410 LECTURE XVI. 



zontal axis by means of a machine, termed a Clinostat, which 

 will be described hereafter. Similar results were obtained by 

 Schwarz in the case of roots. Finally, Elfving found, by 

 experiments with peas pinned to a disc of cork which was 

 made to rotate rapidly, that a considerable centrifugal force, 

 acting along the axis of growth of the roots of seedlings in 

 the direction from the base to the apex, did not cause any 

 material alteration in the rate of their growth as compared 

 with that of roots growing under normal conditions. This 

 result, again, has been confirmed by Schwarz. 



ELECTRICITY. 



The effect upon its rate of growth in length of the passage 

 of constant currents through a growing organ was made the 

 subject of investigation by Elfving. He found that when a 

 current runs through the long axis of a root, its growth is 

 retarded, and more markedly when the direction of the cur- 

 rent is opposed to that of the growth in length of the root. 

 These results have been confirmed by Miiller-Hettlingen. The 

 influence of the current appears to be of a tonic nature. 



PRESSURE AND TRACTION. 



The effects of pressure and traction upon the rate of 

 growth of organs, though for the most part they can hardly 

 be regarded as phenomena of irritability, but as purely 

 mechanical, may be conveniently considered here. In a 

 former lecture (p. 348) we discussed the effect of the tensions 

 of the tissues upon growing cells, and we then learned that 

 pressure limits the growth of the cells in the direction in 

 which it is applied. Generally speaking, the effect of pressure 

 upon a growing organ is the same as that produced upon 

 growing cells by the resistance of the older tissues ; pressure 

 retards the growth of that portion of the organ to which it is 

 applied. Sachs gives a number of illustrative cases of this : 

 if the root of a seedling be grown so that it comes into 

 permanent contact with some solid body, it gradually curves 

 round it, in consequence of the growth in length of that 



