Insectivorous Plants

within their cells is ordinarily in a state of expansion, but is paralysed by irritation, or may be said to suffer temporary death ; the elasticity of the cell-walls then coming into play, and causing the contraction of the stamens. Now the cells on the upper or concave side of the bending part of the tentacles of Drosera do not appear to be in a state of tension, nor to be highly elastic; for when a leaf is suddenly killed, or dies slowly, it is not the upper but the lower sides of the tentacles which contract from elasticity. We may, therefore, conclude that their movements cannot be accounted for by the inherent elasticity of certain cells, opposed as long as they are alive and not irri- tated by the expanded state of their contents.

A somewhat different view has been advanced by other physiologists — namely that the protoplasm, when irritated, contracts like the soft sarcode of the muscles of animals. In Drosera the fluid within the cells of the tentacles at the bending place appears under the microscope thin and homogeneous, and after aggregation consists of small, soft masses of matter, undergoing incessant changes of form and floating in almost colourless fluid. These masses are completely redissolved when the tentacles re-expand. Now it seems scarcely possible that such matter should have any direct mechanical power; but if through some molecular change it were to occupy less space than it did before, no doubt the cell- walls would close up and contract. But in this case it might be expected that the walls would exhibit wrinkles, and none could ever be seen. Moreover, the contents of all the cells seem to be of exactly the same nature, both before and after aggregation; and yet only a few of the basal cell* contract, the rest of the tentacle remaining straight.

though rejected by most others, is that the whole cell, including the walls, actively contracts. If the walls are composed solely of non-nitrogenous cellulose, this view is highly improbable ; but it can hardly be doubted that they must be permeated by proteid matter, at least whilst they are growing. Nor does there seem any inherent improbability in the cell-walls of Drosera contracting, considering their high state of organisa- tion ; as shown in the case of the glands by their power of absorption and secretion, and by being exquisitely sensitive so as to be affected by the pressure of the most minute particles. The cell-walls of the pedicels also allow various impulses to pass through them, inducing movement, increased secretion and aggrega- tion. On the whole the belief that the walls of certain cells contract, some of their contained fluid being at the same time forced outwards, perhaps accords best with the observed facts. If this view is rejected, the next most probable one is that the fluid contents of the cells shrink, owing to a change in their molecular state, with the consequent closing in of the walls. Anyhow, the movement can hardly be attributed to the elasticity of the walls, together with a previous state of tension.

With respect to the nature of the motor impulse which is transmitted from the glands down the pedi- cels and across the disc, it seems not improbable that it is closely allied to that influence which causes the protoplasm within the cells of the glands and ten- tacles to aggregate. We have seen that both forces originate in and proceed from the glands within a few seconds of the same time, and are excited by the same causes. The aggregation of the protoplasm last? almost as long as the tentacles remain inflected, even though this be for more than a week ; but the

protoplasm is redissolved at the bending place shortly before the tentacles re-expand, showing- that the ex- citing cause of the aggregating process has then quite ceased. Exposure to carbonic acid causes both the latter process and the motor impulse to travel very slowly down the tentacles. We know that the aggre- gating process is delayed in passing through the cell- walls, and we have good reason to believe that this holds good with the motor impulse ; for we can thus understand the different rates of its transmission in a longitudinal and transverse line across the disc. Under a high power the first sign of aggregation is the ap- pearance of a cloud, and soon afterwards of extremely line granules, in the homogeneous purple fluid within the cells ; and this apparently is due to the union of molecules of protoplasm. Now it does not seem an improbable view that the same tendency — namely for the molecules to approach each other — should be com- municated to the inner surfaces of the cell-walls which are in contact with the protoplasm ; and if so, their molecules would approach each other, and the cell-wall would contract.

To this view it may with truth be objected that when leaves are immersed in various strong solu- tions, or are subjected to a heat of above 130° Fahr. (54°*4 Cent.), aggregation ensues, but there is no movement. Again, various acids and some other fluids cause rapid movement, but no aggregation, or only of an abnormal nature, or only after a long- interval of time ; but as most of these fluids are more or less injurious, they may check or prevent the aggre- gating process by injuring or killing the protoplasm. There is another and more important difference in the two processes : when the glands on the disc are ex- cited, they transmit some influence up the surrounding

tentacles, which acts on the cells at the bending place, but does not induce aggregation until it has reached the glands ; these then send back some other in- fluence, causing the protoplasm to aggregate, first in the upper and then in the lower cells. The He-expansion of the Tentacles. — This movement is always slow and gradual. When the centre of the leaf is excited, or a leaf is immersed in a proper solu- tion, all the tentacles bend directly towards the centre, and afterwards directly back from it. But when the point of excitement is on one side of the disc, the surrounding tentacles bend towards it, and therefore obliquely with respect to their normal direction ; when they afterwards re-expand, they bend obliquely back, so as to recover their original positions. The ten- tacles farthest from an excited point, wherever that may be, are the last and the least affected, and probably in consequence of this they are the first to re-expand. The bent portion of a closely inflected tentacle is in a state of active contraction, as shown by the following experiment. Meat was placed on a leaf, and after the tentacles were closely inflected and had quite ceased to move, narrow strips of the disc, with a few of the outer tentacles attached to it, were cut off and laid on one side under the microscope. After several failures, I succeeded in cutting off the convex surface of the bent portion of a tentacle. Movement immediately recom- menced, and the already greatly bent portion went on bending until it formed a perfect circle ; the straight distal portion of the tentacle passing on one side of the strip. The convex surface must therefore have pre- viously been in a state of tension, sufficient to counter- balance that of the concave surface, which, when free, curled into a complete ring.

are moderately rigid and elastic ; if bent by a needle, the upper end yields more easily than the basal and thicker part, which alone is capable of becoming in- flected. The rigidity of this basal part seems due to the tension of the outer surface balancing a state of active and persistent contraction of the cells of the inner surface. I believe that this is the case, because, when a leaf is dipped into boiling water, the tentacles suddenly become reflexed, and this apparently indi- cates that the tension of the outer surface is mecha- nical, whilst that of the inner surface is vital, and is instantly destroyed by the boiling water. We can thus also understand why the tentacles as they grow old and feeble slowly become much reflexed. If a leaf with its tentacles closely inflected is dipped into boiling water, these rise up a little, but by no means fully re-expand. This may be owing to the heat quickly destroying the tension and elasticity of the cells of the convex surface ; but I can hardly believe that their tension, at any one time, would suffice to carry back the tentacles to their original position, often through an angle of above 180°. It is more probable that fluid, which we know travels along the tentacles during the act of inflection, is slowly re- attracted into the cells of the convex surface, their tension being thus gradually and continually in- creased. .

A recapitulation of the chief facts and discussions in this chapter will be given at the close of the next chapter. As summaries have been given to most of the chapters, it will be sufficient here to recapitulate, as briefly as I can, the chief points. In the first chapter a preliminary sketch was given of the structure of the leaves, and of the manner in which they capture insects. This is effected by drops of extremely viscid fluid surrounding the glands and by the inward movement of the tentacles. As the plants gain most of their nutriment by this means, their roots are very poorly developed ; and they often grow in places where hardly any other plant except mosses can exist. The glands have the power of absorption, besides that of secretion. They are extremely sen- sitive to various stimulants, namely repeated touches, the pressure of minute particles, the absorption of animal matter and of various fluids, heat, and gal- vanic action. A tentacle with a bit of raw meat on the gland has been seen to begin bending in 10 s., to be strongly incurved in 5 in., and to reach the centre of the leaf in Lalf an hour. The blade of the leaf often becomes so much inflected that it forms a cup, enclosing any object placed on it.

A gland, when excited, not only sends some in- fluence down its own tentacle, causing it to bend, but likewise to the surrounding tentacles, which become incurved ; so that the bending place can be acted on by uu impulse received from opposite directions, namely from the gland on the summit of the same tentacle, arid from one or more glands of the neigh- bouring tentacles. Tentacles, when inflected, re-ex- pand after a time, and during this process the glands secrete less copiously, or become dry. As soon as they begin to secrete again, the tentacles are ready to re-act ; and this may be repeated at least three, probably many more times.

It was shown in the second chapter that animal sub- stances placed on the discs cause much more prompt and energetic inflection than do inorganic bodies of the same size, or mere mechanical irritation; but there is a still more marked difference in the greater length of time during which the tentacles remain in- flected over bodies yielding soluble and nutritious matter, than over those which do not yield such matter. Extremely minute particles of glass, cinders, hair, thread, precipitated chalk, &c., when placed on the glands of the outer tentacles, cause them to bend. A particle, unless it sinks through the secretion and actually touches the surface of the gland with some one point, does not produce any effect. A little bit of thin human hair Vo800 of an inch (-203 mm.) in length, and weighing only TTJ-T-o of a grain (-OOU822 ing.), though largely supported by the dense secre- tion, suffices to induce movement. It is not probable that the pressure in this case could have amounted to that from the millionth of a grain. Even smaller particles cause a slight movement, as could be seen through a lens. Larger particles than those of which the measurements have been given cause no sensation when placed on the tongue, one of the most sensitive parts of the human body.

Movement ensues if a gland is momentarily touched three or four times ; but if touched only once or twice, though with considerable force and with a hard object, the tentacle does not bend. The plant is thus saved from much useless movement, as during a high wind the glands can hardly escape being occasionally brushed by the leaves of surrounding plants. Though insensible to a single touch, they are exquisitely sensi- tive, as just stated, to the slightest pressure if pro- longed for a few seconds ; and this capacity is mani- festly of service to the plant in capturing small insects. Even gnats, if they rest on the glands with their delicate feet, are quickly and securely embraced. The glands are insensible to the weight and repeated blows of drops of heavy rain, and the plants are thus likewise saved from much useless movement.

The description of the movements of the tentacles was interrupted in the third chapter for the sake oi describing the process of aggregation. This process always commences in the cells of the glands, the con- tents of which first become cloudy ; and this has been observed within 10 s. after a gland has been excited. Granules just resolvable under a very high power soon appear, sometimes within a minute, in the cells beneath the glands ; and these then aggregate into minute spheres. The process afterwards travels down the tentacles, being arrested for a short time at each transverse partition. The small spheres coalesce into larger spheres, or into oval, club-headed, thread- or necklace-like, or otherwise shaped masses of proto- plasm, which, suspended in almost colourless fluid, exhibit incessant spontaneous changes of form. These frequently coalesce and again separate. If a gland has been powerfully excited, all the cells down to the base of the tentacle are affected. In cells, especially if filled with dark red fluid, the first step in the

process often is the formation of a dark red, bag- like mass of protoplasm, which afterwards divides and undergoes the usual repeated changes of form. Before any aggregation has been excited, a sheet of colourless protoplasm, including granules (the prim- ordial utricle of Mohl), flows round the walls of the cells; and this becomes more distinct after the con- tents have been partially aggregated into spheres or bag-like masses. But after a time the granules are drawn towards the central masses and unite with them; and then the circulating sheet can no longer be distinguished, but there is still a current of trans- parent fluid within the cells.

Aggregation is excited by almost all the stimulants which induce movement; such as the glands being touched two or three times, the pressure of minute inorganic particles, the absorption of various fluids, even long immersion in distilled water, exosniose, and heat. Of the many stimulants tried, carbonate of ammonia is the most energetic and acts the quickest : a dose of -nrA-o-o of a grain (-00048 mg.) given to a single gland suffices to cause in one hour well- marked aggregation in the upper cells of the tentacle. The process goes on only as long as the protoplasm is in a living, vigorous, and oxygenated condition.

The result is in all respects exactly the same, whether a gland has been excited directly, or has received an influence from other and distant glands. But there is one important difference : when the central glands are irritated, they transmit centri- fugally an influence up the pedicels of the exterior tentacles to their glands; but the actual process of aggregation travels centripetally, from the glands of the exterior tentacles down their pedicels. The ex- citing influence, therefore, which is transmitted from

one part of the leaf to another must be different from that which actually induces aggregation. The process does not depend on the glands secreting more copiously than they did before ; and is inde- pendent of the inflection of the tentacles. It con- tinues as long as the tentacles remain inflected, and as soon as these are fully re-expanded, the little masses of protoplasm are all redissolved ; the cells becoming filled with homogeneous purple fluid, as they were before the leaf was excited.

As the process of aggregation can be excited by a few touches, or by the pressure of insoluble particles, it is evidently independent of the absorption of any matter, and must be of a molecular nature. Even when caused by the absorption of the carbonate or other salt of ammonia, or an infusion of meat, the process seems to be of exactly the same nature. The proto- plasmic fluid must, therefore, be in a singularly un- stable condition, to be acted on by such slight and varied causes. Physiologists believe that when a nerve is touched, and it transmits an influence to other parts of the nervous system, a molecular change is induced in it, though not visible to us. Therefore it is a very interesting spectacle to watch the effects on the cells of a gland, of the pressure of a bit of hair, weighing only y-g-'-jn, °f a grain and largely supported by the dense secretion, for this excessively slight pressure soon causes a visible change in the proto- plasm, which change is transmitted down the whole length of the tentacle, giving it at last a mottled appearance, distinguishable even by the naked eye.

In the fourth chapter it was shown that leaves placed for a short time in water at a temperature of 110° Fahr. (430>3 Cent.) become somewhat inflected ; they are thus also rendered more sensitive to the action of meat than they were before. If exposed to a tem- perature of between 115° and 125° (46°-l— 51°-6 Cent.), they are quickly inflected, and their protoplasm under- goes aggregation ; when afterwards placed in cold water, they re-expand. Exposed to 130° (54°'4 Cent.), no in- flection immediately occurs, but the leaves are only temporarily paralysed, for on being left in cold water, they often become inflected and afterwards re-expand. In one leaf thus treated, I distinctly saw the protoplasm in movement. In other leaves, treated in the same manner, and then immersed in a solution of carbonate of ammonia, strong aggregation ensued. Leaves placed in cold water, after an exposure to so high a tem- perature as 145° (62°'7 Cent.), sometimes become slightly, though slowly, inflected ; and afterwards have the contents of their cells strongly aggregated by car- bonate of ammonia. But the duration of the immer- sion is an important element, for if left in water at 145° (62°-7 Cent.), or only at 140° (60° Cent.), until it becomes cool, they are killed, and the contents of the glands are rendered white and opaque. This latter result seems to be due to the coagulation of the albu- men, and was almost always caused by even a short exposure to 150° (65°'5 Cent.) ; but different leaves, and even the separate cells in the same tentacle, differ con- siderably in their power of resisting heat. Unless the heat has been sufficient to coagulate the albumen, car- bonate of ammonia subsequently induces aggregation. In the fifth chapter, the results of placing drops of various nitrogenous and non-nitrogenous organic fluids on the discs of leaves were given, and it was shown that they detect with almost unerring certainty the presence of nitrogen. A decoction of green peas or of fresh cabbage-leaves acts almost as powerfully as an infusion of raw meat ; whereas an infusion of cabbage-

leaves made by keeping them for a long time in. merely warm water is far less efficient. A decoction of grass-leaves is less powerful than one of green peas or cabbage-leaves. These results led me to inquire whether Drosera possessed the power of dissolving solid animal matter. The experiments proving that the leaves are capable of true digestion, and that the glands absorb the di- gested matter, are given in detail in the sixth chapter. These are, perhaps, the most interesting of all my observations on Drosera, as no such power was before distinctly known to exist in the vegetable kingdom. It is likewise an interesting fact that the glands of the disc, when irritated, should transmit some influence to the glands of the exterior tentacles, causing them to secrete more copiously and the secretion to be- come acid, as if they had been directly excited by an object placed on them. The gastric juice of ani- mals contains, as is well known, an acid and a fer- ment, both of which are indispensable for digestion, and so it is with the secretion of Drosera. When the stomach of an animal is mechanically irritated, it secretes an acid, and when particles of glass or other such objects were placed on the glands of Drosera, the secretion, and that of the surrounding and un- touched glands, was increased in quantity and became acid. But, according to Schiff, the stomach of an animal does not secrete its proper ferment, pepsin, until certain substances, which he calls peptogenes, are absorbed; and it appears from my experiments that some matter must be absorbed by the glands of Drosera before they secrete their proper ferment. That the secretion does contain a ferment which acts only in the presence of an acid on solid animal matter, was clearly proved by adding minute do?es of

an alkali, which entirely arrested the process of diges- tion, this immediately recommencing as soon as the alkali was neutralised by a little weak hydrochloric acid. From trials made with a large number of substances, it was found that those which the secretion of Drosera dissolves completely, or partially, or not at all, are acted on in exactly the same manner by gastric juice. We may, therefore, conclude that the ferment of Drosera is closely analogous to, or identical with, the pepsin of animals.

The substances which are digested by Drosera act on the leaves very differently. Some cause much more energetic and rapid inflection of the tentacles, and keep them inflected for a much longer time, than do others. We are thus led to believe that the former are more nutritious than the latter, as is known to be the case with some of these same sub- stances when given to animals ; for instance, meat in comparison with gelatine. As cartilage is so tough a substance and is so little acted on by water, its prompt dissolution by the secretion of Drosera, and subsequent absorption, is, perhaps, one of the most striking cases. But it is not really more remarkable than the digestion of meat, which is dissolved by this secretion in the same manner and by the same stages as by gastric juice. The secretion dissolves bone, and even the enamel of teeth, but this is simply due to the large quantity of acid secreted, owing, apparently, to the desire of the plant for phosphorus. In the case of bone, the ferment does not come into play until all the phosphate of lime has been decomposed and free acid is present, and then the fibrous basis is quickly dissolved. Lastly, the secretion attacks and dissolves matter out of living seeds, which it some- times kills, or injures, as shown by the diseased state

of the seedlings. It also absorbs matter from pollen, and from fragments of leaves. The seventh chapter was devoted to the action of the salts of ammonia. These all cause the tentacles, and often the blade of the leaf, to be inflected, and the protoplasm to be aggregated. They act with very different power ; the citrate being the least powerful, and the phosphate, owing, no doubt, to the presence of phosphorus and nitrogen, by far the most powerful. T3ut the relative efficiency of only three salts of ammonia was carefully determined, namely the car- bonate, nitrate, and phosphate. The experiments were made by placing half-minims ('0296 ml.) of solutions of different strengths on the discs of the leaves, — by applying a minute drop (about the -^^ of a minim, or •00296 ml.) for a few seconds to three or four glands,— and by the immersion of whole leaves in a measured quantity. In relation to these experiments it was necessary first to ascertain the effects of distilled water, and it was found, as described in detail, that the more sensitive leaves are affected by it, but only in a slight degree.

A solution of the carbonate is absorbed by the roots and induces aggregation in their cells, but does not affect the leaves. The vapour is absorbed by the glands, and causes inflection as well as aggregation. A drop of a solution containing -^-^ of a grain (•0675 mg.) is the least quantity which, when placed on the glands of the disc, excites the exterior ten- tacles to bend inwards. But a minute drop, contain- ing ••, 4 i o o °f a grain ('00445 mg.), if applied for a few seconds to the secretion surrounding a gland, causes the inflection of the same tentacle. When a highly sensitive leaf is immersed in a solution, and there is ample time for absorption, the a 6 8'8 0 0 of a grain

(•00024 mg.) is sufficient to excite a single tentacle into movement. The nitrate of ammonia induces aggregation of the protoplasm much less quickly than the carbonate, but is more potent in causing inflection. A drop contain- ing -3-rVo °f a grain ("027 mg.) placed on the disc acts powerfully on all the exterior tentacles, which have not themselves received any of the solution ; whereas a drop with -^^ of a grain caused only a few of these tentacles to bend, but affected rather more plainly the blade. A minute drop applied as before, and contain- ing .a. a ' o o of a grain (-0025 mg.), caused the tentacle bearing this gland to bend. By the immersion of whole leaves, it was proved that the absorption by a single gland of -B-rrWo of a grain ('0000937 rng.) was sufficient to set the same tentacle into movement.

The phosphate of ammonia is much more powerful than the nitrate. A drop containing 3-^-0 of a grain (•0169 mg.) placed on the disc of a sensitive leaf causes most of the exterior tentacles to be inflected, as well as the blade of the leaf. A minute drop con- taining -rivers- °f a grain ('000423 mg.), applied for a few seconds to a gland, acts, as shown by the move- ment of the tentacle. When a leaf is immersed in thirty minims (1'7748 ml.) of a solution of one part by weight of the salt to 21,875,000 of water, the absorp- tion by a gland of only the 1 9 7 ti'0 0 0 0 of a grain (•00000328 mg.), that is, about the one-twenty-mil- lionth of a grain, is sufficient to cause the tentacle bearing this gland to bend to the centre of the leaf. In this experiment, owing to the presence of the water of crystallisation, less than the one-thirty- millionth of a grain of the efficient elements could have been absorbed. There is nothing remarkable in such minute quantities being absorbed by the glands,

for all physiologists admit that the salts of ammonia, which must be brought in still smaller quantity by a single shower of rain to the roots, are absorbed by them. Nor is it surprising that Drosera should be enabled to profit by the absorption of these salts, for yeast and other low fungoid forms flourish in solution^ of ammonia, if the other necessary elements are pre- sent. But it is an astonishing fact, on which I will not here again enlarge, that so inconceivably minute a quantity as the one-twenty-rnillionth of a grain of phosphate of ammonia should induce some change in a gland of Drosera, sufficient to cause a motor impulse to be sent down the whole length of the tentacle ; this impulse exciting movement often through an angle of above 180°. I know not whether to be most astonished at this fact, or that the pressure of a minute bit of hair, supported by the dense secretion, should quickly cause conspicuous movement. Moreover, this extreme sensitiveness, exceeding that of the most delicate part of the human body, as well as the power of transmit- ting various impulses from one part of the leaf to another, have been acquired without the intervention of any nervous system.

As few plants are at present known to possess glands specially adapted for absorption, it seemed worth while to try the effects on Drosera of various other salts, besides those of ammonia, and of various acids. Their action, as described in the eighth chapter, does not correspond at all strictly with their chemical affinities, as inferred from the classification commonly followed. The nature of the base is far more influential than that of the acid ; and this is known to hold good with animals. For instance, nine salts of sodium all caused well -marked inflection, and none of them were poison- ons in small doses ; whereas seven of the nine corn*-

spending salts of potassium produced no effect, two causing slight inflection. Small doses, moreover, of some of the latter salts were poisonous. The salts of sodium and potassium, when injected into the veins of animals, likewise differ widely in their action. The so-called earthy salts produce hardly any effect on Drosera. On the other hand, most of the metallic salts cause rapid and strong inflection, and are highly poisonous ; but there are some odd exceptions to this rule; thus chloride of lead and zinc, as well as two salts of barium, did not cause inflection, and were not poisonous.

Most of the acids which were tried, though much diluted (one part to 437 of water), and given in small doses, acted powerfully on Drosera ; nineteen, out of the twenty-four, causing the tentacles to be more or less inflected. Most of them, even the organic acids, are poisonous, often highly so ; and this is remarkable, as the juices of so many plants contain acids. Benzoic acid, which is innocuous to animals, seems to be as poisonous to Drosera as hydrocyanic. On the other hand, hydrochloric acid is not poisonous either to animals or to Drosera, and induces only a moderate amount of inflection. Many acids excite the glands to secrete an extraordinary quantity of mucus ; and the protoplasm within their cells seems to be often killed, as may be inferred from the surrounding fluid soon becoming pink. It is strange that allied acids act very differently : formic acid induces very slight in- flection, and is not poisonous ; whereas acetic acid of the same strength acts most powerfully and is poi- sonous. Lactic acid is also poisonous, but causes inflection only after a considerable lapse of time. Malic acid acts slightly, whereas citric and tartaric; acids produce no effect.