Chapter I (1)

Microscopic Forms of Life--Thallophytes--Pteridophyta, Phanerogamæ--Structure and Properties of the Cell.

The time has long since passed by since the value of the microscope as an instrument of scientific research might have been called in question. By its aid the foundation of mycology has been securely laid, and cryptogamic botany in particular has, during the last quarter of a century, made surprising progress in the hands of those devoted to pursuits which confer benefits upon mankind.

Little more than thirty years ago practically nothing was known of the life history of a fungus, nothing of parasitism, of infectious diseases, or even of fermentation. Our knowledge of the physiology of nutrition was in its infancy; even the significance of starches and sugars in the green plant was as yet not understood, while a number of the most important facts relating to plants and the physiology of animals were unknown and undiscovered. When we reflect on these matters, and remember that bacteria were regarded merely as curious animalculæ, that rusts and smuts were supposed to be emanations of diseased states, and that spontaneous generation still-survived among us, some idea may be formed of the condition of cryptogamic botany and the lower forms of animal life some eight or ten years after my book on the microscope made its first appearance (1854).

Indeed, long prior to this time, dating from that of even the earliest workers with the microscope, it was known that the water of pools and ditches, and especially infusions of plants and animals of all kinds, teem with living organisms, but it was not recognised definitely that vast numbers of these microscopic living beings (and even actively moving ones) are plants, growing on and in the various solid and liquid matters examined, and as truly as visible and accepted plants grow on soil and in the air and water. Perhaps the most important discovery in the history of cryptogamic botany was initiated here. The change, then, that has come over our knowledge of microscopic plant life during this last busy quarter of a century has been almost entirely due to the initiation and improvement, first in methods of growing them, and in the methods of “_Microscopic Gardening_”; and secondly, to the greater knowledge gained in the use of the microscope.

“If we look at the great groups of plants from a broad point of view, it is remarkable that the fungi and the phanerogams occupy attention on quite other grounds than do the algæ, mosses, and ferns. Algæ are especially a physiologist’s group, employed in questions on nutrition, reproduction, and cell division and growth; the Bryophyta and Pteridophyta are, on the other hand, the domain of the morphologist. Fungi and Phanerogams, while equally or even more employed by specialists in morphology and physiology, appeal widely to general interest on the ground of utility.

“It is very significant that a group like the fungi should have attracted so much scientific attention, and aroused so general an interest at the same time. But the fact that fungi affect our lives directly has been driven home; and whether as poisons or foods, destructive moulds or fermentation agents, parasitic mildews or disease germs, they occupy more interest than all other cryptogams put together, the flowering plants alone rivalling them in this respect. A marked feature of the period in which we live will be the great advances made in our knowledge of the uses of plants, for, of course, this development of economic botany has gone hand in hand with the progress of geological botany, the extension of our planting, and the useful applications of botany to the processes of home industries.”[49]

The intimate organic structure of the vegetable world is seen to consist of a variety of different materials indeterminable by unassisted vision, and for the most part requiring high magnification for their discrimination. Chemical analysis had, however, shown that vegetables are composed of a few simple substances, water, carbonic acid gas, oxygen, nitric acid, and a small portion of inorganic salts. Out of these simple elements the whole of the immense variety of substances produced by the vegetable kingdom are constructed. No part of the plant contains fewer than three of these universally distributed elements, hence the greater uniformity in their chemical constituents. It will be seen, then, that the methods of plant chemistry are of supreme interest both to the chemist and the physiologist, or biologist. Plants, while they borrow materials from the inorganic, and powers from the physical world, whereby they are enabled to pass through the several stages of germination, growth, and reproduction, could not accomplish these transformations without the all-important aid of light and heat, the combined functions of which are indispensable to the perfect development of the vegetable world.

Light, then, enables plants to decompose, change into living matter, and consolidate, the inorganic elements of carbonic acid gas, water, and ammonia, which are absorbed by the leaves and roots from the atmosphere and earth; the quantity of carbon consolidated being exactly in proportion to the intensity of the light. Nevertheless, light in its chemical character is a deoxidising agent, by which the numerous neutral compounds common to vegetables are formed. It is the principal agent in preparing the food of plants, and it is during the chemical changes spoken of that the specific heat of plants is slowly evolved, which, though generally feeble, is sometimes very sensibly evolved, especially so when flowers and fruits are forming, on account of the increase of chemical energy at this period.

The action of heat is measurable throughout the whole course of vegetable life, although its manifestations take on various forms--those suited to the period and circumstances of growth. Upon the heat generated depends the formation of protein and nitrogenous substances, which abound more directly in the seed buds, the points of the roots, and in all those organs of plants which are in the greatest state of activity. The whole chemistry of plant life, in fact, is manifest in this production of energy for drawing material from its surroundings; therefore the organising power of plants bears a direct ratio to the amount of light and heat acting upon them.

The living medium, then, which possesses the marvellous property of being primarily aroused into life and energy, and which either forms the whole or the greater portion of every plant, is in its earliest and simplest form nothing more than a microscopic cell, consisting of one or two colourless particles of matter, in closest contact, and wholly immersed in a transparent substance somewhat resembling _albumen_ (white of egg), termed _protoplasm_, but differing essentially in its character and properties. This nearly colourless organisable matter is the life-blood of the cell. It is sufficiently viscid to maintain its globular form, and under high powers is seen to have a slightly consolidated film enclosing semi-transparent particles, together with vacuoles which are of a highly refractive nature. These small bodies are termed nuclei, and they appear to be furnished with an extremely delicate enveloping film. In a short time the nuclei increase in number and split up the parent body. The protoplasmic mass, however, is undoubtedly the true formative material, and is rightly regarded as “the physical basis of life” of both the vegetable and animal kingdoms.

There are, however, certain members of the vegetable kingdom which somewhat resemble animals in their dependence upon receiving organic compounds already formed for them, being themselves unable to effect the fixation of the carbon needed to effect the first stage in their after chemical transformations. Such is the case with a large class of flowering plants, among Phanerogams, and the leafless parasites which draw their support chiefly from the tissues of their hosts. It is likewise the case with regard to the whole group of fungi; the lower cryptogams, which derive the greater portion of their nutritive materials from organic matter undergoing some form of histolysis; while others belonging to this group have the power of originating decomposition by a fermentative (_zymotic_) action peculiarly their own. There are many other protophytes which live by absorption, and which appear to take in no solid matter, but draw nourishment from the atmosphere or the water in which they exist.

With regard to motion, this was at one time considered the distinctive attribute of animal life, but many protophytes possess a spontaneity of power and motion, while others are furnished with curious motile organs termed _cilia_, or whip-like appendages, _flagella_, by which their bodies are propelled with considerable force through the water in which they live.

Henceforth this protoplasmic substance was destined to take an important position in the physiological world. It is, then, desirable to enter somewhat more fully into the life history of so remarkable a body. It has a limiting membrane, composed of a substance somewhat allied to starch, termed _cellulose_, one of the group of compounds known as carbo-hydrates. The mode of formation and growth of this cell wall is not yet definitely determined; nevertheless, it is the universal framework or skeleton of the vegetable world, although it appears to play no special part in their vital functions. It merely serves the purpose of a protecting membrane to the globular body called the “_primordial cell_,” which permanently constitutes the living principle upon which the whole fundamental phenomena of growth and reproduction depend.

Sometimes this protoplasmic material is seen to constitute the whole plant; and so with regard to the simplest known forms of animal life--the amœba, for example. That so simple and minute an organism should be capable of independent motion is indeed surprising. Dujardin, a French physiologist, termed this animated matter _sarcode_. On a closer study of the numerous forms of animal life it was found that all were alike composed of this sarcode substance, some apparently not having a cell wall. The same seemed to hold good of certain higher forms of cells, the colourless blood corpuscles for instance, which under high powers of the microscope are seen to change their shape, moving about by the streaming out of this sarcode. At length the truth dawned on histologists that the cell contents, rather than the closing wall, must be the essential structure. On further investigation it became apparent that a far closer similarity existed between vegetables and animals than was before supposed. Ultimately it was made clear that the vegetable protoplasm and the animal sarcode were one and the same structure. Max Schultz found this to be the case, and to all intents and purposes they are identical.

We have now to retrace our steps and look somewhat more closely into the discovery of that important body, the _cell-nucleus_. It was an English botanist, Dr. Robert Brown, who, in 1833, during his microscopical studies of the epidermis of orchids, discovered in their cells “an opaque spot,” to which soon afterwards he gave the name of _nucleus_. Schleiden and Schwann’s later researches led them to the conclusion that the nucleus is the most characteristic formative element in all vegetable and animal tissues in the incipient phase of existence. It then began to be taught that there is one universal principle of development for the elementary parts of all organisms, however different, and that is the formation of cells. Thus was enunciated a doctrine which was for all practical purposes absolutely new, and which opened out a wide field of further investigation for the physiologist, and led up to a fuller knowledge of the cell contents. In fact, it became a question as to whether the cell contents rather than the enclosing wall should not be considered the basis of life, since the cell at this time had by no means lost its importance, although it no longer signified the minute cavity it did when originally discovered by Schwann. It now implied, as Schultz defined it, “a small mass of viscid matter, protoplasm, endowed with the attributes of life.” The nucleus was once more restored to its original importance, and with even greater significance. In place of being a structure generated _de novo_ from non-cellular substance, and disappearing as soon as its function of cell formation is accomplished, the nucleus is now known as the central permanent feature of every cell, and indestructible while the cell lives, and the parent, by division of its substance, of other generations of nuclei and cells. The word _cell_ has at the same time received its final definition as “a small mass of protoplasm supplied with a nucleus.” In short, all the activities of plant and animal life are really the product of energy liberated solely through _histolysis_, or destructive processes, amounting to the combustion that takes place in the ultimate cells of the organisms.

But there are other points of especial interest involved in the question of cell formation beside those already mentioned.

The cell and its contents collectively are termed the _endoplasm_, or when coloured, as in algæ, _endochrome_. With regard to the outer layer of the cell and its growth nothing satisfactory has been clearly determined and finally accepted.

The cell as a whole is a protoplasmic mass, and not an emulsion, as some observers would have us suppose. It is, in fact, a reticulated tissue of the most delicate structure, made up of canaliculate spiral fibrils with hyaline walls capable of expansion and contraction. These fibrils are probably composed of still finer spirals. The visible granulated portion of the protoplasm, the only part that takes a stain under ordinary circumstances, is simply the contents of these canals. It is the chromatin of Flemming, and is capable of motion within the canals. The nucleus, then, is probably nothing more than a granule of the extra-cellular net, and is formed by the junction of the several bands of wall-threads which traverse it in different directions. The cell wall of plants possesses the same structure as protoplasm, and is probably protoplasm impregnated by cellulose.

It is this portion of the protoplasmic mass that is now recognised under the term _octoplasm_, or primordial utricle, and is of so fine and delicate a nature that it is only brought into view when separated from the cell wall either by further developmental changes, or by reagents and certain stains or dyes. It was, in fact, discovered to be a slightly condensed portion of the protoplasmic layer corresponding to the _octosare_ of the lower forms of animal life. The octoplasm and cell wall can only be distinguished from each other by chemical tests. Both nucleus and nucleoli are only rendered visible in the same way, that is, by staining for several hours in a carmine solution, and washing in a weak acetic acid solution.

With the enlargement of the cell by the imbibition of water, clear spaces, termed vacuoles, are seen to occupy a small portion of the cell, while the nucleus and nucleoli lie close to the parietal layer.

The interesting phenomenon of cyclosis, to which I shall have occasion to refer further on, is now believed to be due to the contractility of certain wall-threads stretching from the nucleus to the outermost layers of the cell. An intimate relationship is thereby established between the nucleus, the nucleolus, and the parietal layer. This much has been made clear by the more scientific methods of investigation pursued in the use of the microscope. Nevertheless a large and important class of cells, forming a kind of borderland between the vegetable and animal kingdoms, still remains to be dealt with, in which the cell contents are only imperfectly differentiated, while numerous other unicellular organisms, owing to their extreme minuteness, tenuity, and want of all colour, are apparently devoid of any nucleus, and when present can only be differentiated by a resort to a specially conducted method of preparation and staining. There is, however, a remarkable feature in connection with many micro-organisms--that certain of these protophytes possess motive organs, cilia or flagella, bodies at one time supposed to be characteristic of, and belonging to, the protozoa.

This being the case, the methods of plant chemistry are of supreme interest, the more so because physiologists are in a position to isolate a single bacterial cell, grow it in certain media, and thus devote special attention to it, and keep it for some time under observation. In this way it has become possible to further grasp facts in connection with cell nutrition and the nature of its waste products. We have, then, arrived at a stage when the history of the chemical changes brought about by bacteria can be more definitely determined, as we have here to do with the vegetable cell in its simplest form. The chemical work performed by these micro-organisms has as yet occupied only a few years; nevertheless, the results have been of the most remarkable and encouraging character.

At an earlier period an interesting discovery in connection with the pathogenic action of these bodies was, by the labours of Schöenlein, Robin, and others, brought to the notice of the medical profession, viz., that certain diseases affecting the human body were due to vegetable parasites. In 1856 an opportunity offered itself for a thorough investigation, and the microscopical part of the work fell into my hands, with the result that I was able to add considerably to Schöenlein’s list of parasitic skin diseases. My observations were in the first instance communicated to the medical journals. But the generalisation arrived at was that “If there be any exceptions to the law that parasites select for their sustenance the subjects of debility and decay, such exceptions are rarely to be found among the vegetations belonging to fungi, which invariably derive nutrition from matter in a state of lowered vitality, passing into degeneration, or wherein decomposition has already taken place to a certain extent.... It scarcely admits of a doubt that all diseases observed of late years among plants have been due to parasites of the same class favoured by want of vigour of growth and atmospheric conditions, and that the cause of the various murrains of which so much has been heard is also due to similar causes.”[50]

Herein, then, is to be found the solution of a difficulty that so long surrounded the question, but which subsequently culminated in the specialisation and scientific development of bacteriology, due to the unceasing labours of Pasteur, whose solid genius enabled him to overcome the prejudices of those who were at work on other lines, and who had no conception of the functions that parasitic organisms fulfil in nature.

Going back to my earlier experimental researches to determine the part taken by saccharomycetes and saprophytes in fermentation, I find, from correspondence in my possession, that in 1859 I demonstrated to the satisfaction of Dr. Bell, F.R.S., the then head of the chemical laboratory of Somerset House, that a very small portion of putrefactive matter taken from an animal body, a parasitic fungus (_Achorion Schöenleinii_), a mould (_Aspergillus_ or _Penicillium_), and a yeast (_Torula cerevisiæ_) would in a short time, and indifferently, set up a ferment in sweet-wort and transform its saccharine elements into alcohol, differing only in degree (quantitative), and not in kind or quality. This, then, was the first step in the direction towards proving symbiotic action between these several parasitic organisms. The only apparent difference observed during the fermentative processes was that putrefactive (saprophytic) action commenced at a somewhat earlier stage, and that the percentage of alcohol was also somewhat less.[51]

In 1856, also, the ærobic bacteria attracted my attention, and, together with the late Rev. Lord Sidney Godolphin Osborne, I exposed plates of glass (microscopical slides), covered with glycerine and grape sugar, in every conceivable place where we thought it possible to arrest micro-organisms. The result is known, viz., that fungoid bodies (moulds and bacterial) were taken in great numbers, and varying with the seasons. The air of the hospital and sick-room likewise engaged attention, each of which proved especially rich in parasitic bodies. During the cholera visitation of 1858 the air was rich in ærobic and anærobic bacteria, while a _blue mist_ which prevailed throughout the epidemic yielded a far greater number than at any former period (represented in Plate I., No. 13). This blue mist attracted the especial attention of meteorologists. At a somewhat later period a more remarkable fungoid disease, the fungus foot of India, _mycetoma_, came under my observation, a detailed description of which I contributed to the medical journals, and also, with further details, to the “Monthly Microscopical Journal” of 1871. Interlacing mycelia, ending in hyphæ, in this destructive form of parasitic disease were seen to pervade the whole of the tissues of the foot, the bony structures being involved, and it was only possible to stay the action of the parasite by amputation.

So far, then, the study of parasitic organisms had at an early period shared largely in my microscopical work, extending over several years, and with the result that these micro-organisms were found to exhibit on occasions great diversity of character, and that different members of the bacteria in particular flourish under great diversity of action, and often under entirely opposite conditions; that they feed upon wholly different materials, and perform an immense variety of chemical work in the media in which they live.

The study of the chemistry (_chemotaxis_) of bacteria has, however, greatly enlarged our conception of the chemical value and power of the vegetable cell, while it is obvious that no more appropriate or remunerative field of study could engage the attention of the microscopist, as well as the chemist, than that of bacterial life, and which is so well calculated to enlarge our views of created organisms, whether belonging to the vegetable or animal kingdom.

Pathogenic Fungi and Moulds.

It is scarcely necessary to go back to the history of the parasitic fungi to which diseases of various kinds were early attributable. The rude microscopes of two and a half centuries ago revealed the simple fact that all decomposable substances swarmed with countless multitudes of organisms, invisible to ordinary vision. Leuwenhoek, the father of microscopy, and whose researches were generally known and accepted in 1675, tells of his discovery of extremely minute organisms in rain-water, in vegetable infusions, in saliva, and in scrapings from the teeth; further, he differentiated these living organisms by their size and form, and illustrated them by means of woodcuts; and there can be no doubt that his figures are intended to represent leptothrix filaments, vibrios, and spirilla. In other of his writings attempts are made to give an idea of the size of these “animalcules”; he described them as _a thousand times smaller than a grain of sand_. From his investigations a belief sprung up that malaria was produced by “animalcules,” and that the plague which visited Toulon and Marseilles in 1721 arose from a similar cause. Somewhat later on the natural history of micro-organisms was more diligently studied, and with increasing interest. Müller, in 1786, pointed out that they had been too much given to occupy themselves in finding new organisms, he therefore devoted himself to the study of their forms and biological characters, and it was on such data he based a classification. Thus the scientific knowledge gained of these minute bodies was considerably advanced, and the subject now entered upon a new phase: the origin of micro-organisms. It further resolved itself into two rival theories--spontaneous generation, and development from pre-existing germs--the discussion over which lasted more than a century. Indeed, it only ended in 1871, when the originator of the Abiogenesis theory withdrew from the contest, and the more scientific investigations of Pasteur (1861) found general acceptance. This indefatigable worker had been investigating fermentation, and studying the so-called diseases of wines and a contagious disease which was committing ravages among silkworms. Pasteur in time was able to confirm the belief that the “muscadine disease” of silkworms was due to the presence of micro-organisms, discernible only by the microscope. The oval, shining bodies in the moth, worm, and eggs had been previously observed and described by Nägeli and others, but it was reserved for Pasteur to show that when a silkworm whose body contained these organisms was pounded up in a mortar with water, and painted over the leaves of the tree upon which healthy worms were fed, all took the disease and died.

AFTER D^R CROOKSHANK _J. T. Balcomb. del._

TYPICAL FORMS OF BACTERIA, SCHIZOMYCETES, OR FISSION-FUNGI.]

As the contagious particles were transmitted to the eggs, the method adopted for preventing the spread of the disease was as follows:--Each female moth was kept separate from the others, and allowed to deposit her eggs, and after death her body was crushed up in a mortar as before, and a drop of the fluid examined under the microscope. When any trace of muscadine was found present, the whole of the eggs and body were burnt. In this way the disease was combated, and ultimately stamped out.

Pasteur also pointed out that one form or cause of disease must not be confounded with another. For example, muscadine, a true fungus (_Botrytis bassiana_), should not be confounded with another disease known to attack silkworms, termed _pebrin_, this being caused by a bacterium, and, according to the more recent researches of Balbiani, by a Psorospermia. Botrytis is a true mould, belonging to the Oomycetes, and allied to the potato fungus, Peronospora. It is propagated by spores, which, falling on a silkworm, germinate and penetrate its body. A mycelium is then developed, which spreads throughout the body. Hyphæ appear through the skin, and bear white chalky-looking spores; these become detached, and float in the air as an impalpable dust-like smoke. Damp further develops the fungus.

Insects suffer much from the ravages of fungi. The house-fly sticking to the window-pane is seen to be surrounded by the mycelia of _Penicillium racemosum_ (_Sporendonema muscæ_, or _Saprolegnia feræ_). In other cases Cordiceps attacks certain caterpillars belonging to the genera Cossus and Hepialus when they are buried in the sand and before their metamorphosis into chrysalides; they are killed by the rapid development of hyphæ and mycelium in their tissues.

_Sphæria miletaris_, a parasite of _Bombyx pilyocarpa_, the caterpillar of which is found on pine-trees, is one of the few fungi which may be regarded as beneficial to man, since it aids in the destruction of multitudes of these caterpillars, which otherwise would devour the young shoots and pine needles. Giard specialises other parasites of insects, which he terms Entomophoreæ. Others, _E. rimosa_, attack grasshoppers and the diptera, enveloping them in a dense coating of mycelium and spores, which speedily kills the victim.

The study, then, of the life-history of germs, microbes, micro-organisms, or bacteria (as they are indifferently termed), opened up a new science, that of Bacteriology. By the more recent advances in this science we are enabled to understand the very important part these minute organisms fill in the great scheme of Nature, for almost exclusively by their agency the soil is supplied with the requisite nutritive material for plant life. And, as already pointed out, wherever organic matter is present--that is, the dead and useless substances which are the refuse of life--such material is promptly seized upon by micro-organisms, by means of which histolysis is rapidly accomplished.

Bacteria require a power of from 600 to 1,000 diameters or more for the determination of the species to which they belong. The number of species has been so much increased of late that a bulky volume is found to be insufficient for their enumeration. I am, however, by the courtesy of Professor Crookshank, enabled to present my readers with the typical forms of thirty-nine species of Bacteria, Schizomycetes, or fission-fungi, a selection, it will be seen, chiefly taken from among pathogenic organisms--those believed to originate disease. But many of the supposed _Saprophytic_ forms often described as originating disease are merely accidental associates, that is, living in companionship for a time.

_Size._--In ordinary terms of measurement, bacteria are on an average from 1/25000th to about 1/5000th of an inch long. These measurements do not convey a definite impression to the mind. It is calculated that a thousand million of them could be contained in a space of 1/25th of an inch. The best impression of the size of the bacteria is, perhaps, obtained when it is stated that a 1/25-inch immersion objective gives a magnification of nearly 2,200 diameters, and that under this power the bacteria appear to be about the size of very small print. The standard of measurement accepted by bacteriologists is the micro-millimeter. One millimeter is equal to about 1/25000th an English inch. The number of micrococci in a milligramme of a culture of _Staphylococcus pyogenes aurens_ has been estimated by Bujwid by counting at eight thousand millions. Not only do various species differ in dimensions, but considerable differences may be noted in a pure culture of the same species. On the other hand, there are numerous species which so closely resemble each other in size and shape that they cannot be differentiated by microscopic examination alone, and we have to look to other characteristics, as colour, growth in various culture media, pathogenic power, chemical products, &c., in order to decide the question of identity.

_Reproduction._--The reproduction of bacteria takes place for the most part by fission and by spore formation. _Fission_ is a process of splitting up or division, whereby an organism divides into two or more parts, each of which lives and divides in its turn. If certain organisms are watched under the microscope, a coccus or bacillus will be seen to elongate and at the same time become narrower, until its two halves become free, the two individual organisms again dividing and subdividing in their turn. This kind of reproduction is more readily seen in a higher class of unicellular organisms, the desmids. If, however, the new organisms do not break away from each other, but remain connected in groups or clusters, they are termed Staphylococci; if they remain connected in the form of a chain, or like a string of beads, they are termed Streptococci. If the division takes place in one plane, Diplococci are formed; if in two directions Tetracocci, or Tablet-cocci, are formed. On account of this multiplication by fission, the generic name of Schizomycetes, or fission-fungi, has been given to bacteria.

_Spores._--A second method by which bacteria propagate is by spores. These bodies are distinguished by their remarkable power of resistance to the influence of temperature and the action of chemical reagents. Some of them will resist their immersion in strong acid solutions for many hours; also freezing and very high temperatures. Spore formation may take place in two ways: firstly, by “endogenous spores” (internal spores); secondly, by “arthrospores.”

_Endogenous Spores._--When the formation of the spores takes place in the mother-cell, the protoplasm is seen to contract, giving rise to one or more highly refractive bodies, which are the spores. The enclosing membrane of the organism then breaks away, leaving the spores free.

_Arthrospores._--When the spore is not formed in the parent bacillus, but when entire cells (owing to lack of favourable conditions of growth) become converted into spores, the formation is known as “arthrogenous,” the single individual being called an arthrospore. When the conditions are again favourable, spores germinate, giving rise to new bacilli. The germinating spore becomes elongated, and loses its bright appearance, the outer membrane becomes ruptured, and the young bacillus is set free. Certain conditions, such as the presence of oxygen in the case of the anthrax bacillus, give rise to the formation of spores; while various kinds of bacteria secure continuous existence by developing spores when there is lack of proper food material.

With reference to the incredible rapidity with which the bacteria multiply under conditions favourable to the growth and development, Cohn writes as follows:--“Let us assume that a microbe divides into two within an hour, then again into eight in the third hour, and so on. The number of microbes thus produced in twenty-four hours would exceed sixteen and a half millions; in two days they would increase to forty-seven trillions; and in a week the number expressing them would be made up of fifty-one figures. At the end of twenty-four hours the microbes descended from a single individual would occupy 1/40th of a hollow cube, with edges 1/25th of an inch long, but at the end of the following day would fill a space of twenty-seven cubic inches, and in less than five days their volume would equal that of the entire ocean.”

Again, Cohn estimated that a single bacillus weighs about 0·000,000,000,024,243,672 of a grain; forty thousand millions, 1 grain; 289 billions, 1 pound. After twenty-four hours the descendants from a single bacillus would weigh 1/2666th of a grain; after two days, over a pound; after three days, sixteen and a half million pounds, or 7,366 tons. It is quite unneccessary to state that these figures are purely theoretical, and could only be realised if there were no impediment to such rapid increase.

Fortunately, however, various checks, such as lack of food and unfavourable physical conditions, intervene to prevent unmanageable multiplication of these bodies.

These figures show, however, what a tremendous vital activity micro-organisms do possess, and it will be seen later at what great speed they increase in water, milk, broth, and other suitable media.

The following bacilli, among others, have numerous flagella distributed over the whole of the organism: the bacillus of blue milk (_Bacillus cyanogenus_)[52]; the bacillus of malignant œdema; the hay bacillus (_Bacillus subtilis_); _Proteus vulgaris_, &c.

The following have only one or two flagella at the poles: the _Bacillus pyocyaneus_, the _Spirillum finkleri_, the _Spirillum choleræ Asiaticæ_, &c.

The _Spirillum undala_, _Spirillum rubrum_, _Spirillum concentricum_, and _Sarcinæ_, pocket-cocci, have several flagella.

_Micrococcus agilis_ have also several flagella; these possibly arise from one point. As I have already pointed out, the _classification_ of the bacteria is one of great difficulty, since new kinds are being constantly discovered, and at present any attempt made in this direction can only be considered as quite of a provisional nature.

The difficulties which stand in the way may be surmised from the fact that _Sarcinæ_, pocket-cocci, were originally believed to be a single species, described by me, under the name of _Sarcina ventriculi_, in the fourth edition of my book, “as remarkable bodies invading the human and animal stomach, and seriously interfering with its functions.”

The original woodcut of these curious parasites is reproduced in Fig. 268, also in Plate IX., No. 7, and which evidently belong to a different species, numbering thirty-nine altogether. Quite recently Mr. G. H. Broadbent, M.R.C.S., Manchester, sent me a supply of these interesting bodies lately discovered by him in an infusion of cow manure. On examining a drop with a power of 1500 diameters they were discovered moving over the field of the microscope with a gyrating motion by the aid of flagella projecting from each corner of the pocket. After some days, having attained their full growth of four, eight or sixteen in a pocket, they break up, and recommence the formative process. Sarcinæ are certainly pathogenic in their nature. Cocci in groups, or asso-cocci, are similarly associated. These several forms of spiro-bacteria are enclosed in a transparent cell-wall, and are sometimes described as zooglæa.

Of bacteria the most characteristic groups are bacillus, bacterium, and a species of clostridium, a bottle-shaped bacillus. It is, however, difficult to draw a sharp line between so-called _species_.

_Spiro-bacteria_, or _spirilla_, possess short or long filaments, rigid or flexible, and their movements are accordingly rotatory, or in the long axis of the filaments. These bodies are again divided into comma bacilli, or vibrios--a name invented by the older microscopists who first described them--some species of which have a flagellate appendage, to which their movements are due.

=Anthrax=, =Splenic Fever=, has been long known to be prevalent among cattle at certain seasons of the year, and is believed to originate from peculiar conditions of climate and soil. This view of splenic fever on microscopical examination proved an entire fallacy. Bollinger in 1872 discovered that the blood of the affected animal was still virulent after death, owing to the presence of the _spores_ of the bacillus, and that the soil also became infected and impregnated by the disease germs wherever the fever first broke out. In 1877 Dr. Koch made a more careful investigation into the source of the disease, and was able to give a complete demonstration of the life-history of the splenic fever bacillus, and to offer definite proofs of its pathogenic properties. He pointed out that the rods grew in the blood and tissues by lengthening and by cross division. Further, that they not only grew into long leptothrix filaments but they produced enormous numbers of seeds or spores. He watched the fusion of the rods to the formation of spores and the sprouting of fresh rods. He furthermore inoculated a mouse, watched the effect through several generations, and fully demonstrated that in the blood and swollen spleen of the animal the same rods were always present. Pasteur and Paul Bret pursued the same course of investigations, which were always followed with precisely similar results. It was, however, principally due to the researches of Koch that the doctrine of _contagium vivum_ was placed on a scientific basis.

Subsequently Koch formulated methods of cultivation, and dictated the microscopical apparatus needful. Furthermore, he furnished postulates for proving beyond doubt the existence of specific pathogenic micro-organisms.

“The chain of evidence regarded by Dr. Koch as essential for proving the existence of a pathogenic organism is as follows:--1. The micro-organism must be found in the blood, lymph, or diseased tissue of man or animal suffering from, or dead of the disease. 2. The micro-organism must be isolated from the blood or tissue, and cultivated in suitable media--_i.e._, outside the animal body. These pure cultivations must be carried on through successive generations of the organism. 3. Pure cultivation thus obtained must, when introduced into the body of a healthy animal, produce the disease in question. 4. In the inoculated animal the same micro-organism must again be found. The chain of evidence will be still more complete if, from artificial culture, a chemical substance is obtained capable of producing the disease quite independently of the living organism. It is not enough to merely detect, or even artificially cultivate, a bacterium associated with disease. An endeavour must be made to establish the exact relationship of the bacteria to disease processes. In many instances disease bacteria regarded as the actual contagia have been found, on a further searching inquiry, to be entirely misleading. It is almost needless to remind the enthusiast that the actual contagion of the disease must be fully demonstrated.”

_Typhoid Bacillus_ (Fig. 269).--Rods 1 to 3µ in length, and ·5 to ·8µ in breadth, and threads. Spore-formation has not been observed, but the protoplasm may be broken up, producing appearances which may be mistaken for spores. Actively motile, provided, some with a single and others with very numerous flagella, which are from three to five times as long as the bacillus itself. They stain readily in aqueous solutions of aniline dyes; and grow rapidly at a temperature of about 60° Fahr. In plate cultivations minute colonies are visible in thirty-six to forty-eight hours; they are circular or oval, with an irregular margin. On agar they form a whitish transparent layer, and they flourish in milk.

_The Plague_ (_Pestis Bacillus_).--The Bombay plague of 1897-98 will ever be remembered as one of the most appalling visitations ever known. The number of deaths will never be accurately determined, as the native population, among whom the disease chiefly prevailed and became so fatal, concealed their dead or carried them away by night. The outbreak from the first proved to be most infectious, its incubation lasting from a few hours to a week only. It prevailed in all the over-crowded native quarters of the city. The rats and mice that infested the dwellings of the poor were found to be equally susceptible with human beings, and these vermin also died by hundreds. Those that survived left their holes and made off, in this way helping to spread the infective virus. On examining the bodies of dead rats, they were found to have swollen legs, the blood being filled by bacilli and curious monads, with whip-like appendages. The bacillus of plague was discovered by Kitasato in 1894; it is characterised by short rods with rounded ends, and a clear space in the middle. The bacilli stain readily with aniline dyes, and when cultivated on agar, white transparent colonies are formed which present an iridescent appearance when examined by reflected light. In addition to the bubonic swellings, the neighbouring lymphatic glands were also swollen and blocked by bacilli.

_a_. Monad threading its way among the blood-corpuscles; _b_. Another with pendulum movement attached to a corpuscle; _c_. Angular forms; _d_. Encysted forms; _e_ and _f_. The same seen edgeways.]

My illustration (Fig. 270) is from a micro-photograph taken in 1897, when the death rate stood very high. The general distribution of the bacilli, together with phagocytes and the contents of swollen lymphatic glands, magnified 1,200 ×, is from a preparation made in hospital. The monads from the rat’s blood, 1200 ×, seen threading their way among the blood corpuscles of a rat, and represented in Fig. 271, are somewhat larger than those found in the Bombay rats, but the flagella in the latter were quite as marked, while the encysted forms were wholly absent and the blood corpuscles less crenated. The white bodies (Fig. 270) were in some preparations, together with the lymphatic bodies, more numerous and more swollen.

With regard to the conditions of life of the bacteria, they may be divided broadly into two classes. When the organisms draw their nourishment from some living body or “host,” they are known as “parasites.” These are further termed “obligate” parasites if they exclusively live on their “host.” If the bacteria draw their nourishment from dead organic matter, they are called “saprophytes.” These are also divided into “obligate” and “facultative” saprophytes. Thus it will be apparent that a parasite under certain circumstances may readily become a saprophyte.

Some of the more important saprophytes are those organisms which play an important and useful part in our every-day life, such as, for instance, in the phenomena associated with fermentation, and putrefaction agents which transform dead and decomposing organic matter into their simpler elements, thus completing the great life cycle, and rendering the dead and effete matter again ready for the vital processes.

Among other life manifestations of certain bacteria may be mentioned those which have the property of generating colouring matter, though not chlorophyll. The bacteria themselves are colourless and transparent, and the pigment is merely formed as a product of their metabolism, especially under the influence of light. Many of the bacteria give rise to various gases and odours, particularly the anærobic organisms, which originate those foul putrefactive gases (ammonia, sulphuretted hydrogen, &c.). The blood-rain, _Micrococcus prodigiosus_, gives off an odour resembling trimethylamin. Micro-organisms have the property of producing various changes in the medium on which they are grown. In many cases albuminous bodies are peptonized and gelatine is liquefied. Many bacteria have the faculty of resolving organic bodies into their simplest elements; others, again, have the property of converting ammonia into nitric and nitrous acid. Certain microbes have the property of becoming phosphorescent in the dark. These phosphorescent bacteria are often seen on decaying plants and wood; sometimes in tropical climates the sea becomes luminous owing to the presence of countless numbers of these organisms. Again, they are frequently seen on the surface of dead fish, particularly mackerel, which often become so bright as to strongly illuminate the cupboard in which they lie.

The particular class of fungi that produce disease in man and the higher animals are generally known as “pathogenic.” These pathogenic organisms may exert their pernicious power in several ways. They may be injurious on account of their abstracting nourishment from the blood or tissues, or for the purely mechanical reason of their stopping up the minute capillaries and blood-vessels by their excessive multiplication. But the poisonous action of most of the pathogenic bacteria is due to the chemical products secreted by the organisms, and it is to the circulation and absorption within the body of these poisons that the disturbances of the animal system, which characterise disease, decay, and dissolution of every organism, must be traced.

Parasitic Diseases of Plants.

The subject of fungoid diseases and fungus epidemics are of worldwide interest, if only because of the annual losses to agriculturists from parasitic diseases of plants, amounting to millions of pounds sterling. The history of wheat-rust, and that of oats and rye, each equally susceptible to the ravages of the same Rufus, can be traced back to Genesis. A description of it was given in 1805 by Sir Joseph Banks. He suggested that the germs entered the stomata, and he warned farmers against the use of _rusted_ litter, and made important experiments on the sowing of rusted wheat-grains. A great discussion on the barberry question followed, Fries particularly insisting on the difference between _Æcidium berberidis_ and _Puccinia graminis_. Tulasne confirmed the statement made by Henslow that the uredo and puccinia stages belong to the same fungus, and are not mixed species. De Bary’s investigations in 1860-64 proved that the _sporidia_ of some Uredinieæ (_e.g._, _Coleosporium_) will not infect the plant which bears the spores, and that the æcidia of certain other forms are stages in the life-history of species of Uromyces and Puccinia. Furthermore, De Bary in 1864 attacked the question of wheat rust, and by means of numerous sowings of the telentospores on barberry proved that they bring about the infection.

This led to the discovery of the phenomenon of _Heterœcism_ (colonisation), introducing a new idea, and clearing up many difficulties. In 1890 the rust question entered on a new phase: it was taken up by men of science all over the world, and active inquiries were set on foot. The result has been the confirmation of De Bary’s results, but with the further discovery that our four common cereals are attacked by no less than ten different forms of rust belonging to five separate species or “form species,” and with several physiological varieties, capable of turning the table upon the barberry by infecting it. Some of these are found to be strictly confined to one or other of the four common cereals, infecting two or more of them, while others can infect various kinds of our common wild grasses.

_a._ _Aregma speciosum_; _b._ _Xenodochus paradoxus_; _c._ _P. Amorphæ_; _d._ _Triphœmium dubens_; _e._ Younger spores; _f._ _P. lateripes_; magnified 450 diameters.]

The fact is, that what has usually gone by the name of _Puccinia graminis_ is an aggregate of several species, and that varietal forms of this exist so especially adapted to the host, that, although no morphical differences can be detected between them, they cannot be transferred from one cereal to another, pointing to physiological variations of a kind met with among bacteria and yeasts, but hitherto unsuspected in these higher parasitic fungi. It now appears we must be prepared for similar specialisation of varietal forms among Ustilagineæ as well as among Uredineæ.

Moreover, it has been found that different sorts of wheat, oats, barley, and rye are susceptible to their particular rusts in different degrees, at the bottom of which, it is suggested, there must be some complex physiological causes. De Bary gave proof, in 1886, that Peziza (Plate I., Nos. 1, 4, 5, 6) succeeds in becoming parasitic only after _saprophytic_ culture to a strong mycelium, and that its form is altered thereby--probably by the excretion of a poison. Professor Marshall Ward showed that similar results took place in the case of the lily disease. Reinhardt, in 1892, showed that the apical growth of a peziza is disturbed and interrupted if the culture solution is employed concentrated; and Büsgen, in 1893, showed that _Botrytis cinerea_ excretes poison at the tips of the hyphæ, thus confirming Professor Ward’s results with the lily disease in 1888, and of later years, that a similar excretion occurs in rust-fungus. He further found that the water contents of the infected plant exercises an influence, as in the case of _Botrytis_ attacking chrysanthemums and other plants in the autumn, and that cold increases the germinating capacity of the spores.

Pfeiffer, in his work on “_Chemotaxis_,” shows that bacteria will congregate in the neighbourhood of an algal cell evolving oxygen. He also found that many motile antherozoids, zoospores, bacteria, &c., when free to move in a liquid, are attracted towards a point whence a given chemical substance is diffusing. He was concerning himself less with the evolution of oxygen or movements of bacteria than with a fundamental question of stimulation to movement in general. He found the attractive power of different chemical substances vary with the organism, and that various other bodies beside oxygen attract bacteria--peptone, dextrose, potassium salts, &c.; that swarm spores of the fungus _Saprolegnia_ are powerfully attracted towards the muscles of a fly’s leg placed in the water in which they are swimming about; also, that in many cases where the hyphæ of fungi suddenly and sharply bend out of their original course to enter the body of a plant or animal, the cause of the bending lies in a powerful chemotropic action, due to the attraction of some substance escaping from the body. Professor Ward has seen zoospores of a _Pythium_ suddenly dart out on to the cut surface of a bean-stem, and there fix themselves.

This will be better understood by referring to the course pursued by these bodies generally. When the spore of a parasitic fungus settles on a plant, it frequently behaves as follows:--The spore germinates and forms a slender tube of delicate consistency, blunt at the end, and containing colourless protoplasm, as shown, highly magnified in Fig. 272, and in Figs. 273 and 274 much less magnified. De Bary long ago showed that such a tube--the germinal-hypha--only grows for a short time along the surface of the organ, and its tip soon bends down and enters the plant, either through one of the stomata or by boring its way directly through the cell-walls. Professor Ward says these phenomena suggested to himself that the end of the tube is attracted in some way, and by some force which brings its tip out of the previous direction, and De Bary has suggested that this attraction is due to some chemical substance excreted by the host plant. It is remarkable with what ease the tube penetrates the cell-walls, and which Ward believes to be due to the solvent action of an enzyme, capable of dissolving cellulose.

“Miyoshi carried these observations a step further when, in 1894, he showed that if a leaf is injected with a substance such as ammonium-chloride, dextrine, or cane-sugar (all substances capable of exerting chemotropic attraction on fungus-hyphæ), and spores of a fungus which is _not parasitic_ are then sown on it, the hyphæ of the fungus penetrate the stomata and behave exactly as if the fungus were a true parasite.

“So surprising a result lets in a flood of light on many known cases of fungi, which are, as a rule, _non_-parasitic, becoming so, in fact, only when the host plant is in an abnormal condition, _e.g._, the entry of species of Botrytis into living tissues when the weather is cold and damp and the light dull; the entry of Mucor into various fruits, tomatoes, apples, pears, &c., when the hyphæ meet with a slight crack or wound, through which the juices are exposed. It is exceedingly probable that the rapid infection of potato leaves in damp weather in July is traceable not merely to the favouring effect of the moisture on the fungus, but that the state of super-saturation of the cell-walls of the potato leaf--the tissues of which are now unduly filled with water and dissolved sugars, &c., owing to the dull light and diminished transpiration--is the primary factor which determines the easy victory of the parasite, and, as Professor Ward suggested some time ago, that the suppressed life of Ustilagineæ in the stems of grasses is due to the want of particular carbo-hydrates in the vegetative tissues, but which are present in the grain. A year later Miyoshi carried proof to demonstration, and showed that a fungus-hypha is actually so attracted by substances on the other side of a membrane, and that its tip pierces the latter; for the hyphæ were made to grow through films of artificial cellulose, of collodion, of cellulose impregnated with paraffin, of parchment paper, and even the chitinous coat of an insect, simply by placing the intact films on gelatine impregnated with the attracting substance, and laying the spores on the opposite side of the membrane.

“Now this is obviously a point of the highest importance in the theory of parasitism and parasitic diseases, because it suggests at once that in the varying conditions of the cells, the contents of which are separated only by membranous walls from the fungus-hyphæ, whose entrance means ruin and destruction, there may be found circumstances which sometimes favour and sometimes disfavour the entrance of the hyphæ; and it is, at least, a remarkable fact that some of the substances which experiments prove to be highly attractive to such hyphæ--_e.g._, sugars, the sap of plums, phosphates, nitrates, &c.--are just the substances found in plants; and the discovery that the action depends upon the nature of the substance as well as on the kind of fungus, and is affected by its concentration, the temperature, and other circumstances, only confirms us in this idea.”