CH. vii] knot's experiment. 155 



resistance is supplied by the descent of the root into 

 mercury. The arrangement of the experiment is shown 

 in Sachs' paper on the growth of roots\ A shallow 

 dish of 6 — 8 cm. in diameter is filled with mercury to a 

 depth of 2 — 3 cm., on which is a layer of water 5 — 6 mm. 

 in thickness. A split cork is firmly jammed like a rider 

 on the edge of the dish, and to it a bean is pinned so that 

 the root lies horizontally in the water just touching the 

 surface of the mercury. The whole arrangement is 

 covered with a bell-jar and left for 18 hours. Another 

 way of fixing the bean, which we find convenient, is 

 simply to support the pin, on which the cotyledons are 

 impaled, in a clamp attached to a small heavy stand. 

 We usually keep the cotyledons wet with a strip of filter- 

 paper dipping into the water. 



(200) Knight's experiment^. 



Sachs figures' an apparatus which any one can 

 construct for himself, and by which it may be demonstrated 

 that the geotropic parts of plants bend in relation to 

 centrifugal force. 



A simple plan is to use a water-wheel driven by a 

 strong fine jet of water directed against the wheel from 

 the water-tap. The wheel should stand in a sink fitted 

 with a cover; in this way, — with the help of the spray 

 from the wheel — the experimental plants are kept 

 thoroughly damp. 



' Arbeiten, i. p. 452, fig. 14. 



2 Phil. Trans. 1806. 



3 Physiologie V&gStale, p. 124. 



