HAPTOTROPISM 497 



first proceeds to carry its booty towards the middle of the leaf, but as soon as it 

 reaches the centre a stimulus is transmitted to the other peripheral tentacles 

 which then begin to incurve also. We have thus to discriminate between move- 

 ment induced by a direct stimulus from that resulting from a transmitted 

 stimulus, and we are able to establish the fact that that transmission can be 

 effected only by means of the central tentacles. All the disk tentacles, not 

 merely those which occupy the exact centre, are able to radiate impulses. If the 

 leaf be stimulated half-way between the centre and the margin at two opposite 

 points, half the tentacles bend towards one centre of stimulation, half towards 

 the other, so that, as Darwin says, two wheel-like arrangements are formed, one 

 on either side, whose spokes are formed by the tentacles and whose hubs are the 

 points towards which all the glands concentrate. Let us now study the effect 

 of a direct stimulus on a single tentacle, so as to obtain some idea of the nature 

 of the sensitivity, the perceptive region, and the method of curvature. 



Drosera reacts both to mechanical and chemical stimuli. The actively 

 mechanical stimuli are contact stimuli as in tendrils. Darwin (1876 b) showed 

 that neutral liquids, such as water, produce no effect, although they are driven 

 against the tentacles with considerable force, and Pfeffer (1885) also proved that 

 a rod covered with gelatine (as in the case of tendrils) was ineffective. On the 

 other hand, solid, insoluble bodies, even though extremely light, induce movement, 

 provided only they were capable of penetrating 

 the secretion. The tentacles may be stimulated 

 also by a blow from a pencil or a splinter of wood, 

 although to induce a result several successive con- 

 tacts are necessary. In all these respects Drosera 

 agrees entirely in its behaviour with tendrils. 

 We owe to Darwin the proof of the fact that the 

 sensitivity is localized in the gland exclusively, 

 and Haberlandt (1901) showed that histological 

 structures similar to those which occur in tendrils, 

 i.e. sensitive pits, occur also in the epidermis of the ^.^ ^^^ p^^, ^^ ^ transverse section 

 tentacle. Curvature is entirely confined to the through a leaf of Drosera, showing 



_j™7t. X J.1- J. J. 1 J '11 j.„ li-r, three tentacles, one of which has under- 



staik of the tentacle and more especially to its go^g curvatu/e as the result of a 

 base, which bends sharply while the upper part re- stimulation. After darwin (1876 b). 

 mains quite straight. This is most marked in the 



long-stalked peripheral tentacles (Fig. 157). As to the mechanics of the curva- 

 ture we as yet know nothing. Since, however, Batalin (1877) has shown 

 that growth acceleration takes place during the curving, it is very probable that 

 not only the nature of the sensitivity but also the mode in which the movement 

 is carried out corresponds to that exhibited by tendrils. 



The tentacles respond very rapidly to a mechanical stimulus. After 

 10 seconds Darwin was able, with the aid of a lens, to observe the commence- 

 ment of curvature ; curvature visible to the naked eye was often apparent in less 

 than a minute. As bending proceeds the end of the tentacle in a very short time 

 describes a considerable curve in space, the peripheral tentacles being able not 

 infrequently to curve through an angle of 270'' in the course of an hour. Some 

 time after the completion of the incurving a reverse curvature takes place and 

 a straightening of the tentacle, even if contact with the body acting as a stimulant 

 is continued. This movement certainly takes place much more slowly than in 

 the case of the tendril— according to Darwin, in about 24 hours— yet the factors 

 concerned and the mechanics of the movement would appear to be identical 

 with the analogous processes in tendrils. Immediately after this straightening 

 — possibly even sooner — the tentacle is ready to receive a new stimulus. 



It has already been pointed out that Drosera is also capable of being stimu- 

 lated by chemical agents. Chemical stimuli as a rule act much more vigorously 

 than mechanical, as is shown by the rapidity of the movement and the duration 



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