548 History of the Doctrine of [Book hi. 



position of carbon dioxide which does not take place in the 

 dark, yet he gave an example of genuine scientific investiga- 

 tion and again expressed its true spirit, when he said that the 

 Linnaean phrase, ' the sleep of plants,' was unsuitable, because 

 the sleeping leaves are not relaxed, but continue as stiff as in 

 the day-time. De Candolle also, like Senebier, experimented 

 in 1806 on the influence of light on vegetation, and succeeded 

 in proving that the daily period of leaves may be reversed by 

 artificial illumination ; he was, as we have said above, an 

 adherent of the theory of a vital force, but only made use of 

 it when physical explanations failed him. The same year 

 (1806) is the date of a brilliant discovery, which was extremely 

 inconvenient to the thorough-going adherents of the nature- 

 philosophy and the vital force, and did much to bring the 

 scientific study of the movements of plants back to the right 

 path. Andrew Knight 1 showed by experiment that the ver- 

 tical growth of stems and primary roots is due to gravitation ; 

 he attached germinating plants to a rapidly revolving wheel, 

 and thus exposed them to the centrifugal force, either alone 

 or combined with gravitation ; the radicles, which normally 

 follow gravitation, here took the direction of the centrifugal force, 

 while the stems assumed the opposite direction. The next ques- 

 tion was, why gravitation makes the roots and stems take exactly 

 opposite directions, why, that is, in a plant placed in a hori- 

 zontal direction, the root-end curves downwards and the stem 

 upwards. Knight supposed that the root, being of a semi- 

 fluid consistence, is bent downwards by its own weight, while 

 the nutrient sap in the stem moves to the underside and causes 

 stronger growth there, until by means of the curvature so pro- 

 duced the stem assumes the upright position. Here too, as in 

 Dodart's case, it was no great misfortune that the explanation 

 proved afterwards to be insufficient ; it served at the time to 



1 Thomas Andrew Knight, President of the Horticultural Society, was 

 born at Wormsley Grange, near Hereford, in 1758, and died in London in 

 1838. 



