PHYSIOLOGICAL RESPONSE IN PLANT AND ANIMAL 753 



the tip moved towards it, and was thus destroyed. The 

 sensitive reaction of the root-tip is thus seen to be, not an 

 adaptive act evolved for the advantage of the plant, but 

 merely an example of the general law, that under moderate 

 stimulation it is the indirect effect of stimulus that reaches 

 the growing organ and causes movement away from, while 

 under strong stimulation the direct effect determines move- 

 ment towards, the source of stimulus. Now, it is perfectly 

 true that had any given reaction been such as, under normal 

 conditions, to bring about the self-destruction of large numbers 

 of organisms, we should not have witnessed the survival of 

 organisms characterised by that particular movement. The 

 plant lives because the physiological differentiations induced 

 in it under natural conditions, and the movements induced 

 under periodic changes of those conditions, are in harmony 

 with the fluctuating forces of the environment. Thus the 

 forced rhythm becomes more deeply impressed with repeti- 

 tion, and the greater is the harmony between this rhythm 

 and the environment, the greater will be its stability under 

 given conditions ; the plant persists, that is to say, because 

 it is perpetually in tune, instead of perpetually at war, with 

 its surroundings. We may take it, therefore, in the case of 

 any particular movement, that it constitutes an expression of 

 this stable relation of the plant to its environment, but not 

 that it represents any deliberate adaptation to such an end. 



Reverting to the nyctitropic movement in particular, we 

 find it adduced by Darwin as to a certain extent furnishing 

 an example of the influence of heredity on the individual 

 organism, a view which has been questioned. But if we are 

 prepared to give a sufficiently extended and consistent mean- 

 ing to the word, we must accept this, for heredity is essentially 

 the repetition of a past cycle, the persistence of after-effects ; 

 and there are innumerable degrees of such persistence, be- 

 tween that of a transitory after-effect and the phenomenon 

 of absolute persistence, if such occurs. The diurnal periodi- 

 city of Mimosa, for example, is maintained for several days 

 under unchanging conditions of illumination or of darkness, 



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