224 PLANT PHYSIOLOGY 



cell walls, will permit the passage of oxygen and carbon dioxide. 

 This is extremely important, because it means that in dry weather 

 and under conditions which tend to close the stomata, some oxygen 

 can get through to the tissues beneath and the cells will not be 

 suffocated. The plants will not dry out and yet at the same time 

 will not die from lack of oxygen. 



These investigators have thus shown that the carbon dioxide 

 diffuses through the cell walls in solution in the water which 

 impregnates them. Before it can get into the cell from the inter- 

 cellular spaces it must first dissolve in the water of the cell wall 

 and then diffuse through into the cell in solution. The water 

 combines with the carbon dioxide forming a three-phase system 

 of water, carbonic acid, and carbon dioxide. As the carbonic 

 acid is used up inside the cell, more carbon dioxide diffuses in 

 from the outside, and thus the supply is maintained. 



Work of Blackman. — The third conclusion given above natu- 

 rally leads to the question of the proportion of the carbon dioxide 

 which finds its way into the intercellular spaces of the leaf through 

 the stomata and the proportion which enters through the cuticle. 

 To answer this question Blackman (1895) devised an apparatus by 

 which gases could be led into a small chamber clamped to the 

 side of a leaf. A gas of known composition was passed through 

 the chamber and then analyzed again on leaving to see whether 

 it had been modified by the gaseous exchange of the leaf. Experi- 

 ments with leaves having stomata on both sides and those with 

 stomata on only one, showed that the exchange which accompa- 

 nies respiration occurred only through the surface where the sto- 

 mata were. For example, the oleander (Neriam) gave out 0.065 g. 

 of carbon dioxide from the lower surface and only 0.002 g., or 

 less than one-thirtieth as much, from the upper side. 



That the exchange during photosynthesis also takes place 

 through the stomata and not through the cuticle, can be shown 

 very easily by covering the lower surface with vaseline or wax, 

 which thus plugs up the stomata. Under such conditions, as was 

 shown by Mangin (1887), very little photosynthesis occurs, al- 

 though it is not completely stopped. That the stomata determine 

 largely the entrance of carbon dioxide is also shown in the case 

 of the water plantain {Alisma). Here the lower surface has 74% 

 as many stomata as the upper and absorbs 68% as much carbon 

 dioxide. 



