210 Plant Physiology 



leaf, and there the air chambers are most numerous ; but 

 chlorenchyma is better developed toward the upper sur- 

 face. There are, however, so many factors which influ- 

 ence the structure of the leaf that the apparent inconsis- 

 tency of this arrangement must be regarded as an effective 

 compromise. 



The same stomatal mechanism effects, of course, a 

 rapid elimination of the oxygen produced during photo- 

 synthesis, after this oxygen has diffused into the air spaces 

 from the moist membranes of the cells wherein it is pro- 

 duced. 



114. The amount of carbon dioxid. - The amount of 

 carbon dioxid in the air seems almost infinitesimal when 

 we contemplate the results of its use. The air contains 

 normally about .028 to .03 per cent, although this amount 

 may be temporarily somewhat increased in the neighbor- 

 hood of cities, or of areas where manufacturing is a chief 

 industry. The limited amount of this gas suggests, 

 further, the necessity of broad surfaces and the thorough 

 distribution of chlorophyll. 



It has been found that the normal supply of carbon 

 dioxid is often insufficient for the maximum work of the 

 leaf. Under ordinary conditions, as when the plant is 

 growing in strong light at a temperature of from 20 to 

 25 C., and with a sufficient water-supply, a chief limiting 

 factor in growth is the minimum tension of carbon dioxid. 



It has been shown experimentally that an increase in the 

 amount of this gas to such extent that the air will contain 

 from 1 to 10 per cent or more may be beneficial, provided 

 the other factors permit a maximum activity. The results 

 obtained by Godlewski and Kreusler are not entirely 



