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 



