340 SPONTANEOUS MOVEMENTS IN PLANTS. 



CHAPTER XII. 



OF THE SPONTANEOUS MOVEMENTS WHICH PLANTS EXHIBIT. 



647. Plants, like other living beings, execute certain movements, 

 or changes in the position of their parts, through some inherent 

 powers, which, though far less striking and less varied than in ani- 

 mals, and of a nature wholly different from muscular motion, must 

 not be overlooked. 



648. The Special Directions which the organs of the plant assume 

 belong to this class of manifestations, although the movements are 

 mostly much too slow to be directly observed. Among these are the 

 invariable descent of the root in germination, the invariable ascent 

 of the stem into the light and air, and the turning of branches and 

 the upper surface of leaves towards the light (113, 139, 294). 

 Although these movements are incited by common physical agents 

 (just as analogous kinds of movements are in animals), and can- 

 not be the result of any thing like volition, yet nearly all of them 

 are inexplicable upon mechanical principles. Some of them, at 

 least, are spontaneous motions of the plant or organ itself, due to 

 some inherent power, which is merely put in action by light, attrac- 

 tion, or other external influences. 



649. The external agencies concerned in the descent of the root 

 and the rise of the stem seem chiefly to be, 1st, the attraction of 

 the earth acting upon the root ; and 2d, the influence of light upon 

 the stem. The influence of gravitation, or of a similar force, was 

 proved by the celebrated experiment of Mr. Knight ; who caused 

 the seeds of the Bean to germinate in a quantity of Moss fastened 

 to the circumference of a wheel, which was made to revolve verti- 

 cally at a rapid rate ; where the seeds were subjected to the cen- 

 trifugal force alone, acting like that of gravitation, but in the oppo- 

 site direction. On examination, after some days, the young root 

 and stem were found to have taken the direction of the axis of ro- 

 tation ; the former being turned towards the circumference, and 

 the latter towards the centre of the wheel. The same result took 

 place when the wheel was made to revolve horizontally with con- 

 siderable rapidity ; but when the velocity was moderate, the roots 

 were directed obliquely downwards and outwards, and the stems 



