igS JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



I propose then to-day to confine my remarks to a consideration 

 of the plant in relation to its physical environment, reserving for the 

 next lecture that of the mutual influences of the plants themselves 

 one upon another. 



If we ask ourselves what are the most fundamental of the physical 

 conditions which on the whole affect plant growth, we should not be 

 far wrong in naming temperature and the available water supply as of 

 prime importance. Temperature cannot be ignored, for it affects the 

 nature and rate of chemical change. This is easily seen, in its broader 

 outlines, when one compares the behaviour of temperate and stove 

 plants with each other. The influence of water supply on vegetation 

 in its more superficial aspects is familiar to everybody, but as soon as 

 one gets nearer to the life processes themselves one becomes aware 

 how profoundly the water content may determine the course of 

 development of an organism. For the plant is a chemical laboratory, 

 in which delicate operations of manifold variety are going on, all 

 regulated by an apparatus which itself is readily liable to change 

 under the influence of altering physical conditions. Amid such 

 complexity it is hardly surprising that we have as yet made but 

 little headway, comparatively speaking, in obtaining comprehension of 

 what goes on in a plant ; but we have, nevertheless, made some sure 

 advance, and here and there we have begun to piece together parts of the 

 whole story. The further we proceed the more clearly do we see that 

 success or failure is bound up with providing the conditions requisite 

 for the proper carrying out of those chemical changes that lie behind 

 healthy life, and that, as we are able to replace empirical treatment by 

 a procedure based on a true appreciation of the really relevant facts, 

 our advance will be the more certain and the more rapid. 



As our knowledge grows, we find that the gulf which still separates 

 the living from the non-living world is not so formidably wide as it 

 once seemed to be. By refusing to rest contented with seeing in a 

 particular " adaptation " just a beautiful adjustment of plant structure 

 to its supposed requirements — by substituting for this pleasant 

 fashion of regarding the development of useful variations a more 

 prosaic and more rigorous analysis of the factors concerned — we acquire 

 a new standpoint, and one which is full of promise. The further our 

 understanding can probe the matter, and disentangle the causes 

 and effects, the more certainly shall we get intelligent control over 

 plant growth, and be able to secure with certainty results that we now 

 can only occasionally obtain by chance. 



Chemistry and physics have taught us much, and they have enabled 

 us not only to win a great measure of control over the inorganic world 

 but also to make some inroads into the organic one, and the further 

 we advance on those lines the more surely we find matters that seemed 

 mysterious are susceptible of resolution into simpler components which 

 fit into the scheme of things, so far as they have as yet become under- 

 stood. 



It has long been known that chemical reactions are profoundly 



