FACTORS INFLUENCING PHOTOSYNTHESIS 259 



combustion, and all oxidation processes in maintaining the supply 

 of carbon dioxide for photosynthesis. Roughly estimated, 150 

 square meters of leaf area will use up in one summer all of the 

 carbon dioxide which an average man produces through respira- 

 tion in one year. 



When one considers that the amount of carbon dioxide in the 

 air is only about 0.03 per cent, that is, about 

 3 parts in 10,000 parts of air, it is surpris- 

 ing that plants can make sugar as rapidly 

 as they do. Sometimes, as around cities 

 with many factories, the per cent of carbon 

 dioxide may be a little higher but it is 

 always exceedingly low. Of course carbon 

 dioxide is present in solution in the soil 

 water; but it is easily demonstrated that 

 this carbon dioxide is of practically no help 

 to plants in photosynthesis. To compensate 

 for the limited amount of carbon dioxide, 

 it is obvious that leaves need broad surfaces 

 and a thorough distribution of chlorophyll, 

 so that their absorbing surface may be 

 large. However, with all of these adjust- 

 ments of the plant, it has been demonstrated FIQ 23 g Leaf 



that the normal supply of carbon dioxide showing the effect on 

 is often insufficient for the maximum photosynthesis of clos- 

 amount of photosynthesis; for some plants, in g tne stomata. The 



when surrounded by air in which the stomata on the under 

 f ! ,..,.. , surface of the white area 



amount of carbon dioxide is increased up , , , 



were closed by covering 



to 1 per cent, show a corresponding rise in the epidermis with vase- 

 photosynthetic activity. line, thus filling the 



Since stomata are the openings through stomata and excluding 

 which carbon dioxide enters the leaf, their carbon dioxid e- After 

 number per area of leaf surface and the 

 extent to which they are open affect the amount of this gas that 

 reaches the mesophyll. That photosynthesis is inhibited when 

 stomata are closed is demonstrated by the experiment shown in 

 Figure 236. The experiment shows the necessity of keeping the 

 stomata free from dust and other bodies, such as spores of plants 

 and secretions of insects, that clog the stomatal openings. It is 

 for this reason that we are advised to cover house plants with a 



