86 THE BIOLOGY OF FLOWERING PLANTS 



since stomata are actual free openings, diffusion through 

 these is most important. This conclusion is borne out 

 by experiment for the exchange of carbon dioxide, and 

 transpiration of water vapour. The details of this relation 

 are most conveniently investigated by comparing the rate 

 of gas exchange through the upper and lower epiderms of a 

 leaf with different numbers of stomata on the two surfaces, 

 and comparing the results with the relative number of 

 stomata. A striking rough demonstration for the case 

 of water vapour is given by making use of the change 

 of colour of filter paper soaked in a solution of cobalt 

 chloride. When thoroughly dry this salt is a brilliant 

 blue ; exposed to air it absorbs water rapidly and 

 changes to pale pink. If a leaf of cherry laurel or ivy 

 be placed between two pieces of this cobalt chloride paper, 

 previously dried to the blue colour, and the whole be 

 protected by two sheets of glass, then in the course of a few 

 seconds the paper touching the lower side of the leaf changes 

 to pink ; on the upper side the change takes many minutes. 

 This corresponds to the presence of stomata on the lower 

 surface only. Exact measurements may be made by 

 absorbing the aqueous vapour given off from each surface 

 by a hygroscopic salt (calcium chloride is convenient) and 

 determining the amount by weighing. Using this method, 

 Unger (1862) (quoted from Burgerstein, 1904) obtained 

 results shown in Table XVII. 



TABLE XVII 



Relation of Transpiration to Stomatal Numbers 



