EFFECT OF EXTERNAL AGENCIES ON GROWTH. 607 



heliotropism, in spite of appearing to be due to negative heliotropism. 

 But little is yet known as to these points; but it is surmised that there 

 are in plants two sets of cells, some positively heliotropic, retarded in 

 their growth by light, others negatively heliotropic, whose growth is 

 promoted by exposure to the light. Organs in which growth is finished 

 no longer manifest these heliotropic curvatures, of which the greatest 

 degree occurs in the regions where growth is going on most actively. 

 The curves are not produced immediately on exposure to light, but occur 

 gradually, and disappear gradually, even after the exposure ceases. The 

 position and degree of curvature necessarily vary, according to the angle 

 of incidence of the light, and are proportionate to the intensity of the 

 light. Negative heliotropism occurs in the case of the plasmodium or 

 naked masses of protoplasm of JEthalium (Myxomycetes). Exposure to 

 light causes, according to Baranetzky, a thinning of the plasmodium, 

 and a corresponding thickening or aggregation in the parts to which the 

 light has no access. As in other plants, the most energetic effect is pro- 

 duced by the blue refrangible rays, the heliotropic effect being not obser- 

 vable in the luminous rays. In darkness, the mass of protoplasm ascends, 

 against gravity, the surface of a wall or other support, but its progress 

 upwards is arrested by exposure to light. 



Action of Electricity. The mechanical and chemical processes which 

 go on in the cells of plants are necessarily connected with disturbances 

 of electrical equilibrium. Ranke, Burdon Sanderson, Velten and others 

 have demonstrated the fact that there exists in plants an electric current 

 such as Du BoisReymond has demonstrated in the muscles and nerves of 

 animals, but in an inverse direction. While in animal tissues the current 

 is directed from the longitudinal to the transverse section of the muscles, 

 the direction of the current in vegetable fibre is from the transverse to 

 the longitudinal direction. The currents are made visible by the medium 

 of a galvanometer, the epidermis, a bad conductor of electricity, having 

 been removed from the fragment of tissue examined. The electric cur- 

 rent in plants, says Velten, has no relation to the degree of concentration 

 of the sap or the state of aggregation of the protoplasm. 



According to Ranke, the molecules of a plant are embedded in a con- 

 necting substance, and have, according to the direction of the electric 

 current, two positive poles and an equatorial negative zone, just the 

 reverse of what obtains in animal molecules according to Du Bois Rey- 

 mond. 



Plants seem to be regulators of electricity, and restore the balance 

 between the disturbances of the electric tension in the case of the earth 

 and air respectively. Bridgeman has shown (Gardeners' Chronicle, 1873, 

 p. 142) that healthy and luxuriant growth takes place round the nega- 

 tive pole of a galvanic battery, while at the positive pole the direction of 

 the radicles is inverted, and they are thrust upwards, grow feebly, and 

 become the prey to mildew. 



Velten's recent researches on the influence of electrical currents on 

 protoplasm show that constant and inductive galvanic currents exercise 

 the same effects on the protoplasm and its movements. Very weak elec- 

 trical currents in parts of plants which offer great resistance cause an 

 acceleration of the movements of the protoplasm, which may be ascribed 

 to the higher temperature brought about by the stream. A very weak 



