Insectivorous Plants

by Charles Darwin

* ‘Bot. Zeitung,’ 1860, p. 234. [page 231]

that cutting and pricking the leaf does not excite movement. The petiole of the leaf is quite insensible.

The backs of the leaves bear numerous minute papillae, which do not secrete, but have the power of absorption. These papillae are, I believe, rudiments of formerly existing tentacles together with their glands. Many experiments were made to ascertain whether the backs of the leaves could be irritated in any way, thirty-seven leaves being thus tried. Some were rubbed for a long time with a blunt needle, and drops of milk and other exciting fluids, raw meat, crushed flies, and various substances, placed on others. These substances were apt soon to become dry, showing that no secretion had been excited. Hence I moistened them with saliva, solutions of ammonia, weak hydrochloric acid, and frequently with the secretion from the glands of other leaves. I also kept some leaves, on the backs of which exciting objects had been placed, under a damp bell-glass; but with all my care I never saw any true movement. I was led to make so many trials because, contrary to my previous experience, Nitschke states* that, after affixing objects to the backs of leaves by the aid of the viscid secretion, he repeatedly saw the tentacles (and in one instance the blade) become reflexed. This movement, if a true one, would be most anomalous; for it implies that the tentacles receive a motor impulse from an unnatural source, and have the power of bending in a direction exactly the reverse of that which is habitual to them; this power not being of the least use to the plant, as insects cannot adhere to the smooth backs of the leaves.

I have said that no effect was produced in the above

* ‘Bot. Zeitung.’ 1860, p. 437. [page 232]

cases; but this is not strictly true, for in three instances a little syrup was added to the bits of raw meat on the backs of leaves, in order to keep them damp for a time; and after 36 hrs. there was a trace of reflexion in the tentacles of one leaf, and certainly in the blade of another. After twelve additional hours, the glands began to dry, and all three leaves seemed much injured. Four leaves were then placed under a bell-glass, with their footstalks in water, with drops of syrup on their backs, but without any meat. Two of these leaves, after a day, had a few tentacles reflexed. The drops had now increased considerably in size, from having imbibed moisture, so as to trickle down the backs of the tentacles and footstalks. On the second day, one leaf had its blade much reflexed; on the third day the tentacles of two were much reflexed, as well as the blades of all four to a greater or less degree. The upper side of one leaf, instead of being, as at first, slightly concave, now presented a strong convexity upwards. Even on the fifth day the leaves did not appear dead. Now, as sugar does not in the least excite Drosera, we may safely attribute the reflexion of the blades and tentacles of the above leaves to exosmose from the cells which were in contact with the syrup, and their consequent contraction. When drops of syrup are placed on the leaves of plants with their roots still in damp earth, no inflection ensues, for the roots, no doubt, pump up water as quickly as it is lost by exosmose. But if cut-off leaves are immersed in syrup, or in any dense fluid, the tentacles are greatly, though irregularly, inflected, some of them assuming the shape of corkscrews; and the leaves soon become flaccid. If they are now immersed in a fluid of low specific gravity, the tentacles re-expand. From these [page 233] facts we may conclude that drops of syrup placed on the backs of leaves do not act by exciting a motor impulse which is transmitted to the tentacles; but that they cause reflexion by inducing exosmose. Dr. Nitschke used the secretion for sticking insects to the backs of the leaves; and I suppose that he used a large quantity, which from being dense probably caused exosmose. Perhaps he experimented on cut-off leaves, or on plants with their roots not supplied with enough water.

As far, therefore, as our present knowledge serves, we may conclude that the glands, together with the immediately underlying cells of the tentacles, are the exclusive seats of that irritability or sensitiveness with which the leaves are endowed. The degree to which a gland is excited can be measured only by the number of the surrounding tentacles which are inflected, and by the amount and rate of their movement. Equally vigorous leaves, exposed to the same temperature (and this is an important condition), are excited in different degrees under the following circumstances. A minute quantity of a weak solution produces no effect; add more, or give a rather stronger solution, and the tentacles bend. Touch a gland once or twice, and no movement follows; touch it three or four times, and the tentacle becomes inflected. But the nature of the substance which is given is a very important element: if equal-sized particles of glass (which acts only mechanically), of gelatine, and raw meat, are placed on the discs of several leaves, the meat causes far more rapid, energetic, and widely extended movement than the two former substances. The number of glands which are excited also makes a great difference in the result: place a bit of meat on one or two of the discal [page 234] glands, and only a few of the immediately surrounding short tentacles are inflected; place it on several glands, and many more are acted on; place it on thirty or forty, and all the tentacles, including the extreme marginal ones, become closely inflected. We thus see that the impulses proceeding from a number of glands strengthen one another, spread farther, and act on a larger number of tentacles, than the impulse from any single gland.

Transmission of the Motor Impulse.—In every case the impulse from a gland has to travel for at least a short distance to the basal part of the tentacle, the upper part and the gland itself being merely carried by the inflection of the lower part. The impulse is thus always transmitted down nearly the whole length of the pedicel. When the central glands are stimulated, and the extreme marginal tentacles become inflected, the impulse is transmitted across half the diameter of the disc; and when the glands on one side of the disc are stimulated, the impulse is transmitted across nearly the whole width of the disc. A gland transmits its motor impulse far more easily and quickly down its own tentacle to the bending place than across the disc to neighbouring tentacles. Thus a minute dose of a very weak solution of ammonia, if given to one of the glands of the exterior tentacles, causes it to bend and reach the centre; whereas a large drop of the same solution, given to a score of glands on the disc, will not cause through their combined influence the least inflection of the exterior tentacles. Again, when a bit of meat is placed on the gland of an exterior tentacle, I have seen movement in ten seconds, and repeatedly within a minute; but a much larger bit placed on several glands on the disc does not cause [page 235] the exterior tentacles to bend until half an hour or even several hours have elapsed.

The motor impulse spreads gradually on all sides from one or more excited glands, so that the tentacles which stand nearest are always first affected. Hence, when the glands in the centre of the disc are excited, the extreme marginal tentacles are the last inflected. But the glands on different parts of the leaf transmit their motor power in a somewhat different manner. If a bit of meat be placed on the long-headed gland of a marginal tentacle, it quickly transmits an impulse to its own bending portion; but never, as far as I have observed, to the adjoining tentacles; for these are not affected until the meat has been carried to the central glands, which then radiate forth their conjoint impulse on all sides. 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.