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. 



