THE CONSTITUENTS OF THE ASH OF PLANTS 187 
somewhat widely spread enzyme known as Laccase, whose 
normal function is, however, at present obscure. 
The elements of the last group are numerous; they 
vary with the composition of the soil in which the plants 
are growing, and appear to subserve no useful purpose, 
Many of them in even moderately dilute solutions are 
extremely poisonous, so that they must be absorbed in a 
high state of tenuity. Their presence shows that the 
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 formation 
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 forma- 
tion of the flowers and subsequently the seeds. Thus the 
composition 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 
