284 The Physiology of Plants BOOK in 



A somewhat animated controversy ensued for a few years, 

 but it cannot be said to have advanced our knowledge of 

 the subject. 



/ The stomatal theory received some support from the 

 work of Wiesner, in 1879, on the diffusion of gas through 

 small apertures. He determined the rates at which equal 

 volumes of gases of different densities pass through epi- 

 dermis bearing stomata, and found that they agreed with 

 the rates required on the theory that they pass by gaseous 

 diffusion through the apertures. 



In 1880 Pfeffer called attention to the effect of the 

 diminution of turgidity in the guard cells of the stomata, 

 and affirmed that it must render the absorption of car- 

 bon dioxide extremely difficult and hence markedly 

 diminish the photosynthetic activity of a subaerial leaf. 

 Pfeffer, however, did not oppose the cuticular theory ; 

 on the other hand, he held it to play a certain if sub- 

 ordinate part. 



In 1884 Haberlandt attacked the view that the inter- 

 cellular spaces are associated with gaseous interchanges, 

 holding them to be more particularly adapted to direct the 

 outflow of plastic substances from the constructing cells 

 and confine it to the most direct routes. He accordingly 

 favoured the theory of cuticular absorption. 



As a result of careful experiments on the diffusion of 

 gases through isolated cuticle, or perhaps epidermis, Mangin^ 

 in 1887, was led to support the stomatal theory. He calcu- 

 lated the amount of carbon dioxide that passed by osmosis 

 through specially prepared membranes consisting of the 

 outermost layers of certain leaves, isolated by the action 

 of bacilli, and found it to be insufficient to account for the 

 whole of the quantity which he determined in independent 

 experiments to take part in the gaseous interchanges in 

 similar living leaves. He computed further that during 

 the appropriation of carbon dioxide, the tension of that gas 



