THE PLANT AND ITS PHYSICAL ENVIRONMENT. 207 



in the soil, as well as by the supply of available water. Nevertheless 

 a knowledge of the factors involved will certainly result in complete 

 control over the colouring being ultimately gained, especially when the 

 apples are grown in pots for exhibition or other special purpose. 



Similarly, we ought to be able to control, better than we can at 

 present, the colour development of many flowers. Recent investiga- 

 tions have shown that ferments, and in particular those which induce 

 oxidative changes, are largely concerned in the matter, but although 

 as yet we are entirely in the dark as to the precise nature of any 

 ferment whatever, we are fully aware that the chemical processes which 

 they stimulate are materially affected by the conditions of the environ- 

 ment. Probably light, and all it stands for, plays an important part, 

 as it does in other vegetable processes. But it is also certain that the 

 addition of certain constituents to the soil are by no means negligible. 

 If anyone doubts this, let him experiment with some of the easy 

 alpines, e.g. Saxifraga oppositifolia or Saponaria ocymoides. The 

 addition of phosphate of potash to the soil will materially alter the 

 colour of the flowers, and will, so far at least as my own experience 

 goes, tend to cause them to approximate more closely to the vivid 

 hues of the flowers when growing in their natural montane homes. 



Time will not allow my attempting to deal with many other 

 aspects of this part of the subject. We may, however, inquire how 

 it is that the responses to the exigencies of the physical environment 

 are often so strikingly apt, and it is worth while asking further if 

 this is really as general as is often supposed, and indeed whether the 

 apparent universality of adaptation is not liable to lead us by fallacious 

 argument to unsound conclusions, 



[As examples of these, the instance of intercellular spaces was 

 discussed, and the difhcuities occasioned by excretion of sugar by lime 

 trees (honey-dew) and in other vegetative parts of plants were con- 

 sidered.] 



The variety of response to similar environmental claims emphasizes 

 still another aspect of the general problem, and the general conclusion 

 arrived at is, to put it shortly, that the more closely we are enabled 

 to analyse the response of a plant to the demands of the physical 

 environment, the more we find the laws, so far as we know them, 

 of chemistry, molecular physics, surface tension, and so forth hold 

 good, and furnish us with the clue to the unravelling of the secrets 

 of living things. F^ven auto-regulation, that wonderful property 

 so commonly displayed by plants (and animals), can be shown, in 

 at least some critical instances, to be explicable as the result 

 of the operation of limiting factors not essentially different in kind 

 from those which control chemical actions in vitro. In these directions 

 lies our hope of gaining control over vital processes — a control which 

 in a small measure we have begun to secure, though we have every 

 reason to hope and expect that as our knowledge grows so will our 

 power increase also. 



