§2] UPON THE DIRECTION OF GROWTH 413 



the rays in the same fashion, but not so powerfully, as they 

 would have curved from light. Thus, electric waves produce 

 in Phycomyces a negative electrotropisra. 



4. Magnetropism. — It was stated in an earlier chapter * 

 that magnetism has no clearly established effect upon proto- 

 plasm. The alleged effect on growth is, consequently, worthy 

 of attention. Tolombi ('93) placed over a glass vessel con- 

 taining germinating peas a large horseshoe magnet, connected 

 with a battery composed of eight Daniell cells. The germi- 

 nating organs bent away from the centre of the magnetic field ; 

 by an appropriate position of the magnet the roots might be 

 forced to grow upwards. Tolomei also asserts that young 

 plants are diamagnetic, i.e. tend to place their long axes at 

 right angles to the lines of force of the magnet. These results 

 are interesting enough to deserve confirmation. 



5. Explanation of Electrotropism and Summary. — Is elec- 

 trotropism the direct result of the current, or is it, like the 

 effect upon the rate of growth, an indirect result, due, for 

 instance, to chemical agents produced by the current ? If the 

 action is indirect, it may be either of a mechanical or of a 

 chemical nature. RiSCHAWi, who proposed the very clever 

 explanation of positive curvature on the ground of katophoric 

 action, offered a similar explanation for negative electrotro- 

 pism. He finds that a cylinder made of coagulated yolk, and 

 placed in the water transverse to the current, bends at first 

 convex to the anode, owing to the more rapid diffusion at first 

 of water into that side ; but, later, when the whole mass 

 becomes permeated, it bends so as to be concave towards the 

 anode. So, in the root, a weak current can induce a weak dif- 

 fusion of the external water into the cells on the anode side. 

 This theory does not, however, meet the conditions ; for, first, 

 a long-continued weak current does not induce the positive 

 curvature, and, secondly, because the decapitated root does not 

 turn from the anode, and the irritation of the tip alone can 

 induce the result. The mechanical theory must be rejected. 



There remains only the other theory, that the negative tro- 

 pism is a response to a stimulus of some sort applied at the tip. 



« Chapter VI, § 1. 



