Chap. III.] tllC Movements of Pldllts. 549 



explain as much as was then known of the matter. /Hie spirit 

 of true scientific research displayed in Knight's explanation of 

 geotropism was expressed in many other contributions which he 

 made to vegetable physiology ; two only must he mentioned 

 here. He showed in 181 1 that under suitable conditions roots 

 are diverted from the vertical direction by moist earth, an 

 vation which was confirmed by Johnson in 1828 and afterwards 

 forgotten. More attention was excited by his discovery in 

 18 1 2, that the tendrils of Vitis and Ampelopsis arc negatively 

 heliotropical, that is, that they turn away from the source of 

 light. A few other cases of this kind of heliotropism have 

 since been discovered, and they are highly interesting, because 

 they teach that there is the same opposition in the relations of 

 plants to light as in their relations to gravitation. Knight 

 possessed some of the direct and bold reasoning power of his 

 countryman Hales ; he defied the vital force, and was always 

 ready with a mechanical explanation, if it was at all possible to 

 find one. Thus he explained the twining of tendrils by sup- 

 posing that the pressure of the support drives the juices to the 

 opposite side, which consequently grows more vigorously and 

 causes the curvature, which makes the tendril wind round the 

 support. This theory was at all events better than the one 

 which von Mohl sought to put in its place in 1827, and no better 

 one was suggested till very recently. Much the same may he 

 said of Knight's explanation of geotropic curvatures ; it is true 

 that Johnson showed in 1828 that the ends of roots as they 

 curve downwards set in motion a heavier weight than them- 

 selves, and therefore do not simply sink down, and Pinot in 

 1829, that they force their way even into quicksilver, and that 

 consequently Knight's theory, at least as regards the r. 

 unsatisfactory; but no better theory has yet been found, and 

 his view also of the progress in the upward curvature of the 

 stem has not given place to any one that can he said to he 

 more generally accepted. 



It was the commonly received opinion till after 1820 that the 



