Part 2

Descending the scale to the very bottom, we reach a class of animals, the Protozoa, which cannot be separated in many cases from the Protophyta by these distinctions, since many of the former have no digestive cavity, nor the slightest trace of a nervous system, while many of the latter possess the power of active locomotion. As to external configuration, no certain rules can be laid down for separating animals and plants, many of the lower plants, either in their earlier stages, or in their maturity, being exactly similar in form to some of the lower animals. This is the case with some of the Algæ, which resemble very closely in form certain Infusorian animalcules. Again, many undoubted animals, which are rooted to solid objects in their adult state, are so plant-like in appearance as to be popularly regarded as vegetables. The Sea-firs, and the more highly organized Flustras or Sea-mats, which are usually considered as sea-weeds by sea-side visitors, are a few of many examples that might be taken from the so-called Hydroid Zoöphytes. No decided distinction between animals and plants can be drawn as to their minute internal structure, both alike consisting of molecules, of cells, or of fibres. Some decided, though not universal, differences exist in chemical composition. Plants exhibit a decided predominance of ternary compounds, or compounds which, like sugar, starch and cellulose, are made up of the three elements, carbon, hydrogen and oxygen, but are, comparatively speaking, poorly supplied with quaternary compounds, or those which contain an additional element of nitrogen. Animals, on the contrary, are rich in quaternary nitrogenized compounds, such as albumen or fibrin. Still, in both kingdoms we find nitrogenized and non-nitrogenized compounds, and it is only in the proportion which these sustain to each other in the organism that animals differ in any way from plants.

Before the invention of the microscope, no independent voluntary movements, if we except the opening and closure of flowers, and their turning towards the sun, the drooping of the leaves of sensitive plants under irritation, and some other kindred phenomena, were known in plants. Now, however, we know of many plants which are endowed, either when young or throughout life, with the power of effecting voluntary movements apparently as spontaneous and independent as those performed by the lower animals, the movements being brought about by means of little vibrating cilia, or hairs, with which a part or the whole of the surface is furnished. When it is added that many animals are permanently rooted, in their fully-grown condition, to solid objects, it will at once be apparent that no absolute distinction can be made between animals and plants merely because of the presence or absence of independent locomotive power.

There is, however, a test, the most reliable of all that have been discovered, by which an animal may be distinguished from a plant, and that is the nature of the food and the products which are elaborated therefrom in the body. Plants live upon such inorganic substances as water, carbonic acid and ammonia, and they have the power of manufacturing out of these true organic materials, and are therefore the great producers of nature. All plants which contain green coloring matter, technically called chlorophyll, break up carbonic acid in the process of digestion into its two constituents of carbon and oxygen, retaining the former and setting the latter free. And as the atmosphere always contains carbonic acid in small quantities, the result is that plants remove carbonic acid therefrom and give out oxygen. Animals, on the other hand, have no power of living on water, carbonic acid and ammonia, nor of converting these into the complex organic substances of their bodies. That their existence may be maintained animals require to be supplied with ready-made organic compounds, and for these they are all dependent upon plants, either directly or indirectly. In requiring as food complex organic bodies, which they ultimately reduce to very simply inorganic ones, animals are thus found to differ from plants. Whilst plants are the great manufacturers in nature, animals are the great consumers. Another distinction, arising from the nature of their food, is that animals absorb oxygen and throw out carbonic acid, their reaction upon the atmosphere being exactly the reverse of that of plants. There are organisms, it must be understood, which are genuine plants so far as their nutritive processes are concerned, but which, nevertheless, are in the possession of characters which could locate them among the animals. Volvox, so abundant in our streams during the proper seasons, affords a splendid illustration of the truth of this statement. Plants, which are devoid of chlorophyll, as is the case with the Fungi, do not possess the power of decomposing carbonic acid under the influence of sunlight, but are like animals in requiring organic compounds for their food. Two points must therefore be borne in mind in regarding the general distinctions between plants and animals which we have thus briefly outlined, and these are that they cannot often be applied in practice to ambiguous microscopic organisms, and certainly not to plant-forms that are destitute of chlorophyll.

That life should manifest itself certain conditions are essential, but some of which, though generally present, are not absolutely indispensable. One condition, however, seems to be very necessary, and that is that the living body should be composed of a certain material. This material, which forms the essential and fundamental parts of everything living, whether vegetable or animal, is technically called protoplasm. Other substances than it are often found in living bodies, but it is in protoplasm only that vitality appears to be inherent.

But whether it is the same in plants as in animals is a matter of opinion. One thing, however, seems reasonably certain, and that is that it is the medium or vehicle through which vital force is made manifest. Used in its general sense, protoplasm is chemically related in its nature to albumen, and generally has the character of a jelly-like, semi-fluid, transparent material, which, in itself, exhibits no definiteness of structure. When heated to a certain temperature it coagulates, just as the white of an egg does when boiled. Living protoplasm has the power of movement, of increasing in size or of maintaining its existence by the assimilation of fresh and foreign materials, and of detaching portions of itself which may subsequently develop into fresh masses. Though protoplasm be present in the ova of animals and the seeds of plants, yet there is no external and visible manifestation of life. There is in them what is called a dormant vitality, which may remain for a long time unchanged, until altered external circumstances cause the organism to pass into a state of active life.

Generally, certain external conditions must be present before any external vital phenomena can be manifested. The presence of atmospheric air, or rather of free oxygen, is in an ordinary way essential to active life. Life, that is its higher manifestations, is only possible between certain ranges of temperature, varying from near the freezing point to about 120° Fahrenheit. As water is a necessary constituent of protoplasm in its living state, so it becomes an absolutely essential requisite to the carrying on of vital processes of all kinds, for the mere drying of an animal or plant will, in most cases, kill it outright, and will always bring about a suspension of all visible life-phenomena.

While the large majority of living beings are organized, or composed of different parts, called organs, which sustain certain relations with one another, and which discharge different offices, yet it must not therefore be concluded that organization is a necessary accompaniment of vitality, or that all living creatures are organized. Innumerous low forms of life, so low that they occupy the very lowest place in the scale of animated existences, absolutely exhibit no visible structure, and cannot, therefore, be said to be organized, but they, nevertheless, discharge all their vital functions just as well as though they possessed special organs for the purpose. Concluding our theme, we are forced to admit that animals are organized, or possess structure, because they are alive, and not that they live because they are organized. By carefully comparing the morphological and physiological differences between different animals and plants, naturalists have divided the entire animal and vegetable kingdoms into a number of divisions, whose leading characteristics may be found in almost every text-book. All that we promised ourselves when this work was first thought of was a brief treatment of a few of the most interesting life-forms of this planet of ours in the light of their ways and doings, and the direction of human thought to those traits of character and manifestations of conscious intelligence which fit them to become partakers with man of that new life which awaits him beyond the grave.

PLANTS THAT FEED ON INSECTS.

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.

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.