Chapter X — 143 — Drosera 



sarily limited to one side, but the difference of rate is obvious and 

 produces the same result. Since the movement of the tentacle is a 

 matter of growth, and since there is a limit of total growth, the num- 

 ber of times bending may be repeated is limited. Darwin found the 

 number is three, and this was confirmed by Hooker. Though the two 

 movements constitute practically a continuous reaction, at least when 

 a single brief stimulus is originally applied, the unbending reaction fol- 

 lows a stimulus inherent in the internal conditions (such as tissue ten- 

 sions) set up during bending, and is tropic (autotropic, autonomous, 

 Behre) in nature. Since the entire armament of tentacles may not be 

 used in any one grasping of prey, the leaf as a whole may react more 

 than three times, even though a single tentacle cannot. The short 

 radially structured tentacles of the disc do not react by bending to 

 stimuli applied directly to the glands, but only to stimuli received 

 through the glands of other tentacles. Hooker regards the response as 

 tropic while the original response of the lateral tentacles is evidently 

 nastic, though the unbending response is tropic. Both Darwin and 

 NiTSCHKE recorded their behef that marginal tentacles when stimu- 

 lated indirectly bend toward the point of stimulation. Hooker takes 

 exception to this, saying that he was unable to get evidence of it, 

 and thinks that they normally bend toward the middle of the disc, that 

 is, nastically. 



Exceptions to this he thought "to be purely accidental." None- 

 theless, Hooker was sufficiently impressed with his observations 

 to state that "most of the marginal tentacles which reacted to the 

 conducted impulse" from the discal tentacles "in bending toward the 

 center of the leaf bent hkewise in the direction of the source of excite- 

 ment. " The bending of the discal tentacles is, however, always toward 

 the point of original stimulation, and cannot be stimulated directly. 

 The response is tropic, but "in all probability" the movements "are 

 likewise the result of differential growth on opposite sides" of the 

 tentacle base. The method used was not applicable to the determina- 

 tion of this fact. 



Behre (1929) admitted that Hooker's conclusions were nearly 

 right, but was evidently impressed by the discrepancies admitted 

 by him. He, therefore, attacked the problem at this point, and ana- 

 lysed the movement of the tentacles more rigorously, controlling his ob- 

 servation by means of a horizontal measuring microscope with a scale 

 in the field. He recorded accurately the movements of tentacles rela- 

 tive to each other and to the position of the source of stimulation, and 

 made them available to the reader by means of maps showing the 

 paths of movements. 



In the case of D. rotundifolia he found that, according to their be- 

 havior the tentacles can be divided into three groups, namely, {a) mar- 

 ginal, the outermost standing exactly on, or very near to the leaf mar- 

 gin; {h) an outer zone of discal tentacles of one to three rows, called by 

 him the "surface outer tentacles"; and (c) the discal tentacles within 

 (&), or "central tentacles". With some sHght differences due to the 

 posture of the tentacles, the same holds for other species investigated 

 {D. binata, intermedia, capensis, spathulata). His observations yielded 

 the following results, and here it may be injected that he used in most 



