It is quite certain, that if a healthy seed, or leaf-bud, be placed in such a situation, that, according to the laws known, it will in all likelihood germinate, if all the elements for its sustenance exist in the soil, and the temperature and hygrometric {144} condition of the atmosphere are adapted to it, a healthy plant will be the result. Light, heat, moisture, and soil are therefore to be considered as the agents required to exist in a certain balance, or proportion, in reference to the health or power of vitality of the plant. Within a certain amount of variation, health may persist in virtue of the power of selection, which appertains to the spongioles of the root in absorbing nutriment; and also as regards light, from the tendency which most plants have to accommodate themselves to any deficiency of this element, by presenting their leafy expansion in that direction where the most of its influence may be obtained. But beyond a certain limit an unhealthy condition sets in. If the soil contain not the inorganic elements, which are absolutely indispensable for the tissues of the plant, or even if they be there and not in a state to be absorbed, a dwindling and degeneration ensue; if light be deficient in quant.i.ty, pallor, feebleness, and elongation of tissue follow, with more fluidity and general softness of texture. These conditions of plants have their a.n.a.logues in the ill-fed and ill-nourished children in some of our manufacturing districts; they are stunted and diseased. Transport a healthy country lad, with the bloom of health on his cheek, from his native hills and valleys, or woods and fields, to the stool behind a desk for eight hours a day, in a narrow street in any city, where the rays of the sun rarely penetrate, it will not be long before {145} the skin of the animal and the cuticle of the plant may be submitted for comparison, when both will testify to the importance of the solar rays, as an indispensable agent in supporting the normal processes of organic life. So far common observation is competent to a solution of the facts; but beyond this we come to the enquiry, what resemblances are there in the early conditions of plants and animals. Each originates from nucleated cells, endowed by the All-seeing Power with a blind impulse of progressive development; the most simple cell of a vegetable multiplies itself by a generation of new cells within it, when the parent dies, and liberates the offspring. Here progression is simply multiplication; it is, as it were, progression in length only. The original cell, however, of animals, which is styled the germinal vesicle, extends or becomes developed into dissimilar parts; and whatever may be the variety, all alike proceed from the original germ cell, and the _tout ensemble_ of parts const.i.tutes the one and indivisible whole; in this instance there is addition besides multiplication, tissues and organs are added in all variety, until the maximum of organic development is attained in the wonderful being, man.

Yet how many points of resemblance are there between the vegetable cell and the fully developed human being, in a physiological and pathological point of view. There must be nourishment to sustain both; both require a certain amount of light {146} and heat for their growth and increase, and are dependent upon various unknown causes for active and healthy existence; and when a certain time has expired, all alike return to a condition, in which the particles composing them are subject only to the dominion of the laws which preside over inorganic matter.

But during the existence of plants and animals, we discover other features of comparison; plants, as well as animals, are liable to disease; they are subject to functional and organic affections. The former, among plants, are usually traceable to atmospheric vicissitudes or irregularities, changes of situation, &c.; and in man to irregularities of diet, and mental and bodily excesses, as well as to atmospheric vicissitudes.[59]

The organic diseases of plants and animals depend upon a repet.i.tion, or continuance, of functional derangement. As a consequence of this, the nutrition and reproduction of tissues lose their normal and definite character, wherefrom an indefinite and abnormal result is obtained. There is a limit to abnormal productions, and they are apparently {147} subject to laws, though not yet understood. In animals, they may be either excessive development of natural tissue in natural localities, as obesity and fatty tumours; they may be natural products in unnatural situations, as fatty degenerations of muscular tissue; or altogether new and unnatural products, as tubercle and cancer.

In plants, from their greater simplicity of structure, organic affections are perhaps entirely limited to the two first forms of animal organic disease; viz. to undue development of tissue in natural situations, and to the formation of natural tissue in parts of a plant where they are not usually found in a state of nature. The variety of excrescences seen on the stems, branches, and twigs of plants, may be given as instances of the former; and the conversion of stamina into petals, as in double flowers, as an instance of the latter.

We derive our sustenance from vegetables, and they from us; they produce for us the soothing opiate and the deadly strychnia; we for them the animating ammonia, and the distortions and sterility of excessive culture; we engender in them, by the latter, debility, disease, and death; and in our turn we become their prey. All this indeed is but a cycle of events, that requires no learned mind to fathom, and to comprehend; it is a matter of every day occurrence, and, though perhaps not entirely unheeded, is not dwelt upon in the fulness of its bearings and importance. {148}

Let us now consider the diseases of plants, as a study progressive to those of man; and as their physiology has so extensively served us, we may possibly also find in their pathology much material for instruction; not that it will be attempted to shew that the same diseases affect both kingdoms, but that diseases, though dissimilar in effects, may have similar sources.

Unfortunately, there are not many men in this country, who need go further than their own gardens to find abundance of disease among their fruit trees and vegetables. The vine, the apple and the potato, common to most gardens, will furnish specimens.

It is an error of a serious kind to suppose, that the parasites which infest plants are not essentially the cause, or, perhaps, more properly speaking, the elements of disease. I confine myself here to disease of parasitic origin, as that is the subject of which I am chiefly treating.

That parasitic growths are the elements of disease in some instances, is now beyond dispute. The experiments of Mr. Ha.s.sall, detailed in Part II. of the Transactions of the Microscopical Society of London, are most conclusive; and they are of that simple nature, that any one may convince himself of their accuracy, by a repet.i.tion of them from the directions there laid down.

He says, the decay is communicable at will "to any fruits of the apple and peach kind, no matter {149} how strong their vital energies may be, by the simple act of inoculation of the sound fruit with a portion of decayed matter, containing filaments of the fungi. We may use with success the sporules of such fungi; but in this case the decomposition does not set in so quickly; in the one case, the smaller filaments of the fungi have advanced several stages in their growth; while in the other, the sporules have yet to pa.s.s through the several stages of their development."

Mr. Ha.s.san, however, seems to speak doubtfully as to the mode in which the disease becomes naturally introduced;[60] how the spores enter the fruit, "is not very clear--though probably, it is by insinuating themselves between the cells of which the cuticle is composed, or perhaps by means of the stomata, where they are present. I may here state that the experiments were made on fruit, while living, and attached to the tree."

But why should there be a doubt as to the parts by which the sporules of minute fungi enter the plant, when it is clear, that not only can they enter {150} by the spongioles, but by the stomata of the leaves, and mingle with the sap. It is true, that they make their appearance and grow upon the leaves and the fruit; but these are the situations most adapted for their fructification. I have seen the spores of the fungi which attack the cuc.u.mber and vegetable-marrow, in the cells of the hairs, and even their filamentous prolongations; these appropriate the fluids conveyed to the cells of the hair, rupture them, and at length fructify.

On referring to Dr. Lindley's Medical and Economic Botany, I find that many fungi are the active elements of disease, and in a manner which renders it highly improbable that they are so in any other way, than by obtaining an entrance to the sap of the plants. Of the microscopic fungus which destroys wheat, the Uredo caries of De Candolle, we find the habitat to be within the ovary of the corn, and that 4,000,000 may be contained in a grain of wheat,--now this and another fungus, the Lanosa nivalis, are said to destroy whole crops of corn: we cannot imagine that such an extensive affection, can have any other source than by means of the spores through the sap, seeing that bruising of the surface, or rupture of the cuticle of the apple, a comparatively soft fruit is necessary to produce the disease artificially in them; besides, a grain of corn containing vibriones, when grown and having fruited, the new fruit also contains them--now here, as this is I believe almost invariably the {151} case, either they or their ova must be carried with the sap to the new germs.

It is rather a remarkable fact, that these entophytes appropriate the nutriment destined for the plant in which they grow, they are consequently the means in many instances of its entire destruction, though only partially so in others.

There are many Fungi which have this tendency. The Puccinia gramienis, "preys upon the juices of plants, and prevents the grain from swelling."

The aecidium urticae, common on nettles, deprives the plant on which it grows, of the organizable matter, intended for its own nutrition. The Erysiphe communis, overruns and destroys peas. The Botrytis infestans, "attacks the leaves and stems of potatoes." The Oidium abortifaciens, attacks the ovaries of gra.s.ses--and the Oidium Tuckeri, "a formidable parasite, destroys the functions of the skin, of the parts it attacks." The latter has been most injurious to the vines, during the last two years. I have known instances in which the vines have been cut down, and every means taken to rid the houses of the disease; but this year, it has made its appearance, with all its former virulence, in the new shoots.

This, however, is sufficient to shew that plants are liable to disease, depending upon parasitic growths, which affect their vital powers, and deprive them of their natural nutritive fluids.

But somewhat similar diseases belong also to {152} warm climates; in a letter from Cuba, dated Dec. 1843,--Mr. Bastian writes, "_a plague_ has appeared among the orange trees--a mildew attacking the leaves and the blossoms, which finally dry up. It most frequently kills the trees. None of the orange family are exempt; lemons, limes, and their varieties, with the shaddock and forbidden fruit, have all suffered." This disease has continued without intermission, till the present year,--when the same gentleman writes, Feb. 20th, 1850: "The evil exists, although in a diminished degree, so much so, as to have allowed the trees to produce me 30,000 oranges again. In old times, the same plantations produced me 100,000."

The West India sugar-canes are also liable to a disease, which the Rev. Mr.

Griffiths, in his Natural History of the Island of Barbadoes, speaks of, in the following manner: "This, among diseases peculiar to canes, as among those which happen to men, too justly claims the horrible precedence." This disease is called the Yellow Blast. It is difficult to distinguish the Blast in its infancy, from the effect of dry weather.

There are often seen on such sickly canes, many small protuberant k.n.o.bs, of a soft downy substance. It is likewise observable, that such blades will be full of brownish decaying spots. The disease is very destructive to the canes. It is observed, that the Blast usually appears successively in the same fields, and often in the very same spot of land. {153}

This Blast is often found far from "infected places," and the infection always spreads faster to the leeward, or with the wind.

"_It is remarkable if canes_ have been once infected with the Blast, although they afterwards to all appearance, seem to recover; yet the juice of such canes will neither afford so much sugar, nor so good of its kind, as if obtained from canes which were never infected."

I may here allude to the circ.u.mstance, that in the island of Cuba, the destructive mildew is commonly called, _la pesta_.

It were needless to multiply instances of other endemic and epidemic diseases of vegetables; they are well known by practical observers to be very numerous, and I believe, in most instances, depending upon fungoid growths. The destruction of vegetables by insects, is of a very different nature to that produced by the fungi; it would be as unreasonable to consider the consumption of corn and herbage by locusts, as a disease of vegetation, as the ma.s.sacre and devouring of human beings by cannibals, a disease of the human body.

It is true that insects are exceedingly destructive to plants, but as far as I am able to obtain information, they appear to be so chiefly by their voracious propensities; they consume the structure of the plant in its ent.i.ty, and do not primarily interfere with its vitality. The instance of the vibriones, before-mentioned, seems at first to be an exception {154} to the rule, but this is rather apparent, than real; and it may be made to apply more as a confirmation, than an obstacle to the vegetable theory: for if we may fairly compare the diseases of animals with those of plants, the existence of entozoa in the latter, would be considered an essential point to be substantiated.

Having now considered the question as to the infeasibility of supposing that chemical fermentation is the basis upon which a theory of diseases can be sustained, and having shewn that life is inseparable from infection, and miasmatic generation;--having explained the phenomena of the dispersion of diseases by comparison with the dispersion of plants, and finally, having demonstrated that the physiology and pathology of plants bear so close a relation to each other, and that their epidemic affections depend upon minute organic germs, I submit to the judgment of my readers, whether there is not much reasonableness in the application of the facts to the inference--that living germs are the cause of epidemic disease in man and animals.

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CHAPTER IV.

RESULTS IN PROOF OF THE TENABLENESS OF THE PROPOSITION.

SECTION I.

OBSERVATIONS ON SOME OF THE LAWS OF EPIDEMIC DISEASES.

The results obtained by comparing certain facts connected with Epidemic Affections of animals, with a.n.a.logous affections in plants, afford, from the few instances I shall here notice, a very strong presumption, that a.n.a.logous causes operate in the production of these affections. I have already quoted from Hecker, to shew that previously to, and during the Epidemics of the Middle Ages, the minuter forms of animal and vegetable life appeared to be called into existence, much more abundantly than usual; that famines prevailed in consequence of failure of cereal crops, no doubt depending then, as now, upon the various forms of fungiferous growth. I cannot refrain quoting here, a pa.s.sage or two from our old friend Virgil; for he confirms not only the fact of peculiar showers in {156} connexion with diseases, but he also refers to the rust of corn, thus:

150. "Mox et frumentis labor additus; ut mala culmos Esset rubigo ...

... Intereunt segetes."

_Georg. 1._

Then:

311. "Quid tempestates autumni et sidera dicam?

322. "Saepe etiam[61] immensum coelo venit agmen aquarum Et foedam glomerant tempestatem imbribus atris Collectae ex alto nubes."

_Georg. 1._

The occurrence of black showers in this country has been observed during the present year, and I understand that in the fenny countries of the East, the corn has suffered much from the Uredo. I am not mentioning the circ.u.mstances as cause and effect, but merely to call attention to the fact, that unusual phenomena of this kind have been generally a.s.sociated with disease of the animal and vegetable tribes.

The same causes also predispose plants as well as animals, to epidemic attacks of disease. The repeated observations in the public journals on the subject of ventilation, drainage, and over-crowding, render all notice from me needless, to shew that these, though they do not produce the diseases {157} treated of, yet that under the influence of bad air, bad drainage, and over-crowding, epidemics are fostered and spread.

Lastly, says the Count Philippo Re, "I would remark that if _bad cultivation, and especially bad drainage, does not produce bunt or s.m.u.t, it is certain that those fields, the worst treated in these respects, suffer the most from these diseases_."

It has been remarked by many observers, that a greater fecundity has attended upon Pestilences, and this has been proved by comparison, that the births in proportion have far exceeded the ordinary limit.[62] In juxtaposition with this observation, I will place the following, not as a proof, but as a remark made quite independently of the subject of which I am treating. "From the first the diseased ears are larger than the healthy ones, and are sooner matured. What appears singular, but which I have not, perhaps, sufficiently verified, is _that the seeds are more abundant than in a sound ear_."

{158}

Now these are facts which require amplification, and if these two alone should be shewn upon an extensive field of observation, to apply not only to corn, but to other members of the vegetable kingdom, as I doubt not will be the case, though I am not fully prepared to prove it, it would be difficult to dissociate the fertility of the two living kingdoms from the operations of one and the same, or an a.n.a.logous law.

The epidemic diseases of plants are both infectious and contagious, at times they are observed to be endemic only, and then depending particularly upon some local causes. This is a law of diseases which applies equally to those of men and animals. In connexion with this law is another, which, as far as I am aware, has not hitherto been noticed in connexion with plants.

The potato disease, which excited so much interest and created so much anxiety for the poorer cla.s.ses of society, led the Government of this country to employ the most learned men to investigate the subject, in the hope of propounding some reasons which should explain the cause of the calamity, and thereby deduce a method of eradicating the evil, or, in other words, discover a cure for the disease. Many were the opinions as to the cause of the distemper, which it were useless here to recount, but a method was suggested, to which most people, I believe, looked forward with great antic.i.p.ations, and this was to obtain native seed, and to sow it on virgin soil. Was the end accomplished? No. {159} For though the seed was sown, and the plants grew, the disease still appeared among the newly imported individuals, to as great an extent, as among the native or domesticated plants.