SPONTANEOUS MOVEMENTS IN PLANTS. 341 



666. The Special Directions which the parts of all plants assume are 

 the result of self-caused movements, although such movements are 

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

 most universal are the descent of the root in germination, the ascent 

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

 the upper surface of leaves towards the light (120, 131, 294). 

 These directions evidently are not the result of mere growth. It is 

 not that the root grows downwards and the stem upwards ; but the 

 root end of the elongating radicle bends or curves in the course of 

 its growth so as to point downwards if not already in that position, 

 and the other extremity, with the plumule, curves upwards, and the 

 young stem, after reaching the light, if unequally illuminated, bends 

 towards the stronger Ught. 



667. Strenuous attempts have been made to explain these changes 

 of direction upon mechanical principles. Mr. Knight thought that 

 the descent of the root and the ascent of the stem were caused by 

 gravitation ; and he seemed to show this by his celebrated experi- 

 ments of removing germinating seeds from the influence of gravita- 

 tion, and causing the root and stem to take a different direction in 

 obedience to a different force. lie fixed some beans ready to ger- 

 minate in a quantity of moss upon the circumference of a wheel, and 

 made it to revolve vertically at a rapid rate ; replacing the effect of 

 gravity by centrifugal force. On examination, after some days, the 

 young root was found to have turned towards the circumference, and 

 the stem 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 

 obliquely upwards and inwards, in obedience both to the centrifugal 

 force and the power of gravitation, acting at right angles to each 

 other. It remained for Mr. KJnight to explain how the same force, 

 gravitation, could produce such opposite effects, causing the stem to 

 ascend as well as the root to descend. This he ingeniously at- 

 tributed to their different mode of growth. The root growing at its 

 extremity only, he supposed that the soft substance of the growing 

 j)oint would be acted upon by gravity like an imperfect solid, and 

 accumulated on the lower side ; while the stem, growing by the 

 elongation of an internode or a series of internodes already formed, its 

 solid tissues would be unaffected by gravity, which could affect only 

 its nutritive juices, causing their accumulation on the lower side of a 



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