CH. III. DOWNWARD GROWTH OF THE ROOTS. 



99 



wliere the earth protects them from the hght and air. 

 When denuded, the herbaceous envelope begins to 

 turn green in about a fortnight.* 



But an admirable experiment of Knight's furnishes 

 the main fact from which it has been asserted that both 

 the ascent of the stem and the descent of the root 

 should be referred to gravity. When he subjected 

 beans to a strong centrifugal force by making their 

 seeds grow on the rims of wheels whirled rapidly by 

 water, their roots grew from the centres of the wheels, 

 and their stems towards the centres of the wheels. 

 When the wheel was vertical, the growth of the plants 

 was precisely as stated. When the wheel was horizontal, 

 the growth of the plants was nearly horizontal ; but 

 the stems inchned upward and the roots downward in 

 an inverse ratio as the degree of centrifugal force. 

 Eichard tells us that this experiment was repeated by 

 Dutrochet, and the only difference was, that, in the 

 case of the horizontal wheel, ' the inclination was much 

 greater, and the radicles and gemmules had become 

 almost horizontal.' This last witness appears to me to 

 prove too much : for, granting the effects of gravity on 

 the growth of plants to cease directly as the centrifugal 

 force applied, the centrifugal force applied to each part 

 of the plants in this experiment would diminish directly 



* I have omitted to observe whether the under bark of the 

 old roots of plants grown in water is green or white ; if it is 

 green, the colour must be owing to the action of light, indejpen- 

 dently of atmospheric air. 



H 2 



