Francis E. Lloyd — 140 — Carnivorous Plants 



several of the outer tentacles were inflected; in 5 hours all the sub- 

 marginal tentacles were fully inflected; next morning after an interval of 



22 hours they were fully expanded I then put a dead fly in the 



center of (a) leaf, and next morning it was closely clasped; five days 

 after the leaf reexpanded and the tentacles, with their glands sur- 

 rounded by secretion, were ready to act again." 



A given stimulus acting somewhere on one side of the leaf will affect 

 the marginal tentacles on that side sooner than those of the other side 

 further away; or indeed, only one side of the leaf may be called into 

 action. In the case of a cup-shaped peltate leaf {D. gigantea) I have 

 observed that the total result of such movements is to bring the prey 

 into the depths of the cup, where, in the course of time, only the chitin- 

 ous remains of the captured insects are to be found. This result is 

 perhaps contributed to by the surface tension of the drop of secretion 

 which more or less fills the cup. 



It was thought by Nitschke that even the back of the leaf could 

 accept stimuli and transmit them to the tentacles, but Darwin was 

 unable to cause any response by stimulating the leaf blade proper, on 

 the front or the back. In order to locate the sensitive or sense per- 

 ceptive points, Darwin removed the gland from a tentacle, whereupon 

 the latter made a brief response by slightly bending but soon regained 

 its erstwhile posture. When stimulus was applied to the cut tentacle, 

 no response followed. But if now the disc tentacles were stimulated, 

 the amputated tentacle responded, as if the head were not missing. 

 The stalk of a tentacle, no more than the leaf or petiole, can receive a 

 stimulus. In any event, the marginal tentacles are not so sensitive as 

 the rest, nor are they affected by rain drops. Small (1939) denies 

 this. That the disc tentacles are more sensitive may appear to be the 

 case because the stalks of these are very short, and the tentacles are 

 closer together so that a given stimulus does not have to travel so far 

 to elicit response. And although the stimulus travels radially from a 

 point of stimulation, Darwin found that it travels more readily longi- 

 tudinally than transversely across the leaf blade. The stimulus may 

 travel quite across the blade so that when it is applied to the tentacles 

 on one margin, those of the opposite may respond; but in spite of 

 repetition of the stimulus, the opposite tentacles will open again, from 

 which Darwin argued that the "motor discharge must be more power- 

 ful at first then afterward." It was asked by Darwin whether the 

 motor impulse travels through the vascular tissue, but this turned out 

 not to be the case, certainly "not exclusively," for the tentacles of a 

 group surrounding the point of stimulus will respond all at a uniform 

 rate notwithstanding the fact that the vascular connections are very 

 unequal in length; indeed the course of the vascular tissues in the leaf 

 as a whole does not permit the view in question when the uniformity 

 of response of the tentacles is considered. 



The intensity of the stimulus necessary to procure response was a 

 matter of much concern to Darwin. He endeavored to get some 

 measure of intensity by weighing small pieces of hair, etc., which would 

 prove efficient. The following quotation embodies an expression of his 



reflections on this " it is an extraordinary fact that a little bit of 



soft thread 1/50 of an inch in length and weighing 1/8197 of a grain, 



