82 Chapters in Modern Botany chap. 



But it is difficult for us to form any conception of the 

 reason why different plants or different parts of the same 

 plants should be affected by the light in opposite ways. 

 Thus the flower-stalks of the ivy-leaved toad-flax {Lmaria 

 Cyinbalarid)^ which so often drapes our old walls with 

 beauty, at first bend towards the light, but turn in the 

 opposite direction after the flowers are fertilised, and the 

 time approaches for planting the seed in a crevice. 



As to the causes of the light-seeking or light-avoiding 

 movements, in their usual forms they are possible only as 

 long as the moving parts continue to grow. Now as light 

 usually exerts a retarding influence on growth, the side of 

 the plant which is shaded grows more quickly than the 

 side which is lighted, hence the plant bends towards the 

 light. But this will not explain light-avoiding, and so we 

 are led to recognise, as before, that the effect of the light 

 on the living matter of different parts of the plant or of 

 different plants is not always the same. 



Rationale of Light-seeking and Light-avoiding Move- 

 ments. — But when we inquire more precisely into the in- 

 fluence of the light, a hundred difficulties beset us. How far 

 was Darwin right in supposing that light simply modifies an 

 existing spontaneous movement of circumnutation ? How 

 far are others warranted in assuming that there are in 

 different parts of the plant specially contractile cells which 

 are affected in various ways by stimuli .? in other words, 

 how far may the movements be independent of growth ? 

 Or if the movements be entirely dependent upon growth, 

 how far is this the result of a change in the water-tension 

 or turgescence of the cells exerting pressure on their in- 

 ternal surfaces, as Sachs and De Vries would say ; or is 

 Klebs wholly wrong in regarding the turgescence rather as 

 a symptom than as a cause, and in referring growth to un- 

 known properties of the protoplasm ? 



