532 SUMMABY OF CURRENT RESEARCHES RELATING TO 



bearing the same title, and in a great many points comes to a more or 

 less different conclusion, his arguments being in all cases founded on 

 actual experiments. The following are some of the more important 

 points in which he differs from Darwin. 



As regards circumnutation, Wiesner doubts its existence in roots, 

 attributing this apparent phenomenon to the antagonism between 

 geotropism and the natural tendency to curvature existing in roots, 

 first one and then the other of these forces getting the upper hand, 

 and thus moving the tip of the root backwards and forwards. In the 

 case of stems, also, he considers that there are some plants which do 

 not exhibit circumnutation. Some leaves also, he states, grow in 

 absolutely straight lines without circumnutating, the apparent cir- 

 cumnutation being here again due to the varying action of opposing 

 forces, viz. epinasty, apogeotropism, apheliotropism, and gravitation. 



Wiesner's explanation of heliotropism agrees with that of De 

 Candolle, that the convex side grows more quickly simply because it 

 is in shade. 



The author disputes the value of Darwin's experiments which are 

 alleged to prove the sensitiveness of the tips of radicles, attributing 

 the observed phenomena to injury resulting from the means em- 

 ployed. 



Electrical Researches on Plant Forms.* — The absorption of 

 water by porous bodies is accompanied by electric currents. When 

 a porous earthenware cell is partly filled with water, and a current 

 completed through a galvanometer by means of electrodes in the 

 water and in contact with the outer wall of the cell, a current passes. 

 The intensity continually diminishes, until it finally ceases, and then 

 a current begins in the opposite direction from the cell-wall through 

 the water. This reversal of current is due to the incomplete state of 

 saturation of the walls of the cell. These phenomena are employed 

 by A. J. Kunkel to account for various electric phenomena observed 

 in plants. 



In regard to the electromotive action of the upper surface of green 

 leaves, the difference of tension of the various parts was determined 

 by a systematic method of contact over the whole surface, with the 

 result that the leaf-veins are generally positive towards the rest of 

 the leaf, but the direction of the current is reversed if the spot on the 

 leaf where the electrode is placed is wetted before the other electrode 

 is placed on the vein. Also a spot long moistened is positive 

 towards one freshly wetted. "When the electrodes rest on the epi- 

 dermis of a plant and a wound is made near the electrode, then that 

 electrode will be negative to the other. The same result is obtained 

 by bending the plant, and the current formed is the more intense the 

 greater the amount of bending, the electrode near the bend being 

 negative to the other. Sometimes plants also show the existence of 

 electric currents, which when the plant moves cause the galvanometer 

 needle to oscillate. 



* Bied. Centr., 1882, pp. 28-30. Cf. Journ. Chem. Soc. Abstr., xlii. (1882) 

 p. 638. 



