PRESIDENTIAL ADDRESS. 25 
- These distinctive changes, which can now be somewhat 
readily followed, suggest the evolutionary formation of the 
several species and groups. The extended period which 
must have elapsed before the species succeeded in reaching 
such definiteness in general features, indicates also a con- 
siderable age for the genus, and suggests the probability 
that these changes have been by slow and almost imper- 
ceptible stages. 
The evidences which have so far accumulated appear to 
point to chemical influences being largely the direct cause 
of these distinctive changes, and our knowledge of the . 
groups would be greatly advanced if it were possible to 
discover the mechanism of these chemical reactions. That 
they are physiological in effect is evident, and it is to the 
results of extended study in this branch of science that we 
may hope to understand more and more those complex 
chemical reactions, ever at work building up distinct organic 
material from very simple substances. 
The results of Plant Metabolism are, perhaps, more often 
considered as directly traceable to the effects of organic 
influences, than to those in which inorganic constituents 
appear to play a more important part. It may even be 
suggested that the deeper consideration of these latter 
processes has so far been neglected; certainly this is so in 
comparison with those studies carried out from the strictly 
organic side. Itis generally recognised, however, that the 
available food of plants is intimately associated with the 
presence of certain elements, and metabolism does not 
appear to be able to proceed satisfactorily without their 
assistance. 
The methods whereby these inorganic elements—often 
present in most minute quantities—are able to assist in 
the synthetic production of these complex organic bodies 
are certainly not understood, and the presence of manganese 
