 our knowledge of the composition of water and of carbon 

 dioxide, would seem to suggest that a process of synthesis 

 occurs in the green parts of plants. One method of proving 

 the composition of water is to produce it by synthesis 

 i.e. by bringing together hydrogen and oxygen gases and 

 making them unite to form water. Many complex substances, 

 e.g. sugars, can be produced by synthesis. 



When starch is heated, carbon dioxide and water are given 

 off. This suggests that possibly carbon dioxide and water 

 are brought together, in the green leaf in sunlight, to form 

 starch. This is not exactly what happens, but the fact that 

 a plant can, when supplied with water and carbon dioxide, 

 make starch (containing carbon, hydrogen, and oxygen) 

 suggests that water must be concerned in the process. The 

 formation of starch in a plant exposed to light is clearly seen 

 in a simple water-plant like Spirogyra (Art. 98) ; this plant, 

 placed in water containing a few simple salts (Art. 143) and 

 carbon dioxide, produces starch within a few minutes after 

 exposure to light after having been kept in darkness. 



It is therefore obvious that in some way carbon dioxide 

 and water are broken up and their elements rearranged so 



