LECT. XL ACTION OF GRAVITY. 213 



But I wish to speak of a peculiar phenomenon which 

 presents itself in the development of vegetables, and in 

 which it is impossible not to recognise the effect of 

 gravity. 



In general, the seeds of all vegetables germinate and 

 shoot, by manifesting the tendency which their roots have 

 to descend, and their stems to ascend. Experience proves 

 that the opposite direction which these parts of the plant 

 take, is owing neither to the moisture of the soil, nor to the 

 action of light or of atmospheric air. The roots continue 

 to descend, and the stems to ascend, even when their 

 natural position is inverted; that is to say, when the latter 

 is .placed in contact with the earth, and the former is sub- 

 mitted to the action of light. We are indebted to Knight 

 for some ingenious experiments, which, if they do not 

 entirely clear up this subject, at least have demonstrated 

 the existence of one of the causes which preside over this 

 phenomenon. 



Hunter was the first who observed, that if a barrel filled 

 with earth, in the centre of which were some beans, be 

 rotated, for several days, horizontally, the roots pointed in 

 a direction parallel to the axis of rotation. 



Knight fixed some garden beans on the circumference of 

 a wheel, supplied them with moisture, and kept the wheel 

 revolving for a considerable time. He found, that when 

 the wheel was vertical, the radicles or roots of the young 

 plants pointed towards the circumference, and the stems 

 towards the centre of the wheel ; but when the wheel was 

 horizontal, the roots and the stems pointed obliquely, the 

 roots being always directed towards the circumference. 



By considering Knight's experiment in connexion with 

 the first one quoted, and which demonstrates that the direc- 

 tion of stems and roots is under the influence of gravity, it 

 follows, that in the second experiment these point obliquely, 



