PLATE IX.AFTER DRCROOKSHANKJ. T. Balcomb. del.TYPICAL FORMS OF BACTERIA, SCHIZOMYCETES, OR FISSION-FUNGI.
PLATE IX.
AFTER DRCROOKSHANKJ. 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, thewhole 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, termedpebrin, 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 ofPenicillium racemosum(Sporendonema muscæ, orSaprolegnia 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 ofBombyx 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 uselesssubstances 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 supposedSaprophyticforms 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 from1⁄25000th to about1⁄5000th 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 of1⁄25th of an inch. The best impression of the size of the bacteria is, perhaps, obtained when it is stated that a1⁄25-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 about1⁄25000th an English inch. The number of micrococci in a milligramme of a culture ofStaphylococcus pyogenes aurenshas 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.Fissionis 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. Ifcertain 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 occupy1⁄40th of a hollow cube, with edges1⁄25th 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 weigh1⁄2666th 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: theBacillus pyocyaneus, theSpirillum finkleri, theSpirillum choleræ Asiaticæ, &c.
TheSpirillum undala,Spirillum rubrum,Spirillum concentricum, andSarcinæ, pocket-cocci, have several flagella.
Micrococcus agilishave also several flagella; these possibly arise from one point. As I have already pointed out, theclassificationof 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 thatSarcinæ, pocket-cocci, were originally believed to be a single species, described by me, under the name ofSarcina ventriculi, in the fourth edition of my book, “as remarkable bodies invading the human and animal stomach, and seriously interfering with its functions.”
Fig. 268.—Sarcinæ.
Fig. 268.—Sarcinæ.
The original woodcut of these curious parasites is reproduced inFig. 268, also inPlate 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-calledspecies.
Spiro-bacteria, orspirilla, 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 microscopistswho 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 thesporesof 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 ofcontagium vivumwas 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 ofa 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.”
Fig. 269.—Micro-Photograph of Typhoid Fever Bacteria. Magnified 1000 ×. Taken by Leitz’s oil immersion1⁄12-inch ocular No. 4, and sunlight exposure of one minute.
Fig. 269.—Micro-Photograph of Typhoid Fever Bacteria. Magnified 1000 ×. Taken by Leitz’s oil immersion1⁄12-inch ocular No. 4, and sunlight exposure of one minute.
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 temperatureof 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.
Fig. 270.—Plague Bacillus, Bombay, 1897. Magnified 1200 ×.
Fig. 270.—Plague Bacillus, Bombay, 1897. Magnified 1200 ×.
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 wayhelping 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.
Fig. 271.—Monads in Rat’s Blood, 1,200 ×. (Crookshank.)a. Monad threading its way among the blood-corpuscles;b. Another with pendulum movement attached to a corpuscle;c. Angular forms;d. Encysted forms;eandf. The same seen edgeways.
Fig. 271.—Monads in Rat’s Blood, 1,200 ×. (Crookshank.)
a. Monad threading its way among the blood-corpuscles;b. Another with pendulum movement attached to a corpuscle;c. Angular forms;d. Encysted forms;eandf. 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 inFig. 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 drawtheir 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.
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 ofrustedlitter, 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 betweenÆcidium berberidisandPuccinia 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 thesporidiaof 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 ofHeterœcism(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 areattacked 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.
Fig. 272.—Puccinia, displayinguredosporesandtelentospores.a.Aregma speciosum;b.Xenodochus paradoxus;c.P. Amorphæ;d.Triphœmium dubens;e.Younger spores;f.P. lateripes; magnified 450 diameters.
Fig. 272.—Puccinia, displayinguredosporesandtelentospores.
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 ofPuccinia graminisis 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 aftersaprophyticculture 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 thatBotrytis cinereaexcretespoison 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 ofBotrytisattacking 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 fungusSaprolegniaare 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 aPythiumsuddenly 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 inFig. 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 whichbrings 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 isnot parasiticare 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.”
Moreover, there is one other fact which it is important to notice, viz., that there are substances which repel instead of attract the hyphæ. Is it not, then, asks Professor Ward, natural to conclude that the differences in behaviour of different parasites towards different host-plants, and towards the same host-plant under different conditions, probably depend on the chemotropic irritability of the hyphæ towards the substance formed in the cells on the other side of the membranous cell-walls? And when, as often happens, the effusion of substances, such as the cells contain, to the exterior is facilitated by over-distension and super-saturation, or by actual wounds, we cannot be surprised at the consequences when a fungus, hitherto unable to enter the plant, suddenly does so. To this proposition my answer is emphatically in the affirmative, since in my investigations into the “fungus-foot disease” (“Mycetoma”), 1871, of India, the entry of the fungus was in almost every case shown to be through an abrasion of the skin or a direct open wound; the majority of the cases reported were among the agricultural classes. When, then, as often happens, the effusion of substances, such as the cells contain, to the exterior is facilitated by over-distension and super-saturation, or by actual wounds, we cannot be surprised at the consequences when a fungus, hitherto unable to enter the plant, suddenly does so. Nevertheless, it must be admitted that the knowledge gained of parasites does not satisfactorily account for epidemic visitations over large areas.
Fig. 273.—Fungi and Moulds.Description of Figures.—a.Fungi Spores, taken in a sick chamber;b.Aspergillus glaucus;c.Yeast, recent state;d.Exhausted yeast, budding;e.Penicillium spores more highly magnified;g.Aerobic spores and mould mycelium;h.Aspergillus spore, grown on melon.
Fig. 273.—Fungi and Moulds.
Description of Figures.—a.Fungi Spores, taken in a sick chamber;b.Aspergillus glaucus;c.Yeast, recent state;d.Exhausted yeast, budding;e.Penicillium spores more highly magnified;g.Aerobic spores and mould mycelium;h.Aspergillus spore, grown on melon.
Habitat.—The habitat of vegetable parasitic fungi is extremely variable. Fungi are found everywhere, living and flourishing on all the families of the vegetable and animal kingdoms. They attack our houses, foods, clothes, utensils of every kind, wall papers and books, the paste of which, to my astonishment, affords a sufficient supply of nourishment. Members of the parasitic tribe of bacteria, by a combined effort of countless myriads, have given rise to a sense of supernatural agency.Bacillus prodigiosus, described also asPalmella mirificaandZoogalactina imetropia, from its attacking milk and other alimentary substances, the spores of which are often of a deep red colour, have been found to cover whole tracts of country in a single night with what is called a “gory dew,” changing in daylight to a deep green colour. This was at one time regarded with superstitious awe as a miracle, as it has been known to attack bread and even the sacred wafer, and which in mediæval ages was described as the “bleeding-host.” This parasitic plant belongs to anærobic bacteria, and is only developed in the dark. The nitrogen required for nutritionmust be derived from the air. An algal form gives rise to the red scum seen in ponds and reservoirs in the autumn. The discharge from wounds is coloured blue byBacterium pyocyanine. There are many other forms, some of which have an orange colour, and the genus is recognised as “chromogenic microbes.”
Fig. 274.—Fungi and Moulds.Description of Figures.—d.Puccinia graminison wheat;c.Polycystis spore of rye-smut;f.Alder fungus spores,Microspheria penicellula;g.Dactylium roseum, rose-coloured mould;h.Verticillium distans, whorled mould found on herbaceous plants;i.Botrytis, vine and lily fungus;j, j′.Peronospora infestans, potato fungus;k.P. gangliformis, mould of herbaceous plants;l.VariousPenicilliumand other spores taken in a bean-field.
Fig. 274.—Fungi and Moulds.
Description of Figures.—d.Puccinia graminison wheat;c.Polycystis spore of rye-smut;f.Alder fungus spores,Microspheria penicellula;g.Dactylium roseum, rose-coloured mould;h.Verticillium distans, whorled mould found on herbaceous plants;i.Botrytis, vine and lily fungus;j, j′.Peronospora infestans, potato fungus;k.P. gangliformis, mould of herbaceous plants;l.VariousPenicilliumand other spores taken in a bean-field.
A cryptogam belonging to anærobic bacteria, described asProtococcus invalis, on being set aside in a bottle, and a little rain water added, was seen to set up spontaneous fermentation, and in a very short time exhibited remarkable activity. The colour of the infusion changed, it assumed a delicate pink hue in direct light, which deepened to a red in reflected light. The fluid contents were now observed to be dichoric, and the spectroscopic appearance subsequently presented was one of much interest. The spectrum was a well-marked one, and might be taken to determine the presence of a nitrogenous element or of glucose.
Among all the various plants known to suffer from the attacks of parasites, the vine has been the greatest sufferer. The oïdium, orErysiphe Tuckeri, so called from the name of the discoverer by whom it was first described, has been longest known to the vine grower. This really belongs to the group Ascomycetes, and appears to have been brought from America in 1845, whence it was passed on to France, where it soon threatened to entirely destroy the vineyards. This was followed by another parasite, belonging in this instanceto the animal kingdom,Phylloxera vastatrix. This oïdium appears on the grape in the form of greyish filaments, terminating in an enlarged head, which contains an agglomeration of spores, not free or in a chaplet, as in Aspergillus (Fig. 273). These spores when ripe burst from the capsule as fine dust, and are diffused by the air in all directions, thus spreading the disease far and away. Another of the parasitic moulds,Peronospora viticola, is a kind of mildew, differing from oïdium. The hyphæ penetrate more deeply than that of oïdium. On the upper surface of the leaf brown patches appear; these branch out and ramify as seen in the potato-fungus,P. infestans(Fig. 274). The parasite destroys the tissue of the leaf, and it withers and dies. There are other well-known parasites, the black-rot,Phomauvicola, belonging to the Ascomycetes. This appears in early shoots in the form of round black spots, and gradually spreads over leaves and young fruit. This same rot, one year, devastated the American vineyards.
Fig. 275.—Fungi, Moulds.a.Clustered Spores,Gonatobotrys simplex;b.Spore ofPuccinia coronata, the mildew of grapes;c.Barley smut;d.Puccinia althæa;e.Penicillium glaucum;m.Ixodes farinæ, found in damaged flour together with smut.
Fig. 275.—Fungi, Moulds.
a.Clustered Spores,Gonatobotrys simplex;b.Spore ofPuccinia coronata, the mildew of grapes;c.Barley smut;d.Puccinia althæa;e.Penicillium glaucum;m.Ixodes farinæ, found in damaged flour together with smut.
Cereals, wheats and grasses, suffer from other well-known forms of microscopic fungi termedrustsandsmuts, which cover the blades or infect the full ear of the fruit. The name given indicates their colour, and these belong, for the most part, to the genus Uredo and the family of the Basidiomycetes. They have no endogenous spores but as many as four forms of exogenous. This is also the case with wheat and barley, whereby they are distinguished asUredoorPuccinia graminis(see Figs. 273 and 274, andPlate I., Nos. 19 and 22,Æcidium berberidis). For a long time it was believed thatUredo linearisandPuccinia graminiswere so many distinct species, but it is now known that there are only three successive phases of the developmental stages of a single species—that, as a matter of fact, puccinia presents the phenomenon of alternation of generations, that is, that the complete development of the fungus is only effected by its transference from one plant to another. Other uredines, Ustilago and Tilletia smuts, are more apt to affect the ears of wheat, rye, and other grasses than puccinia. Bread made from wheat affected by smut has an acrid and bitter taste, while that made from rye flour often produces a serious form of disease. The propagation of either, then, should be stopped as quickly as possible by destroying all barberry bushes growing near or within the vicinity of corn fields, and by other means. The ergot of rye is due to distinct species of fungi having endogenous spores enclosed in a sac orascus, hence the name of the family, Ascomycetes orTuberaceæ, which are reproduced by the spores contained in these asci. Truffles belong to this family. But other members of the same family have several forms of spores, and these again present us with the phenomenon of alternation of generations.
Fig. 276.—Fungi, Moulds.p.Spores ofTilletia caries;q.Spores ofTilletia caries, when germinating, produce a fœtid olive-coloured spore in cereal grains;r.Telentospores ofPuccinia graminis;s.Crystopus candidus, spores growing in chains;t.Petronospora infestans, mildew of turnips, &c.;u.A transverse section of ergot of rye, showing spores in masses;v.Claviceps purpuræ, associated with ergoted rye.
Fig. 276.—Fungi, Moulds.
p.Spores ofTilletia caries;q.Spores ofTilletia caries, when germinating, produce a fœtid olive-coloured spore in cereal grains;r.Telentospores ofPuccinia graminis;s.Crystopus candidus, spores growing in chains;t.Petronospora infestans, mildew of turnips, &c.;u.A transverse section of ergot of rye, showing spores in masses;v.Claviceps purpuræ, associated with ergoted rye.
Ergot of rye is used in medicine, but if not used with care it will produce a dangerous disease. This parasitic fungi consists of minute microscopic masses of spores, which cover the young flower of therye with a white flocculent mass, formerly termedsphacelium. The mycelium formed spreads over the ear of corn in thick felt-like masses, termedsclerotis. The sphacelium changes its form in the following spring. Other changes are brought about, and it seems to pass through a cycle of alternations of generations.
Bread made from rye so infested is known to produce grave consequences, soon to become fatal if not detected in time. The disease is termed ergotism, and gangrene of the extremities takes place among people of the north of France and Russia, who consume bread made from rye flour. Ergot of maize will also cause similar diseases. Fowls and other animals fed upon this cereal become in a short time poisoned, and the cause of death is not rightly suspected. There is another fungus belonging to the same group of Ascomycetes, known asEurotium repens, which appears upon leather when left in a damp place, and also upon vegetable or animal substances if badly preserved, and gradually destroys it. This mould is of a darkish green colour.
The minute spores display themselves as rows of beads when fully ripe on the erect mycelium.Aspergillus glaucusrepresents the white exogenous spores of the sphacelium of the ergot of rye; and those subsequently produced in the yellow balls correspond with the asci developed in sclerotis, the endogenous species. Many of the parasitic species belonging to the generaErysiphe,Sphæria,Sordaria,Penicillium, &c., have a similar mode of propagation, and affect a large number of plants.
In the microscopical examinations especially given to the elucidation of parasitic diseases of the skin, previously referred to, I discovered more varieties of spores and filaments of certain cryptogamic plants associated with a larger number of specific forms of fungi than any previous observer. I did not, however, feel justified in concluding, with Küchenmeister, Schœnlein, and Robin, that these fungoid growths were the primary cause of the diseases referred to. Indeed, the foremost dermatologists of the period utterly refused to entertain the specific germ theory of the German investigators. Nevertheless, I contended, “the universality of theirdistribution is in itself a fact of very considerable importance, and one pointing to the belief that they are scavengers ever ready to fasten on decaying matter, and, on finding a suitable soil, spread out their invisible filaments in every direction in so persistent a manner as to arrest growth and overwhelm the plant in destruction.”53
Special forms of fungi are given inPlate I., Nos. 10-14, and those of the ascomycetes in Nos. 17-21.
Fig. 277.—Healthy fresh Yeast, from a large Brewery, in an active stage of formation, × 400.
Fig. 277.—Healthy fresh Yeast, from a large Brewery, in an active stage of formation, × 400.
Oïdium albicansaffects both animals and plants. It often attacks the mucous membrane of the mouths of young children. The spores become elongated and converted into hyphæ, and ramify about in all directions, producing a troublesome form of disease. This parasitic fungus is better known under another name,Saccharomyces mycoderma. Oïdium resemble algæ in their mode of life, as they are mostly found in a liquid media. The structure of all ferments is very simple: each plant is composed of a single cell, either of a spherical, elliptical, or cylindrical form, varying in size, and filled with protoplasmic and nucleated matter. This grows, and is seen to bud out and divide into two or more parts, all resembling the mother cell.
Fig. 277represents the healthy cells of yeast,Saccharomyces cerevisiæ, freshly taken from a brewer’s vat, and in an active stage of growth. The mode of multiplication continues as long as the plant remains in a liquid favourable to its nutrition.
The changes from one stage to another are rapid, as will be noticed on reference to the consecutive formative processes the cells are known to pass through,Fig. 278(1859).
If the development of the plant is arrested by want of a saccharine or nitrogenous substance, and the liquid dries up, theprotoplasm contained in the cell contracts, and the spores, or endogenous reproductive organs, of the plant will remain in a state of rest, become perfectly dry, and yet retain life. They are not easily killed, even when subjected to a very high or low temperature, they do not lose the power of germination when favourable conditions present themselves, and at once take on a new birth.
There are, however, many other ferments besides that of beer-yeasts, such as alcoholic and wine ferments, the commonest of which, according to Pasteur, isSaccharomyces ellipsoideus.