Insectivorous Plants

Cellulose. – I did not obtain this substance in a separate state, but tried angular bits of dry wood, cork, sphagnum moss, linen, and cotton thread. None of these bodies were in the least attacked by the secretion, and they caused only that moderate amount of inflection which is common to all inorganic objects. Gun-cotton, which consists of cellulose, with the hydrogen replaced by nitrogen, was tried with the same result. We have seen that a decoction of cabbage-leaves excites the most powerful inflection. I therefore placed two little square bits of the blade of a cabbage-leaf, and four little cubes cut from the midrib, on six leaves of Drosera. These became well inflected in 12 hrs., and remained so for between two and four days; the bits of cabbage being bathed all the time by acid secretion. This shows that some exciting matter, to which I shall presently refer, had been absorbed; but the angles of the squares and cubes remained as sharp as ever, proving that the framework of cellulose had not been attacked. Small square bits of spinach-leaves were tried with the same result; the glands pouring forth a moderate supply of acid secretion, and the tentacles remaining inflected for three days. We have also seen that the delicate coats of pollen grains are not dissolved by the secretion. It is well known that the gastric juice of animals does not attack cellulose.

Chlorophyll. – This substance was tried, as it contains nitrogen. Dr. Moore sent me some preserved in alcohol; it was dried, but soon deliquesced. Particles were placed on four leaves; after 3 hrs. the secretion was acid; after 8 hrs. there was a good deal of inflection, which in 24 hrs. became fairly well marked. After four days two of the leaves began to open, and the other two were then almost fully re-expanded. It is therefore clear that this chlorophyll contained matter which excited the leaves to a moderate degree; but judging by the eye, little or none was dissolved; so that in a pure state it would not probably have been attacked by the secretion. Dr. Sanderson tried that which I used, as well as some freshly prepared, with artificial digestive liquid, and found that it was not digested. Dr. Lauder Brunton likewise tried some prepared by the process given in the British Pharmacopoeia, and exposed it for five days at the temperature of 37 °Cent. to digestive liquid, but it was not diminished in bulk, though the fluid acquired a slightly brown colour. It was also tried with the glycerine extract of pancreas with a negative result. Nor does chlorophyll seem affected by the intestinal secretions of various animals, judging by the colour of their excrement.

It must not be supposed from these facts that the grains of chlorophyll, as they exist in living plants, cannot be attacked by the secretion; for these grains consist of protoplasm merely coloured by chlorophyll. My son Francis placed a thin slice of spinach leaf, moistened with saliva, on a leaf of Drosera, and other slices on damp cotton-wool, all exposed to the same temperature. After 19 hrs. the slice on the leaf of Drosera was bathed in much secretion from the inflected tentacles, and was now examined under the microscope. No perfect grains of chlorophyll could be distinguished; some were shrunken, of a yellowish-green colour, and collected in the middle of the cells; others were disintegrated and formed a yellowish mass, likewise in the middle of the cells. On the other hand, in the slices surrounded by damp cotton-wool, the grains of chlorophyll were green and as perfect as ever. My son also placed some slices in artificial gastric juice, and these were acted on in nearly the same manner as by the secretion. We have seen that bits of fresh cabbage and spinach leaves cause the tentacles to be inflected and the glands to pour forth much acid secretion; and there can be little doubt that it is the protoplasm forming the grains of chlorophyll, as well as that lining the walls of the cells, which excites the leaves.

Fat and Oil. – Cubes of almost pure uncooked fat, placed on several leaves, did not have their angles in the least rounded. We have also seen that the oil-globules in milk are not digested. Nor does olive oil dropped on the discs of leaves cause any inflection; but when they are immersed in olive oil, they become strongly inflected; but to this subject I shall have to recur. Oily substances are not digested by the gastric juice of animals.

Starch. – Rather large bits of dry starch caused well-marked inflection, and the leaves did not re-expand until the fourth day; but I have no doubt that this was due to the prolonged irritation of the glands, as the starch continued to absorb the secretion. The particles were not in the least reduced in size; and we know that leaves immersed in an emulsion of starch are not at all affected. I need hardly say that starch is not digested by the gastric juice of animals.

Action of the Secretion on Living Seeds.

The results of some experiments on living seeds, selected by hazard, may here be given, though they bear only indirectly on our present subject of digestion.

Seven cabbage seeds of the previous year were placed on the same number of leaves. Some of these leaves were moderately, but the greater number only slightly inflected, and most of them re-expanded on the third day. One, however, remained clasped till the fourth, and another till the fifth day. These leaves therefore were excited somewhat more by the seeds than by inorganic objects of the same size. After they re-expanded, the seeds were placed under favourable conditions on damp sand; other seeds of the same lot being tried at the same time in the same manner, and found to germinate well. Of the seven seeds which had been exposed to the secretion, only three germinated; and one of the three seedlings soon perished, the tip of its radicle being from the first decayed, and the edges of its cotyledons of a dark brown colour; so that altogether five out of the seven seeds ultimately perished.

Radish seeds (Raphanus sativus) of the previous year were placed on three leaves, which became moderately inflected, and re-expanded on the third or fourth day. Two of these seeds were transferred to damp sand; only one germinated, and that very slowly. This seedling had an extremely short, crooked, diseased, radicle, with no absorbent hairs; and the cotyledons were oddly mottled with purple, with the edges blackened and partly withered.

Cress seeds (Lepidum sativum) of the previous year were placed on four leaves; two of these next morning were moderately and two strongly inflected, and remained so for four, five, and even six days. Soon after these seeds were placed on the leaves and had become damp, they secreted in the usual manner a layer of tenacious mucus; and to ascertain whether it was the absorption of this substance by the glands which caused so much inflection, two seeds were put into water, and as much of the mucus as possible scraped off. They were then placed on leaves, which became very strongly inflected in the course of 3 hrs., and were still closely inflected on the third day; so that it evidently was not the mucus which excited so much inflection; on the contrary, this served to a certain extent as a protection to the seeds. Two of the six seeds germinated whilst still lying on the leaves, but the seedlings, when transferred to damp sand, soon died; of the other four seeds, only one germinated.

Two seeds of mustard (Sinapis nigra), two of celery (Apium graveolens) – both of the previous year, two seeds well soaked of caraway (Carum carui), and two of wheat, did not excite the leaves more than inorganic objects often do. Five seeds, hardly ripe, of a buttercup (Ranunculus), and two fresh seeds of Anemone nemorosa, induced only a little more effect. On the other hand, four seeds, perhaps not quite ripe, of Carex sylvatica caused the leaves on which they were placed to be very strongly inflected; and these only began to re-expand on the third day, one remaining inflected for seven days.

It follows from these few facts that different kinds of seeds excite the leaves in very different degrees; whether this is solely due to the nature of their coats is not clear. In the case of the cress seeds, the partial removal of the layer of mucus hastened the inflection of the tentacles. Whenever the leaves remain inflected during several days over seeds, it is clear that they absorb some matter from them. That the secretion penetrates their coats is also evident from the large proportion of cabbage, raddish, and cress seeds which were killed, and from several of the seedlings being greatly injured. This injury to the seeds and seedlings may, however, be due solely to the acid of the secretion, and not to any process of digestion; for Mr. Traherne Moggridge has shown that very weak acids of the acetic series are highly injurious to seeds. It never occurred to me to observe whether seeds are often blown on to the viscid leaves of plants growing in a state of nature; but this can hardly fail sometimes to occur, as we shall hereafter see in the case of Pinguicula. If so, Drosera will profit to a slight degree by absorbing matter from such seeds.]

Summary and Concluding Remarks on the Digestive Power of Drosera.

When the glands on the disc are excited either by the absorption of nitrogenous matter or by mechanical irritation, their secretion increases in quantity and becomes acid. They likewise transmit some influence to the glands of the exterior tentacles, causing them to secrete more copiously; and their secretion likewise becomes acid. With animals, according to Schiff,33 mechanical irritation excites the glands of the stomach to secrete an acid, but not pepsin. Now, I have every reason to believe (though the fact is not fully established), that although the glands of Drosera are continually secreting viscid fluid to replace that lost by evaporation, yet they do not secrete the ferment proper for digestion when mechanically irritated, but only after absorbing certain matter, probably of a nitrogenous nature. I infer that this is the case, as the secretion from a large number of leaves which had been irritated by particles of glass placed on their discs did not digest albumen; and more especially from the analogy of Dionaea and Nepenthes. In like manner, the glands of the stomach of animals secrete pepsin, as Schiff asserts, only after they have absorbed certain soluble substances, which he designates as peptogenes. There is, therefore, a remarkable parallelism between the glands of Drosera and those of the stomach in the secretion of their proper acid and ferment.

The secretion, as we have seen, completely dissolves albumen, muscle, fibrin, areolar tissue, cartilage, the fibrous basis of bone, gelatine, chondrin, casein in the state in which it exists in milk, and gluten which has been subjected to weak hydrochloric acid. Syntonin and legumin excite the leaves so powerfully and quickly that there can hardly be a doubt that both would be dissolved by the secretion. The secretion failed to digest fresh gluten, apparently from its injuring the glands, though some was absorbed. Raw meat, unless in very small bits, and large pieces of albumen, &c., likewise injure the leaves, which seem to suffer, like animals, from a surfeit. I know not whether the analogy is a real one, but it is worth notice that a decoction of cabbage leaves is far more exciting and probably nutritious to Drosera than an infusion made with tepid water; and boiled cabbages are far more nutritious, at least to man, than the uncooked leaves. The most striking of all the cases, though not really more remarkable than many others, is the digestion of so hard and tough a substance as cartilage. The dissolution of pure phosphate of lime, of bone, dentine, and especially enamel, seems wonderful; but it depends merely on the long-continued secretion of an acid; and this is secreted for a longer time under these circumstances than under any others. It was interesting to observe that as long as the acid was consumed in dissolving the phosphate of lime, no true digestion occurred; but that as soon as the bone was completely decalcified, the fibrous basis was attacked and liquefied with the greatest ease. The twelve substances above enumerated, which are completely dissolved by the secretion, are likewise dissolved by the gastric juice of the higher animals; and they are acted on in the same manner, as shown by the rounding of the angles of albumen, and more especially by the manner in which the transverse striae of the fibres of muscle disappear.

The secretion of Drosera and gastric juice were both able to dissolve some element or impurity out of the globulin and haematin employed by me. The secretion also dissolved something out of chemically prepared casein, which is said to consist of two substances; and although Schiff asserts that casein in this state is not attacked by gastric juice, he might easily have overlooked a minute quantity of some albuminous matter, which Drosera would detect and absorb. Again, fibro-cartilage, though not properly dissolved, is acted on in the same manner, both by the secretion of Drosera and gastric juice. But this substance, as well as the so-called haematin used by me, ought perhaps to have been classed with indigestible substances.

That gastric juice acts by means of its ferment, pepsin, solely in the presence of an acid, is well established; and we have excellent evidence that a ferment is present in the secretion of Drosera, which likewise acts only in the presence of an acid; for we have seen that when the secretion is neutralised by minute drops of the solution of an alkali, the digestion of albumen is completely stopped, and that on the addition of a minute dose of hydrochloric acid it immediately recommences.

The nine following substances, or classes of substances, namely, epidermic productions, fibro-elastic tissue, mucin, pepsin, urea, chitine, cellulose, gun-cotton, chlorophyll, starch, fat and oil, are not acted on by the secretion of Drosera; nor are they, as far as is known, by the gastric juice of animals. Some soluble matter, however, was extracted from the mucin, pepsin, and chlorophyll, used by me, both by the secretion and by artificial gastric juice.

The several substances, which are completely dissolved by the secretion, and which are afterwards absorbed by the glands, affect the leaves rather differently. They induce inflection at very different rates and in very different degrees; and the tentacles remain inflected for very different periods of time. Quick inflection depends partly on the quantity of the substance given, so that many glands are simultaneously affected, partly on the facility with which it is penetrated and liquefied by the secretion, partly on its nature, but chiefly on the presence of exciting matter already in solution. Thus saliva, or a weak solution of raw meat, acts much more quickly than even a strong solution of gelatine. So again leaves which have re-expanded, after absorbing drops of a solution of pure gelatine or isinglass (the latter being the more powerful of the two), if given bits of meat, are inflected much more energetically and quickly than they were before, notwithstanding that some rest is generally requisite between two acts of inflection. We probably see the influence of texture in gelatine and globulin when softened by having been soaked in water acting more quickly than when merely wetted. It may be partly due to changed texture, and partly to changed chemical nature, that albumen, which had been kept for some time, and gluten which had been subjected to weak hydrochloric acid, act more quickly than these substances in their fresh state.

The length of time during which the tentacles remain inflected largely depends on the quantity of the substance given, partly on the facility with which it is penetrated or acted on by the secretion, and partly on its essential nature. The tentacles always remain inflected much longer over large bits or large drops than over small bits or drops. Texture probably plays a part in determining the extraordinary length of time during which the tentacles remain inflected over the hard grains of chemically prepared casein. But the tentacles remain inflected for an equally long time over finely powdered, precipitated phosphate of lime; phosphorus in this latter case evidently being the attraction, and animal matter in the case of casein. The leaves remain long inflected over insects, but it is doubtful how far this is due to the protection afforded by their chitinous integuments; for animal matter is soon extracted from insects (probably by exosmose from their bodies into the dense surrounding secretion), as shown by the prompt inflection of the leaves. We see the influence of the nature of different substances in bits of meat, albumen, and fresh gluten acting very differently from equal-sized bits of gelatine, areolar tissue, and the fibrous basis of bone. The former cause not only far more prompt and energetic, but more prolonged, inflection than do the latter. Hence we are, I think, justified in believing that gelatine, areolar tissue, and the fibrous basis of bone, would be far less nutritious to Drosera than such substances as insects, meat, albumen, &c. This is an interesting conclusion, as it is known that gelatine affords but little nutriment to animals; and so, probably, would areolar tissue and the fibrous basis of bone. The chondrin which I used acted more powerfully than gelatine, but then I do not know that it was pure. It is a more remarkable fact that fibrin, which belongs to the great class of Proteids,34 including albumen in one of its sub-groups, does not excite the tentacles in a greater degree, or keep them inflected for a longer time, than does gelatine, or areolar tissue, or the fibrous basis of bone. It is not known how long an animal would survive if fed on fibrin alone, but Dr. Sanderson has no doubt longer than on gelatine, and it would be hardly rash to predict, judging from the effects on Drosera, that albumen would be found more nutritious than fibrin. Globulin likewise belongs to the Proteids, forming another sub-group, and this substance, though containing some matter which excited Drosera rather strongly, was hardly attacked by the secretion, and was very little or very slowly attacked by gastric juice. How far globulin would be nutritious to animals is not known. We thus see how differently the above specified several digestible substances act on Drosera; and we may infer, as highly probable, that they would in like manner be nutritious in very different degrees both to Drosera and to animals.

The glands of Drosera absorb matter from living seeds, which are injured or killed by the secretion. They likewise absorb matter from pollen, and from fresh leaves; and this is notoriously the case with the stomachs of vegetable-feeding animals. Drosera is properly an insectivorous plant; but as pollen cannot fail to be often blown on to the glands, as will occasionally the seeds and leaves of surrounding plants, Drosera is, to a certain extent, a vegetable-feeder.

Finally, the experiments recorded in this chapter show us that there is a remarkable accordance in the power of digestion between the gastric juice of animals with its pepsin and hydrochloric acid and the secretion of Drosera with its ferment and acid belonging to the acetic series. We can, therefore, hardly doubt that the ferment in both cases is closely similar, if not identically the same. That a plant and an animal should pour forth the same, or nearly the same, complex secretion, adapted for the same purpose of digestion, is a new and wonderful fact in physiology. But I shall have to recur to this subject in the fifteenth chapter, in my concluding remarks on the Droseraceae.

CHAPTER VII

THE EFFECTS OF SALTS OF AMMONIA

Manner of performing the experiments – Action of distilled water in comparison with the solutions – Carbonate of ammonia, absorbed by the roots – The vapour absorbed by the glands-Drops on the disc – Minute drops applied to separate glands – Leaves immersed in weak solutions – Minuteness of the doses which induce aggregation of the protoplasm – Nitrate of ammonia, analogous experiments with – Phosphate of ammonia, analogous experiments with-Other salts of ammonia – Summary and concluding remarks on the action of salts of ammonia.

THE chief object in this chapter is to show how powerfully the salts of ammonia act on the leaves of Drosera, and more especially to show what an extraordinarily small quantity suffices to excite inflection. I shall, therefore, be compelled to enter into full details. Doubly distilled water was always used; and for the more delicate experiments, water which had been prepared with the utmost possible care was given me by Professor Frankland. The graduated measures were tested, and found as accurate as such measures can be. The salts were carefully weighed, and in all the more delicate experiments, by Borda's double method. But extreme accuracy would have been superfluous, as the leaves differ greatly in irritability, according to age, condition, and constitution. Even the tentacles on the same leaf differ in irritability to a marked degree. My experiments were tried in the following several ways.

[Firstly. – Drops which were ascertained by repeated trials to be on an average about half a minim, or the 1/960 of a fluid ounce (.0296 ml.), were placed by the same pointed instrument on the discs of the leaves, and the inflection of the exterior rows of tentacles observed at successive intervals of time. It was first ascertained, from between thirty and forty trials, that distilled water dropped in this manner produces no effect, except that sometimes, though rarely, two or three tentacles become inflected. In fact all the many trials with solutions which were so weak as to produce no effect lead to the same result that water is inefficient.

Secondly. – The head of a small pin, fixed into a handle, was dipped into the solution under trial. The small drop which adhered to it, and which was much too small to fall off, was cautiously placed, by the aid of a lens, in contact with the secretion surrounding the glands of one, two, three, or four of the exterior tentacles of the same leaf. Great care was taken that the glands themselves should not be touched. I had supposed that the drops were of nearly the same size; but on trial this proved a great mistake. I first measured some water, and removed 300 drops, touching the pin's head each time on blotting-paper; and on again measuring the water, a drop was found to equal on an average about the 1/60 of a minim. Some water in a small vessel was weighed (and this is a more accurate method), and 300 drops removed as before; and on again weighing the water, a drop was found to equal on an average only the 1/89 of a minim. I repeated the operation, but endeavoured this time, by taking the pin's head out of the water obliquely and rather quickly, to remove as large drops as possible; and the result showed that I had succeeded, for each drop on an average equalled 1/19.4 of a minim. I repeated the operation in exactly the same manner, and now the drops averaged 1/23.5 of a minim. Bearing in mind that on these two latter occasions special pains were taken to remove as large drops as possible, we may safely conclude that the drops used in my experiments were at least equal to the 1/20 of a minim, or .0029 ml. One of these drops could be applied to three or even four glands, and if the tentacles became inflected, some of the solution must have been absorbed by all; for drops of pure water, applied in the same manner, never produced any effect. I was able to hold the drop in steady contact with the secretion only for ten to fifteen seconds; and this was not time enough for the diffusion of all the salt in solution, as was evident, from three or four tentacles treated successively with the same drop, often becoming inflected. All the matter in solution was even then probably not exhausted.

Thirdly. – Leaves cut off and immersed in a measured quantity of the solution under trial; the same number of leaves being immersed at the same time, in the same quantity of the distilled water which had been used in making the solution. The leaves in the two lots were compared at short intervals of time, up to 24 hrs., and sometimes to 48 hrs. They were immersed by being laid as gently as possible in numbered watch-glasses, and thirty minims (1.775 ml.) of the solution or of water was poured over each.

Some solutions, for instance that of carbonate of ammonia, quickly discolour the glands; and as all on the same leaf were discoloured simultaneously, they must all have absorbed some of the salt within the same short period of time. This was likewise shown by the simultaneous inflection of the several exterior rows of tentacles. If we had no such evidence as this, it might have been supposed that only the glands of the exterior and inflected tentacles had absorbed the salt; or that only those on the disc had absorbed it, and had then transmitted a motor impulse to the exterior tentacles; but in this latter case the exterior tentacles would not have become inflected until some time had elapsed, instead of within half an hour, or even within a few minutes, as usually occurred. All the glands on the same leaf are of nearly the same size, as may best be seen by cutting off a narrow transverse strip, and laying it on its side; hence their absorbing surfaces are nearly equal. The long-headed glands on the extreme margin must be excepted, as they are much longer than the others; but only the upper surface is capable of absorption. Besides the glands, both surfaces of the leaves and the pedicels of the tentacles bear numerous minute papillae, which absorb carbonate of ammonia, an infusion of raw meat, metallic salts, and probably many other substances, but the absorption of matter by these papillae never induces inflection. We must remember that the movement of each separate tentacle depends on its gland being excited, except when a motor impulse is transmitted from the glands of the disc, and then the movement, as just stated, does not take place until some little time has elapsed. I have made these remarks because they show us that when a leaf is immersed in a solution, and the tentacles are inflected, we can judge with some accuracy how much of the salt each gland has absorbed. For instance, if a leaf bearing 212 glands be immersed in a measured quantity of a solution, containing 1/10 of a grain of a salt, and all the exterior tentacles, except twelve, are inflected, we may feel sure that each of the 200 glands can on an average have absorbed at most 1/2000 of a grain of the salt. I say at most, for the papillae will have absorbed some small amount, and so will perhaps the glands of the twelve excluded tentacles which did not become inflected. The application of this principle leads to remarkable conclusions with respect to the minuteness of the doses causing inflection.