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 

 that the normal supply of carbon dioxide FIG. 236. Leaf, 

 is often insufficient for the maximum showing the effect on 

 amount of photosynthesis; for some plants, photosynthesis of clos- 



when surrounded by air in which the ing the sio ^- T he 



....... , stomata on the under 



amount of carbon dioxide is increased up surface of the white area 



to 1 per cent, show a corresponding rise in were closed by covering 



photosynthetic activity. the epidermis with vase- 



Since stomata are the openings through line > thus fillin g the 



which carbon dioxide enters the leaf, their stomata and excluding 



. r- i /. / 1,1 carbon dioxide, 



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 deposits of insects, that close the stomatal openings. It is 

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



