182 VEGETABLE PHYSIOLOGY 
selective power of plants is not necessarily connected with 
the development of normal metabolic functions, but is 
mainly physical and only to a slight extent physiological. 
From what has already been advanced, it is evident 
that the time is not ripe for a detailed discussion of the 
parts played by the constituents of the ash of plants. Nor 
will it be till we have ascertained much more fully how 
the various metabolic processes are carried on. Certain 
broad statements of a somewhat general character are all 
that are at present justified, and these concern only some 
of the mineral matters which are absorbed. The meta- 
bolism not only depends on the presence of certain elements, 
but is largely influenced by the relative quantities of each 
which the active cells contain. 
The vegetative activity of, at any rate, herbage plants 
is associated with a plentiful supply of nitrogen. In the 
absence of sufficient potassium vegetative luxuriance may 
be obtained, but the degree of development of the plant is 
limited by such deficiency. In the event of sufficient 
supplies of potassium being afforded, the relative abundance 
of the nitrogen has an important influence on the forma- 
tion of carbohydrates, which are then produced in greater 
quantities. Coincidently the plants go on to maturity ; 
the luxuriance of the leafy parts becomes curtailed, and 
the development proceeds normally, leading to the formation 
of the flowers and subsequently the seeds. Thus the com- 
position of the supplies in the soil determines largely the 
character of the development of the plants growing in it. 
It has also considerable influence upon the variety of the 
species of these plants, owing to the various ways in 
which particular constituents may influence different 
individuals. 
In the absence or the deficiency of particular salts, 
others may be absorbed in proportions very different from 
those which would have been found had the missing ele- 
ment or elements been present, 
