Chapter X — 139 — Drosera 



vigorous ones. He found, however, no response to simple mechanical 

 stimulation, but this was found later to be wrong. Equally so his view 

 that a stimulating body attached to the back of a leaf induced re- 

 sponses whereby the tentacles turned backward to embrace the body 

 quite as well as forward. He found that the leaf may repeat the per- 

 formance after recovery on renewal of secretion, and further that the 

 effect of a given stimulus depends on the distance it has to travel. The 

 movements can take place under water and in response to soHd bodies 

 and acids in weak solution. The rate of response is aflfected by tem- 

 perature but not by Hght. It is then chiefly to Nitschke and to Dar- 

 win that we owe many original observations which furnish a picture of 

 the direction and rapidity of the movements of the leaf and tentacles. 

 The general facts first and most readily observed are the following. 

 If a suitable stimulus is received by any group of leaf tentacles, say 

 near the middle of the leaf, or on or near the "disc" in the case of 

 D. rotundifolia, in the course of a few minutes a bending of the near- 

 by tentacles is to be observed until, the stimulus evidently travelling 

 radially, it reaches even the extreme marginal tentacles which then 

 bend over. If the stimulus is sufficient even the leaf blade responds 

 in like manner. Goebel figures a leaf of D. intermedia which had com- 

 pletely folded over to embrace the body of a large fly which had been 

 caught. D. capensis was found to be particularly good at this. I 

 placed a single Drosophila flylet on a leaf and in the course of time the 

 marginal tentacles, as well of course as those nearby, had responded. 

 Finally the whole apex of the leaf bent over (ij — 8). With regard to 

 the leaf blade not all the species of Drosera behave in this way. Goe- 

 bel observed that D. hinata does not, nor does D. dichotoma, and 

 probably others. From such observations it is evident that the stim- 

 ulus received by a tentacle travels to its base and radially from there 

 to neighboring tentacles, which then respond. A casual glance at a 

 leaf displaying these responses, one in which the tentacles are bent 

 over towards the middle of the disc (speaking of D. rotundifolia) sug- 

 gests that the normal movement of the tentacles is along radial lines. 

 The dorsiventral flatness of the tentacles would seem to condition them 

 to move thus. But Nitschke saw that the matter is not so simple. 

 He said that the tentacles receiving the stimulus bend over in the di- 

 rection of the point at which the stimulus was received, irrespective of 

 its position, so that, if a fly is caught at some eccentric point, the 

 tentacles affected bend over toward this point and not toward the 

 center of the disc. Apparently the direction of movement of the stim- 

 ulus determined the appropriate direction of movement of the tentacle. 

 There is an apparent exception to be noted. Darwin found that 

 when a marginal tentacle is stimulated, it bends over, but no response 

 is called forth in the neighboring marginal tentacles. Only when the 

 marginal tentacle originally stimulated brings its glands with its stimu- 

 lating material into contact with the glands of the disc, is a stimulus 

 provided by the latter which now calls forth a response of the mar- 

 ginal tentacles hitherto not affected. 



The duration of the response depends on the nature of the stimu- 

 lus. Here I quote from Darwin (p. 19) "The central glands of a leaf 

 were irritated with a small camel's hair brush, and in 70 minutes 



