On four occasions leaves were prepared by removing some days previously all the glands from the centre, so that these could not be excited by the bits of meat brought to them by the inflection of the marginal tentacles; and now these marginal tentacles re-expanded after a time without any other tentacle being affected. Other leaves were similarly prepared, and bits of meat were placed on the glands of two tentacles in the third row from the outside, and on the glands of two tentacles in the fifth row. In these four cases the impulse was sent in the first place laterally, that is, in the same concentric row of tentacles, and then towards the centre; but not centrifugally, or towards the exterior tentacles. In one of these cases only a single tentacle on each side of the one with meat was affected. In the three other cases, from half a dozen to a dozen tentacles, both laterally and towards the centre, were well inflected or sub-inflected. Lastly, in [page 236] ten other experiments, minute bits of meat were placed on a single gland or on two glands in the centre of the disc. In order that no other glands should touch the meat, through the inflection of the closely adjoining short tentacles, about half a dozen glands had been previously removed round the selected ones. On eight of these leaves from sixteen to twenty-five of the short surrounding tentacles were inflected in the course of one or two days; so that the motor impulse radiating from one or two of the discal glands is able to produce this much effect. The tentacles which had been removed are included in the above numbers; for, from standing so close, they would certainly have been affected. On the two remaining leaves, almost all the short tentacles on the disc were inflected. With a more powerful stimulus than meat, namely a little phosphate of lime moistened with saliva, I have seen the inflection spread still farther from a single gland thus treated; but even in this case the three or four outer rows of tentacles were not affected. From these experiments it appears that the impulse from a single gland on the disc acts on a greater number of tentacles than that from a gland of one of the exterior elongated tentacles; and this probably follows, at least in part, from the impulse having to travel a very short distance down the pedicels of the central tentacles, so that it is able to spread to a considerable distance all round.
Whilst examining these leaves, I was struck with the fact that in six, perhaps seven, of them the tentacles were much more inflected at the distal and proximal ends of the leaf (i.e. towards the apex and base) than on either side; and yet the tentacles on the sides stood as near to the gland where the bit of meat lay as did those at the two ends. It thus appeared as [page 237] if the motor impulse was transmitted from the centre across the disc more readily in a longitudinal than in a transverse direction; and as this appeared a new and interesting fact in the physiology of plants, thirty-five fresh experiments were made to test its truth. Minute bits of meat were placed on a single gland or on a few glands, on the right or left side of the discs of eighteen leaves; other bits of the same size being placed on the distal or proximal ends of seventeen other leaves. Now if the motor impulse were transmitted with equal force or at an equal rate through the blade in all directions, a bit of meat placed at one side or at one end of the disc ought to affect equally all the tentacles situated at an equal distance from it; but this certainly is not the case. Before giving the general results, it may be well to describe three or four rather unusual cases.
[(1) A minute fragment of a fly was placed on one side of the disc, and after 32 m. seven of the outer tentacles near the fragment were inflected; after 10 hrs. several more became so, and after 23 hrs. a still greater number; and now the blade of the leaf on this side was bent inwards so as to stand up at right angles to the other side. Neither the blade of the leaf nor a single tentacle on the opposite side was affected; the line of separation between the two halves extending from the footstalk to the apex.