igoo-i.] Effects of Water on Foliage Leaves. 265 



for a reasonable explanation for the phenomena chiefly under discussion 

 in this chapter. 



The nature of the substance diffusing out through the leaf tissue, to 

 cause this alkaline reaction, is discussed in more detail in the foregoing 

 chapter. Several phenomena developed during the course of the 

 following experiment, which are interesting and important. On 

 examining the recorded details of the experiments, one notices first that 

 the time required to cause a sufficient change of colour of the litmus is 

 an appreciable interval, and it varies widely with different plants, as 

 might be expected. The time required to cause a distinctly alkaline 

 reaction of the water applied to the surface, depends upon the 

 permeability of the cell walls, especially upon that of the cutin upon 

 the epidermal layer, upon the diff'usibility of the salts extracted by the 

 external water, and directly upon the readiness with which the acid 

 contents of the cells make their way out and neutralize the alkaline 

 substances taken from the tissue by diffusion. The COo in the 

 atmosphere, and in and about the leaf surface, is no unimportant factor 

 in determining the colour of the test paper. As is shown in the preceding 

 chapter the substance which diffuses out is largely K0CO3, CaH^ (CO;,).., 

 and probably some potassium oxalate. Two of these substances, 

 potassium carbonate and potassium oxalate, have a reaction rather 

 strongly alkaline, while the other is slightly acid to litmus test paper- 

 If the last-mentioned salt (CaH.., (COsJa) predominate strongly, there will 

 be a weakly acid reaction, as is shown in some plants. This substance, 

 being so very unstable, breaks down (upon evaporation of the solution), 

 into CaCOs, CO2 and H2O, leaving as a residue the carbonate of lime. 

 If, however, water is present and more CO2 available for absorption into 

 the solution, it would become gradually more and more acid, as is shown 

 in the results of the experiment. 



One very important difficulty in the way of success in demonstrating 

 this phenomenon, is that reddened litmus paper will often become 

 slightly blue if placed in distilled water which is allowed to evaporate 

 down to dryness. That there might be some slight action between the 

 water and the sodium or the potassium of the glass, is barely possible. 

 Small quantities of ammonia in the air may have some effect. An 

 experiment was performed to test this phenomenon. Two well-cleaned 

 panes of glass, 160 by 210 mm, were placed face to face together, with a 

 few drops of distilled water and a few strips of red litmus paper between. 

 Owing to the adhesion of the water for the glass and the slow 

 evaporation, the water remained there several days, and the colouring of 



