132 PROFESSOR JOHN GIBSON ON 
Kinax, IS small. It is therefore in solutions where K,,,,, is greatest, that is, in concentrated 
solutions of the strongest acids, that the clearest evidence of the existence of the 
tendency towards increased conductivity is to be found. At first sight this would seem 
to lead far away from the chemistry of plant life, where mineral acids, and more 
particularly concentrated solutions of free mineral acids, are characteristically absent. 
When the strong chemical affinities are not balanced, but directly brought into play 
as in the action of acids on bases, and generally wherever the strong tendency of the 
charged ions Hl and OH to neutralise each other predominates, there the weaker 
tendency towards increased conductivity is masked. Although the systems hitherto 
discussed comprised strong mineral acids, strong chemical affinities were uniformly 
more or less completely counterbalanced. Thus the examples discussed have com- 
prised, among others, the action of nitric acid on nitric oxide; the action of 
hydrochloric acid on chromic acid and on aldehyde; the action of iodine on hydrogen 
sulphide, and the action of sulphuric acid on formic acid and on cane-sugar. 
Chemical systems in which the tendency towards increased conductivity is very marked 
and clearly recognisable resemble the chemical systems characteristic of plant life in 
this, that they are conducting systems containing good electrolytes, but with the strong 
affinities in abeyance. In plant chemistry the place of strong acids is taken by salts 
derived from the soil or, in the case of marine plants, from sea water. There is this 
further resemblance, that the solutions of many of the salts specially useful to plants, 
show maxima of specific conductivity, as do the strong acids. The conductivity of 
solutions of such salts have a specific conductivity of the same order as those of 
the strong mineral acids of corresponding concentration, their conductivities ranging in 
general from about one-third to one-sixth of those of the strong acids. So far as the 
insufficient data permit, it would appear that by merely selecting those salts which 
give the best conducting solutions and exhibit maxima of specific conductivity, we 
obtain an indication of the kind of salts most generally useful in plant chemistry. 
From the salts whose conductivities are given by KoHLrauscH and Hoxpory, 
calcium, magnesium, lithium, and manganese chlorides, potassium carbonate, fluoride 
and acetate, are thus singled out. (See Fig. [.) 
Reactions characteristic of plant chemistry are generally represented by equations 
which do not include good electrolytes. The fact that in plant metabolism the presence 
of gocd electrolytes, z.e. metallic salts, is indispensable has not hitherto been explained. 
The suggestion is here made that the tendency towards increased specific conductivity is 
an essential and determining factor in plant chemistry. It is not easy to obtain experi- 
mental evidence in favour of this suggestion in respect of foliage leaves, owing to the 
structural complexity of the tissues in which the reactions take place. The chemical 
reactions occurring in plants must be associated with changes in the local concentration 
of the sap solutions, which constitute the medium in which they occur, and consequently 
they must be associated with local changes in specific conductivity. 
There is in foliage leaves a striking periodic alternation between dehydration and 
