MOVEMENTS OF PLANTS 41 



not act as a magnet acts on iron, or to take a better 

 example, it does not simply act as gravity acts on 

 the plumb-line in which the string is kept in a 

 vertical line by the weight. It might be supposed 

 that in some occult way the stem was mechanically 

 kept straight hke the string, and this indeed was 

 the view formerly held about such roots as grow 

 straight down into the earth. But it is not so ; 

 the thing is not explicable mechanically. Gravita- 

 tion is nothing more than a sign-post or signal to the 

 plant — a signal which the plant interprets in the 

 way best suited to its success in the struggle for 

 life, just as what we see or hear gives us signals of 

 changes in the exterior world by which we regulate 

 our conduct. 



It may be said that this is hard to prove, and 

 indeed, like other biological theories, it can only be 

 shown to be true by its explaining a number of 

 facts. It is interesting to try to explain the facts 

 without the assumption in question. If gravity 

 does not act indirectly as a signal it must act 

 directly ; and we must find a reason why, in the case 

 of the mustard seedhng above referred to, the stem 

 has grown up and the root down. There is abso- 

 lutely nothing in their structure or manner of 

 growth to help us to see why this difference of 

 behaviour under identical conditions should exist. 

 And if, instead of placing the mustard seedling in 

 the dark we had grown it near the window,^ we 

 should have come across another remarkable 

 phenomenon, namely, that the stem grows towards, 



1 The root must of course be in a glass of water, and therefore 

 exposed to light. 



