Читать книгу Intelligence in Plants and Animals - Thomas G. Gentry - Страница 7

Table of Contents

Perhaps it would be difficult to find in the whole range of vegetable creation anything more curious than the carnivorous or flesh-eating plants. That animals eat plants creates in us no emotion of curiosity, for this is the common law of nature. But that plants should devour animals is a marvel to which few minds uninitiated in science would give credence. Though these strange forms of vegetable life have been known for about a century, yet it has been but a few years since the attention of naturalists was first specially called to their habits and character. No one has probably done more to explain the life and operations of the flesh-eating plants than Mr. Darwin.

For centuries strange rumors had been circulated of the existence of huge plants in the more remote and unvisited parts of Asia which would imprison and destroy large animals and men that would venture within reach of their great quivering leaves armed with hooked spines, the flesh of the dead victim being absorbed into their structure, but all these giant flesh-eating trees or plants have so far proved to be mere myths. Science has discovered, however, that there is some foundation for these exciting fictions, and it has not been obliged to go to the distant East to find it, for flesh-eating plants are by no means uncommon in this country and Europe. But these plants confine their destructive propensities to the crawling and flying insects which are beguiled by some tempting reward to rest on their leaves. Such a strange provision of nature is no less interesting than if these plants had the power to destroy the larger animals, for it is the fact itself which startles the attention by its seeming reversal of natural laws.

No better example of carnivorous plants could be taken than Dionæa muscipula, or to use the common name, Venus’s Fly-trap. It is a species that is indigenous to North Carolina and the adjacent parts of South Carolina, affecting sandy bogs in the pine forests from April to June, and a representative of the Droscraceæ, or Sundew Family. One cannot fail after once seeing it of becoming impressed with its peculiar characteristics. It is a smooth perennial herb with tufted radical leaves on broadly-winged, spatulate stems, the limb orbicular, notched at both ends, and fringed on the margins with strong bristles. From the centre of the rosette of leaves proceeds at the proper time a scape or leafless stalk which terminates in an umbel-like cyme of from eight to ten white bracted flowers, each flower being one inch in diameter. The roots are small and consist of two branches each an inch in length springing from a bulbous enlargement. Like an epiphytic orchid, these plants can be grown in well-drained damp moss without any soil, thus showing that the roots probably serve for the absorption of water solely. Three minute pointed processes or filaments, placed triangularly, project from the upper surface of each lobe of the bi-lobed leaf, although cases are observed where four and even ten filaments are found. These filaments are remarkable for their extreme sensitiveness to touch, as shown not only by their own movement, but by that of the lobes also. Sharp, rigid projections, diminutive spikes as it were, stand out from the leaf-margins, each of which being entered by a bundle of spiral vessels. They are so arranged that when the lobes close they interlock like the teeth of an old-fashioned rat-trap. That considerable strength may be had, the mid-rib of the leaf, on the lower side, is quite largely developed.

Minute glands, of a reddish or purplish color, thickly cover the upper surface of the leaf, excepting towards the margins, the rest of the leaf being green. No glands are found upon the spikes or upon the foliaceous footstalk. From twenty to thirty polygonal cells, filled with purple fluid, constitute each gland. They are convex above, somewhat flattened underneath, and stand on very short pedicels, into which spiral vessels do not enter. They have the power of secretion under certain influences, and also that of absorption. Minute octofid projections, of a reddish-brown color, are scattered in considerable numbers over the footstalk, the backs of the leaves and the spikes, with a few on the upper surfaces of the lobes.

The sensitive filaments, which are a little more than one-twentieth of an inch in length, and thin, delicate and tapering to a point, are formed of several rows of elongated cells, filled with a purplish fluid. They are sometimes bifid or even trifid at the apex, and towards the base there is a constriction formed of broader cells, and beneath the constriction an articulation, supported on an enlarged base, consisting of differently shaped polygonal cells. As the filaments project at right angles to the surface of the leaf, they would have been in danger of being broken off whenever the lobes closed together had it not been for the articulation, which allows them to bend flat down. So exquisitely sensitive are these filaments, from their tips to their bases, to a momentary touch, that it is hardly possible to touch them even so lightly or quickly with any hard object without causing the lobes to close, but a piece of delicate human hair, two and a-half inches in length, held dangling over a filament so as to touch it, or pinches of fine wheaten flour, dropped from a height, produce no effect. Though not glandular, and hence incapable of secretion, yet the filaments by their sensitiveness to a momentary touch, which is followed by the rapid closure of the lobes of the leaf, assure to Dionæa the necessary supply of insect food for all its wants.

Inorganic bodies, even of large size, such as bits of stone, glass and such like, or organic bodies not containing nitrogeneous matter in a soluble condition, as bits of cork, wood, moss for examples, or bodies containing soluble nitrogeneous matter, if perfectly dry, such as small pieces of meat, albumen, gelatine, etc., may be long left on the lobes, and no movement is excited. But when nitrogeneous organic bodies, which are all damp, are left on the lobes, the result is widely different, for these then close by a slow and gradual movement and not in a rapid manner as when one of the sensitive filaments is touched by a hard substance. Small purplish, almost sessile glands, as has already been stated, thickly cover the upper surface of the lobes. These have the power both of secretion and absorption, but they do not secrete until excited by the absorption of nitrogeneous matter. No other excitement, as far as experiments show, produces this effect. When the lobes are made to close over a bit of meat or an insect, the glands over the entire surface of the leaf emit a copious discharge, as in this case the glands on both sides are pressed against the meat or insect, the secretion being twice as great as when the one or the other is laid on the surface of a single lobe; and as the two lobes come into almost close contact the secretion, containing dissolved animal matter, diffuses itself by capillary attraction, causing fresh glands on both sides to begin secreting in a continually widening circle. The secretion is almost colorless, slightly mucilaginous, moderately acid, and so copious at times in the furrow over the mid-rib as to trickle down to the earth. But all this secretion is for the purposes of digestion. Be the animal matter which the enclosed object yields ever so little, it serves as a peptogene, and the glands on the surface of the leaf pour forth their acid discharge, which acts like the gastric juice of animals.

VENUS’S FLY-TRAP.

How It Captures Insects.

Now as to the manner in which insects are caught by the leaves of Dionæa muscipula. In its native country they are caught in large numbers, but whether they are attracted in any special way no one seems to know. Both lobes close with astonishing quickness as soon as a filament is touched, and as they stand at less than a right angle to each other, they have an excellent chance of capturing any intruder. The chief seat of the movement is near the mid-rib, but is not restricted to this part. Each lobe, when the lobes come together, curves inwards across its whole breadth, the marginal spikes alone not becoming curved. From the curving inwards of the two lobes, as they advance towards each other, the straight marginal spikes intercross by their apices at first, and ultimately by their bases. The leaf is then completely shut and encloses a shallow cavity. If made to shut merely by the touching of one of the sensitive filaments, or by the inclusion of an object not yielding soluble nitrogeneous matter, the two lobes retain their inwardly concave form until they re-expand. The re-expansion, when no organic matter is enclosed, varies according to circumstances, a leaf in one instance being fully re-expanded in thirty-two hours.

But the lobes, when soluble nitrogeneous matter is included, instead of remaining concave, thus containing within a concavity, slowly press closely together throughout their entire breadth, and as this takes place the margins gradually become a little everted, so that the spikes, which at first intercrossed, at last project in two parallel rows. So firmly do they become pressed together that, if any large insect has been caught, a corresponding projection is clearly visible on the outside of the leaf. When the two lobes are thus completely closed, they resist being opened, as by a thin wedge driven with astonishing force between them, and are generally ruptured rather than yield. If not ruptured, they close again with quite a loud flap. The slow movement spoken of, excited by the absorption of diffused animal matter, suffices for its final purpose, whilst the movement brought on by the touching of one of the sensitive filaments is rapid, and thus indispensable for the capturing of insects.

Leaves remain shut for a longer time over insects, especially if the latter are large, than over meat. In many instances where they have remained for a long period over insects naturally caught, they were more or less torpid when they reopened, and generally so much so during many succeeding days that no excitement of the filaments caused the least movement. Vigorous leaves will sometimes devour prey several times, but ordinarily twice, or, quite often, once is enough to render them unserviceable.

What purpose the marginal spikes, which form so conspicuous a feature in the appearance of the plant, subserve was unknown until the genius of Darwin solved the mystery. It was he that showed that elongated spaces between the spikes, varying from one-fifteenth to one-tenth of an inch in breadth according to the size of the leaf, are left open for a short time before the edges of the lobes come into contact, consequent upon the intercrossing of the tips of the marginal spikes first, thus enabling an insect whose body is not thicker than these measurements to escape, when disturbed by the closing lobes and the increasing darkness, quite easily between the crossed spikes. Moderately sized insects, if they try to escape between the bars, will be pushed back into the horrid prison with the slowly closing walls, for the spikes continue to close more and more until the lobes are brought into contact. Very strong insects, however, manage to effect their release. It would manifestly be a great disadvantage to the plant to remain many days clasped over a minute insect, and as many additional days or weeks in recovering its sensibility, inasmuch as a very small insect would afford but little nourishment. Far better would it be for the plant to wait until a moderately large insect was captured, and to allow the little ones to escape, and this advantage is gained by the slow intercrossing of the marginal spikes, which, acting like the large meshes of a fishing-net, allow the small and worthless fry to pass through.

Touching any one of the six filaments is sufficient to cause both lobes to close, these becoming at the instant incurved throughout their entire breadth. The stimulus must therefore radiate in all directions from any one filament, and it must also be transmitted with considerable rapidity across the leaf, for in all ordinary cases, as far as the eye can judge, both lobes close at the same time. Physiologists generally believe that in irritable plants the excitement is transmitted along, or in close connection with, the fibro-vascular bundles. Those in Dionæa seem at first sight to favor this belief, for they run up the mid-rib in a great bundle, sending off small bundles almost at right angles on each side, which bifurcate occasionally as they stretch towards the margin, the marginal branches from adjoining branches uniting and entering the marginal spikes. Thus a continuous zigzag line of vessels runs round the whole circumference of the leaf, while in the mid-rib all the vessels are in close contiguity, so that all parts of the leaf seem to be brought into some degree of communication. The presence of vessels, however, is not necessary for the transmission of the motor impulse, for it is transmitted from the apices of the sensitive filaments, which are hardly one-tenth of an inch in length, into which no vessels are seen to enter. Slits made close to the bases of the filaments, parallel to the mid-rib, and thus directly across the course of the vessels, sometimes on the inner and sometimes on the outer sides of the filaments, do not interfere with the transmission of the motor impulse along the vessels, and conclusively show that there is no necessity for a direct line of communication from the filament, which is touched towards the mid-rib and opposite lobe, or towards the outer parts of the same lobe. With respect to the movement of the leaves, the wonderful discovery made by Dr. Burdon Sanderson, and published in 1874, offers an easy explanation. There is, says this distinguished authority, a normal electrical current in the blade and footstalk, which, when the leaves are irritated, is disturbed in the same manner as is the muscle of an animal when contraction takes place.

After contraction has endured for a greater or less time, dependent upon circumstances which we do not well understand, re-expansion of the leaves is effected at an insensibly slow rate, whether or not any object is enclosed, both lobes opening in all ordinary cases at the same time, although each lobe may act to a certain extent independently of the other. The re-expansion is not determined by the sensitive filaments, for these may be cut off close to their bases, or be entirely removed, and re-expansion occur in the usual manner. It is believed that the several layers of cells forming the lower surface of the leaf are always in a state of tension, and that it is owing to this mechanical state, aided probably by fresh fluid being drawn into the cells, that the lobes begin to separate as soon as the contraction of the upper surface diminishes.

Six known genera, Drosophyllum, Roridula, Byblis, Drosera, Dionæa and Aldrovanda comprise the Droseraceæ, all of which capture insects. The first three genera effect this purpose solely by the viscid fluid secreted from their glands, and the last, like Dionæa, which has already been described, through the closing of the blades of the leaf. In these last two genera rapid movement makes up for the loss of viscid secretion. But of all the genera none is more interesting than the typical Sundews.

Growing in poor peaty soil, and sometimes along the borders of ponds where nothing else can grow, certain low herbaceous plants, called Droseras, abound. So small and apparently insignificant are they, that to the ordinary observer they are almost unnoticed. But they have peculiarities of structure and nature that readily distinguish them. Scattered thickly over their leaves are reddish bristles or tentacles, each surmounted by a gland, from which an extremely viscid fluid, sparkling in the sunlight like dew, exudes in transparent drops. Hence the common name of Sundew by which the half-dozen species found in the United States east of the Mississippi River are known. A one-sided raceme, whose flowers open only when the sun shines, crowns a smooth scape, which is devoid of tentacles. Drosera rotundifolia, our commonest species, has a wide range, being indigenous to both Europe and America. In the United States it extends from New England to Florida and westward, and is occasionally associated with Drosera longifolia, a form with long strap-shaped leaves, but whose distribution is mostly restricted to maritime regions, from Massachusetts to Florida.

ROUND-LEAVED SUNDEW.

Leaves Acting as Stomachs.

All of the species are remarkably similar in habits, capturing insects, and digesting and absorbing the soft parts, a circumstance which explains how these plants can flourish in an extremely poor soil where mosses, which depend almost entirely upon the atmosphere for their nourishment, only can live. Although the leaves of the Droseras at a hasty glance do not appear green, owing to the purple color of the tentacles, yet the superior and inferior surfaces of the blade, the stalks of the central tentacles, and the petioles contain chlorophyll, rendering the best of evidence that the plants obtain and assimilate carbon dioxide from the air. But when the poverty of the soil where these plants grow is considered, it is at once apparent that their supply of nitrogen would be exceedingly small, or quite deficient, unless they had the power of obtaining it from some other source. From captured insects this important element is largely obtained, and thus we are prepared to understand how it is that their roots, which consist of only two or three slightly divided branches, from one-half to one inch in length, and furnished with absorbent hairs, are so poorly developed. From what has been stated it would seem that the roots but serve to imbibe water, but there is no doubt that nutritious matters would also be absorbed were they present in the soil.

With the edges of its leaves curled so as to form a temporary stomach, and with the glands of its closely-inflected tentacles pouring forth their truly acid secretion, which dissolves animal matters that are subsequently absorbed, Drosera may be said to feed like an animal. But, unlike an animal, it drinks by means of its roots, and largely, too, for it would not be able to supply its glands with the necessary viscid fluid. The amount needed is by no means an inconsiderable quantity, as two hundred and seventy drops may sometimes be exposed during a whole day to a glaring sun. Such a profuse exudation implies preparations for hosts of insect visitors. In this Drosera has not miscalculated. Its bright pink blossoms and brilliant, glistening dew lure vast numbers of the smaller kinds, and the larger ones, too, to certain death. But the wholesale destruction of life that goes on is much in excess of what the plant requires for food. While the smaller flies remain adherent to the leaves, affording them the needed aliment, the larger insects, after death, fall around the roots, where they decay and fertilize the soil with nitrogen, which doubtless through the proper channels makes its way into the body of the plant, thus helping to give it tone and vigor. There are times when these plants work better than at others, but whether this is caused by the electrical condition of the atmosphere, or the amount of its contained moisture, is a question which science has not positively determined.

Drosera longifolia folds it leaves entirely around its victim, from the apex down to the petiole after the manner of its vernation, but in Drosera rotundifolia, whose marginal tentacles are longer, the tentacles simply curve around the object, the glands touching the substance, like so many mouths receiving nourishment. Experimented upon with raw beef, the tentacles of healthy leaves, from within to without, but in periods of time varying from six to eight or nine hours, clasp firmly the beef, almost concealing it from view. Equally vigorous leaves, however, made no move towards clasping a bit of dry chalk, a chip of flint, or a lump of earth. Bits of raw apple cause a curving of the tentacles, but very few of the glands are seen touching them. It would seem, therefore, that these plants are really carnivorous, preferring animal substances, which they, by the aid of some ferment analogous to pepsin, which is secreted by the glands, are able to absorb. A minute quantity of already soluble animal matter is the exciting cause, and this must be taken in by the glands, or there is no secretion of the fermenting material.

In all ordinary cases the glands alone are susceptible to excitement. When excited, they do not themselves move or change form, but transmit a motor impulse to the bending part of their own and adjoining tentacles, and are thus carried towards the centre of the leaf. Stimulants applied to the glands of the short tentacles on the disc indirectly excite movement of the exterior tentacles, for the stimulus of the glands of the disc acts on the bending part of the latter tentacles, near their bases, and does not first travel up the pedicels to the glands, to be then reflected back to the bending place. Some influence, however, does travel up to the glands, causing them to secrete most copiously, and the secretion to become acid, just such an influence as that which in animals is transmitted along the nerves to glands, modifying their power of secretion, independently of the condition of the blood-vessels. Over organic substances that yield soluble matter the tentacles remain clasped for a much longer time than over those not acted upon by the secretion, or over inorganic objects. That they have the power of rendering organic substances soluble, that is, that they have the power of digestion, is no longer a question of dispute. They certainly have this power, acting on albuminous compounds in exactly the same manner as does the gastric juice of mammals, the digested matter being afterwards absorbed. In animals the digestion of albuminous compounds is effected by means of a ferment, pepsin, together with weak hydrochloric acid, though almost any acid will serve, yet neither pepsin nor an acid by itself has any such power. It has been observed that when the glands of the disc are excited by the contact of any object, especially of one containing nitrogeneous matter, the outer tentacles and often the blade become inflected, the leaf thereby becoming converted into a temporary cup or stomach. The discal glands then secrete more copiously, the secretion becoming acid, and, moreover, some influence being transmitted by them to the glands of the exterior tentacles, causing them to emit a more abundant secretion, which also becomes acid. This secretion is to a certain extent antiseptic, as it checks the appearance of mould and infusoria, and in this particular acts like the gastric juice of the higher animals, which is known to arrest putrefaction by destroying the microzymes.

With animals, according to Schiff, mechanical irritation excites the glands of the stomach to secrete an acid, but not pepsin. There is strong reason to believe, too, that the glands of Drosera, which are continually secreting viscid fluid to replace the losses by evaporation, do not secrete the ferment proper for digestion when mechanically irritated, but only after absorbing certain matters of a nitrogeneous nature. The glands of the stomachs of animals secrete pepsin only after they have absorbed certain soluble substances designated peptogenes, showing a remarkable parallelism between the glands of Drosera and those of the stomach in the secretion of their appropriate acid and ferment.

Not only animal matter, but also the albumen of living seeds, which are injured or killed by the secretion, are acted upon by the glands of Drosera. Matter is likewise absorbed from pollen, and from fresh leaves. The stomachs of vegetable-feeding animals, as is only too well known, possess a similar power of extracting nourishment from such articles. Though properly an insectivorous plant, but as pollen, as well as the seeds and leaves of surrounding plants, cannot fail to be often or occasionally blown upon the glands of Drosera, yet it must be credited with being to a certain extent a vegetable feeder.

That a plant and an animal should secrete the same, or nearly the same, complex digestive fluid, adapted for a similar purpose, is a wonderful fact in physiology, but not more remarkable than the movements of a tentacle consequent upon an impulse received from its own gland, the movement at the bending place of the tentacle being always towards the centre of the leaf, and so it is with all the tentacles when their glands are excited by immersion in a suitable fluid. The short tentacles in the middle part of the disc, however, must be excepted, as these do not bend at all when thus excited. But when the motor impulse comes from one side of the disc, the surrounding tentacles, and even the short ones in the middle of the disc, all bend with precision towards the point of excitement, no matter where it may be located. This is in every way a remarkable phenomenon, for the leaf appears as if endowed with animal sense and intelligence. It is all the more remarkable when the motor impulse strikes the base of a tentacle obliquely to its flattened surface, for then the contraction of the cells must be restricted to one, two or a very few rows at one end, and different sides of the surrounding tentacles must be acted on that all may bend with precision to the point of excitement. The motor impulse, as it spreads from one or more glands across the disc, enters the bases of the surrounding tentacles, and instantly acts on the bending place, but does not first proceed up the tentacles to the glands, causing them to reflect back an impulse to their bases, although some influence is sent up to the glands, whereby their secretion is soon increased and rendered acid. The glands, being thus excited, send back some other influence, dependent neither on increased secretion nor on the inflection of the tentacles, which causes the protoplasm to aggregate in cell beneath cell. This maybe called a reflex action. How it differs from that which proceeds from the nerve-ganglion of an animal, if it differ at all, no one can say. It is probably the only known case of reflex action in the vegetable kingdom.

Concerning the mechanism of the movements and the character of the motor impulse little is known. During the act of inflection fluid surely passes from one part to another of the tentacles. In explanation of the fact it is claimed that the motor impulse is allied in nature to the aggregating process, and that this causes the molecules of the cell-walls to approach each other, as do the molecules of the protoplasm within the cells, thereby causing the cells in all to contract. This is probably the hypothesis that best accords with the observed facts, although some strong objections may be urged against this view. The elasticity of their outer cells, which comes into activity as soon as those on the inner side cease contracting with prepotent force, leads largely to the re-expansion of the tentacles, but there is reason to suspect that fluid is continually and slowly attracted into the outer cells during the act of re-expansion, thus augmenting their tension.

With respect to the structure, movements, constitution and habits of Dionæa muscipula and Drosera rotundifolia, as well as kindred species, little has been made out by patient study and investigation in comparison with what remains unexplained and unknown. Many of their movements, especially of Dionæa and Drosera, seem so sensible and intelligent that the reflecting mind of man can hardly hesitate to assign them high positions in organic nature and the possession, even though in a very small degree, of that consciousness with which animal life is endowed. That man is psychically related to all life is the belief of millions in the old world, and the hope of millions in the new. In this thought is the escape from materialism, that threat of the ignorant and unbelieving. Higher conceptions of beauty and greatness are now being entertained by the multitudes, and we begin to feel that the next great step is being taken when we shall become, instead of poor trembling denizens of a perishable world, proud and conscious citizens of an imperishable universe. That we of the upper ranks of God’s creation alone possess an inner life which shall transcend all change is no longer a general belief, but there is a growing hope that all nature shares it, and that love is its expression and its method. All existence is a unit. Life, law and love are divine. Man, looking calmly about him, cannot set himself apart as something essentially different from nature, but must recognize himself as a part, and include love in the universal scheme of development. All other expressions of life must share with him in the divine love and progress. His dogmas, founded on mistaken traditions, have given way to science, and he cannot but believe that love is in and of the soul, and that all life has some sort of development of soul. Because plant-life has no brain, and therefore has no intelligence, no mind, no soul, is preposterous to contemplate. Who can positively affirm that brain alone is the seat of conscious intelligence? None but He alone, the Giver of all life, who sits enthroned and exalted in the everlasting heavens.