240 Influence of Environment on Plants 



help of any available hypothesis for points of attack, which may enable 

 us to acquire a more complete mastery of physiological methods. 

 To quote a single example ; I may put the question, what internal 

 changes produce a transition from vegetative growth to sexual repro- 

 duction ? 



The facts, which are as clearly established for the lower as for the 

 higher plants, teach us that quantitative alteration in the environ- 

 ment produces such a transition. This suggests the conclusion that 

 quantitative internal changes in the cells, and with them disturbances 

 in the degree of concentration, are induced, through which the 

 chemical reactions are led in the direction of sexual reproduction. 

 An increase in the production of organic substances in the presence 

 of light, chiefly of the carbohydrates, with a simultaneous decrease 

 in the amount of inorganic salts and water, are the cause of the 

 disturbance and at the same time of the alteration in the direction 

 of development. Possibly indeed mineral salts as such are not in 

 question, but only in the form of other organic combinations, par- 

 ticularly proteid material, so that we are concerned with an alteration 

 in the relation of the carbohydrates and proteids. The difficulties 

 of such researches are very great because the methods are not yet 

 sufficiently exact to demonstrate the frequently small quantitative 

 differences in chemical composition. Questions relating to the 

 enzymes, which are of the greatest importance in all these life- 

 processes, are especially complicated. In any case it is the necessary 

 result of such an hypothesis that we must employ chemical methods 

 of investigation in dealing with problems connected with the phy- 

 siology of form. 



II. Influence of environment on the transformation 



OF SPECIES. 



The study of the physiology of form-development in a pure species 

 has already yielded results and makes slow but sure progress. The 

 physiology of the possibility of the transformation of one species into 

 another is based, as yet, rather on pious hope than on accomplished 

 fact. From the first it appeared to be hopeless to investigate physio- 

 logically the origin of Linnean species and at the same time that of 

 the natural system, an aim which Darwin had before him in his 

 enduring work. The historical sequence of events, of which an 

 organism is the expression, can only be treated hypothetically with 

 the help of facts supplied by comparative morphology, the history 

 of development, geographical distribution, and palaeontology 1 . A 

 glance at the controversy which is going on to-day in regard to 

 different hypotheses shows that the same material may lead different 



1 See Lotsy, Vorlesungen (Jena, i. 1906, n. 1908), for summary of the facts. 



