682 



LECTURE XXXIX. 



recognise in gravitation the cause which influences growth in the geolropic 

 curvatures; but this recognition was, as a matter of fact, obtained in quite another 

 manner. It is usually by round-about paths that truth is detected, because the 

 direct path to it in most cases requires greater clearness of thought : so it was here. 

 The fact that it is gravitation which influences the growth of the plant was demon- 

 strated in 1806 by an Englishman named Knight, as follows. He exposed seedlings 

 to the continued action of centrifugal force, by submitting them to rapid rotation 



Fig. 384.— The axis a is kept in continual rapid rotation. To it is fixed the circular 

 disc r r supporting the circular plate of cork k. On this, by means of two needles in each 

 case, the seedlings A and B are fixed, st the plumule : h the primary root. The lateral 

 roots are all curved outwards in consequence of the rapid rotation, g s^ glass cover : x axis 

 of 



either in the vertical or the horizontal plane. It was shown that the growing 

 root-apices behaved in this case exactly as if they were simply projected from 

 the centre of rotation like the weight of a pendulum made with a thread and swung 

 rapidly round in the hand ; while the shoot of the seedling behaved in exactly the 

 contrary manner, and grew towards the centre of rotation. 



In the case of centrifugal force, as in that of gravitation, it is also an effect of 

 mass which comes into play, and it was in this fact that Knight perceived the 

 proof that the geotropic directions of plant-organs generally are produced simply 

 by the action of the mass of matter, and such an action could only be referred to 

 the centre of gravity of the earth. It is clear, however, that this kind of proof 



