8

PROOFS OF VITAL FORCE.

This is shown, a, by the influence exercised over plants by substances such as laudanum and arsenic; b, by the active and visible motions of the fluid contained within their cells; c, by the unerring directions taken by the delicate apparatus which ensures reproduction by seed; and d, by the locomotive power possessed by the reproductive apparatus of the lower classes of plants.

a. It was long ago shown by Marcet, that if the common Kidney Bean, the Lilac, and other plants, were exposed to the action of such poisons as destroy animal life, they will perish not only under their influence, but in a manner analogous to what occurs among animals. If an animal is dosed with arsenic, or corrosive sublimate, or any poisonous metallic salt, it perishes by inflammation or corrosion: plants die in a similar way, their leaves turning yellow and withering, no art sufficing for their recovery. On the other hand, vegetable poisons destroy life by a species of paralysis, leaves bending, and becoming flaccid, and the whole plant rapidly falling into a state resembling stupefaction, and ending in death. Every one knows that if the inner face of the stamens of the common Berberry is touched they suddenly rise upwards and dash their anthers against the stigma; that after a time they fall back, and then become able again to present the same phenomenon. Macaire showed that when a twig of the Berberry in flower is placed in weak Prussic acid, or a solution of opium, the stamens lose their irritability, and become so flexible that they may be moved backwards and forwards without difficulty. When, however, the Berberry is placed in solutions of arsenic or corrosive sublimate, the stamens equally lose their excitability, but instead of becoming flexible, they are made stiff, hard, and brittle. Similar effects are produced upon the Sensitive Plant and other species. Here we have direct proof that the life of a plant is affected by destructive agents in the same manner as animals.

The curious effect of anesthetic substances is the same upon plants as on animals. Dr. Marcet has shown this by means of Chloroform. "If," he says, "a drop or two of pure chloroform be placed on the point of the common petiole of a leaf of the Sensitive Plant, the petiole is soon seen to droop, and directly afterwards the leaflets collapse in succession, pair by pair, beginning with those that are situate at the extremity of each branch. A minute or two afterwards (the time varying with the irritability of the plant), most of the leaves near that on which the chloroform was placed, and situate below it on the same stem, droop one after the other, and their leaflets collapse, although not in so decided a manner as those of the leaf to which the chloroform was applied. After a certain time, which varies with the condition of

VITAL FORCE IN SENSITIVE PLANTS IN CHARA.

the plant, the leaves gradually open; but when touched they can no longer be irritated so as to collapse, as they do in their natural condition. They remain in this passive state, benumbed, as it were, for a considerable time, and generally it is not until some hours have elapsed that they regain their original sensibility. If, however, while in this passive state, the leaves be again touched with chloroform, they collapse as before. It is not till after several doses that they lose their sensibility entirely, or at all events until the next day; sometimes they wither completely after too many applications of the chloroform. The purer the chloroform and the greater the excitability of the plant, the greater are the effects produced."

Similar experiments with rectified ether gave results quite analogous. When the author repeated the experiment with chloroform, he found that the leaflets remained paralysed, as it were, and still unable to open after eighteen hours' rest; they seemed to be dead. This was apparently caused by excess of chloroform, a larger dose than that employed by Dr. Marcet having been used. It is thus seen that overdoses of chloroform kill plants as well as animals, though small doses are innocuous.

b. There grows commonly in ditches and stagnant water a plant called Chara, in the large cells of which a current of green globules steadily rises up one side and falls by another, presenting an appearance calling to mind the motion of an endless chain. If one of these cells is choked by a ligature, then the motion continues exactly as before in each of the two divisions so produced. The singularity of the

motion, and the ease with which it may be observed, have rendered this plant a favourite object of examination by young microscopists. Those philosophers who refused to admit this to be a vital motion analogous to that of the blood, imagined that they had found an explanation in electrical action. But Dutrochet, who once held this opinion, when he attempted to establish his hypothesis upon experiment, found that the magnetic force, even when prodigious, exercises no influence whatever upon the circulation of fluid within the cells of Chara, and he was obliged to admit the presence of a vital force, of the nature of which we are wholly ignorant. Motion of an analogous nature has been now remarked within the cells of so many plants that we cannot doubt it to be a universal phenomenon. It is to be remarked that this kind of movement is wholly independent of the general motion of the sap.

c. When a grain of pollen falls upon a stigma, it protrudes a tube of extreme tenuity. The tube penetrates the stigma, much in the same manner as the root of a seed pierces the earth. Thence it procceds unerringly to the tiny opening which it is destined to enter, passing by all obstacles, turning to the right or to the left, and now ascending, now curving back again, with the same constant certainty as would attend an act of consciousness. Nor is this all; in certain cases the

10

MANIFESTED BY POLLEN-TUBES, ETC.

entrance of the pollen-tube into the young seed through its foramen,* is assisted by movements on the part of the seed itself, as in the common garden Thrift (Armeria) in which a horizontal strap, interposed between the pollen-tube and the foramen, is spontaneously removed in order to enable the former to enter the latter. These phenomena, varying as the structure of plants itself varies, and destined as they evidently are to ensure that great end, the propagation of species, are wholly inexplicable upon any other principle than that of high vitality; so high indeed that they are only less than voluntary actions.

d. But perhaps nothing places the presence of a powerful vital force in stronger evidence than the facts which modern botanists have discovered in connection with the propagation of the lower orders of plants. It has been known from the observations of the younger Agardh, that in fresh-water Confervæ the seeds (technically called spores) swim about with activity in the interior of the cell which generates them, that they eventually force their way through a thin part of the cell wall, and thence darting into the water move about with great activity, the lighter end downwards, and therefore contrary to the force of gravitation. These motions last for several hours. More recently it has been demonstrated that the motion is caused by delicate vibratile cilia or fringes attached to the small and narrow end of the seed (spore). This motion is stopped instantaneously by any poison, such as iodine, being allowed to mingle in the water. Discoveries of a similar nature have been made among other races of plants. Modern research has shown that in the greater part of the lowest forms of vegetable life, and probably in all, minute spiral bodies exist having the power of active locomotion in many cases. These are called ANTHEROZOIDS in consequence of the bodies or antherids which contain them being regarded as analogous to the anthers of the higher orders of plants. Sea-weeds bear both spores and antherozoids. According to M. Thuret, whose observations are not open to doubt, by placing certain seaweeds in a damp atmosphere, the spores and antherids are freely expelled, and remain on the surface of the fronds, from which they can be readily collected and transferred to vessels containing sea-water. M. Thuret found that when put into separate vessels, the antherids placed by themselves immediately emit their antherozoids; the latter move about with great activity, even for two days successively, but on the third, begin to decompose. Spores, also, when placed in sea-water by themselves, retain their vitality for some time, not decomposing in less than a week; they even make attempts at growth, but abortions are the only consequence, and at last they perish also. It is otherwise when the spores and antherozoids are mingled in the same

The foramen is a minute passage through the integuments of an ovule or young seed, into which the pollen-tube must enter in order to vivify the latent embryo.

VEGETABLE IRRITABILITY.

11

vessel of sea-water. Then occurs a series of most curious phenomena. The antherozoids attack the spores, creep, as it were, over their surface, and communicate, by means of their vibratile cilia, a rotatory motion, which is sometimes very rapid. "Nothing," says M. Thuret, "is more curious than the appearance of great brown spores rolling and tumbling about in the midst of a swarm of antherozoids." The result is the fertilisation of the spore, which then begins to grow, and in ten days becomes a little cellular brown oval body, supported by a transparent rootlet. Sea-weeds are by no means the only plants in which these most remarkable phenomena have been detected. Most cryptogamic plants have now been observed to possess locomotive organs, analogous to antherozoids and bearing the same name. Liverworts, Mosses, Lycopods and Ferns themselves are supplied by nature with parts of the same description. When a Fern-seed vegetates it forms a small, thin, two-lobed green plate or scale lying horizontally on the damp surface of the ground. In this scale, called a protothall, lodge antherozoids and spores. By unknown means the former creep up to the latter, and fertilisation is accomplished. Wherever Fern-seeds have fallen, there a crop of these scaly protothalls springs up, as may be seen on the walls, or pots, or damp earth of any Fern-house. In each protothall is lodged an abundance of antherozoids and spores, the former active and capable of moving from place to place, the latter passive and stationary. Nor is there any thing in their structure which enables the observer to say whether the motions are voluntary or involuntary, so much do they resemble what is witnessed in animal life.

So far then as the important phenomena of reproduction are concerned, we have indications not only of vitality, but of such a force being present in plants in great activity.

Evidence of this kind, proving as it does beyond all doubt the presence of a vitality among plants identical with that of animals, though different in its manifestations, is greatly strengthened by the many known cases of what is called vegetable irritability.

That of the Sensitive Plant, which shrinks from the touch; of the Oscillating Saintfoin (Hedysarum gyrans) whose leaves move with as much appearance of spontaneousness as the polype; the sleep of leaves and flowers which close at night and expand in the day; the violent recoil of the column of Stylidium, or of the lip of Drakea, when touched; the oscillation of the labellum of many Bolbophylls and Pterostyles; the snap of the traplike leaf of Dionæa which closes with great force whenever one of its six bristle-shaped springs is disturbed-phenomena familiar to the naturalist—ar -are all intelligible

12

VITAL PRINCIPLE UNIVERSAL.

upon the supposition that they are the result of high vitality, inexplicable if referred to the operation of mere material forces.

One of the causes which have most embarrassed the progress of cultivation is not perceiving the presence among plants of this vital principle. Because plants neither walk, nor fly, nor crawl; because they are not endowed with the sense of pain or pleasure; because they neither struggle nor shriek, we are too apt to forget that they are alive, and consequently to treat them as if but rods of metal or plates of leather. Once grant that they are living beings, that they breathe although we see no mouths, that they digest although no stomachs are discoverable by common eyes, and above all things, that they feel, however low their sensations may be, and half the modes of cultivation employed by unskilful gardeners will stand conspicuous as palpable errors. Only show that plants are endowed with a life, identical in its nature with that of animals, and men must necessarily make it their first business to study the history of that life, and to master all which interferes with its healthy exercise. Such a step once taken, no cultivator would poison plants by a contaminated atmosphere, or paralyse them by an eternal footbath of cold water, or suffocate them in places where no air can reach them, or starve them by withholding the food without which they cannot exist, or cram them with incessant meals of heavy indigestible matter, which can but reduce them to the condition of an apoplectic glutton. That power which causes the bud to sprout, the leaves to form, the pollen to act, the seed to produce its embryo; which enables vegetation to breathe, and feed, and grow; which distinguishes all organised beings from the brute matter of which they consist, is the same as what gives to man the high attributes of his nature. It is VITALITY; a word which so-called philosophers in their ignorance, or presumption, may sneer at, but which in truth is the unknown force that controls the energy of matter, and directs it to special ends. It is only when cultivation is conducted with a full appreciation of this fundamental truth that Horticulture rises above the level of unreasoning custom, and acquires a solid base upon which the rationalia of the practices which experience seems to sanction can be permanently secured.