Fig. 278.—Development of Yeast Cells.1. When first taken; 2. One hour after introducing a few cells into sweet-wort; 3. Three hours after; 4. Eight hours; 5. Forty-eight hours, when the cells become elongated.
Fig. 278.—Development of Yeast Cells.
1. When first taken; 2. One hour after introducing a few cells into sweet-wort; 3. Three hours after; 4. Eight hours; 5. Forty-eight hours, when the cells become elongated.
But yeast-fungi and mould-fungi, like bacteria or fission-fungi, are micro-organisms, belonging to two specific orders, the Saccharomycetes and the Hyphomycetes, which are intimately related to each other, but quite distinct from bacteria. Their germs occur widely distributed in air, soil, and water. Many species are of hygienic, while others are of pathological interest and importance in being either accidentally associated with, or the cause of, disease processes, while others are fermentations of very essential service in various industrial processes. The making of beers, wines, and spirits, as we understand them,constitutes but a small part of the province of fermentation. The life activities of ferments open out a study of vast importance to mankind, and while they have only been regarded in their worst aspect—that of a bane—they are, nevertheless, a boon to mankind. The first clear view we obtained of this was that of Reess, who in 1870 showed there were several species or forms of the yeast-fungus. Hansen followed up this discovery in 1883, and, taking advantage of the strict methods of culture introduced by bacteriologists, found that by cultivating yeast on a solid media from a single spore it was quite possible to obtain constant types of pure yeasts, eachpossessing its own peculiar properties. One consequence of Hansen’s labours was that it now became possible for every brewer to work with a yeast of uniform type instead of with haphazard mixtures, in which serious disease forms might predominate and injure the beer. Among other things made clear was that a true yeast may have a mycelial stage of development. Furthermore, there is the influence exercised by the nucleus of the yeast cell. Many other points of interest arose out of these investigations; one was, that many higher fungi can assume a yeast-like stage of development if submerged in fluids, as, for instance, various species of Mucor, Ustilago, Exoascus, and numerous others. Ascomycetes, and Basidiomycetes as well, are known to form budding cells, and it was thought that the yeasts of alcoholic fermentation are merely reduced forms of these higher fungi, which have become habituated to the budding condition—a conclusion supported by Hansen’s discovery that a true Saccharomyces can develop a feeble, but a true, mycelium.
Fig. 279.—Saccharomyces and Moulds.1. Section from a tomato, showing spores growing from cuticle; 2. Portion detached to show budding-out process; 3. Lateral view of spore sac with oospores issuing forth; 4. Apiculated ferment spores; 6 and 7.Mycoderma cerivisiæin different stages of growth, as seen on wine bottles; 8 and 9.Torulæ diabeticæ, torulæ and fission spores.
Fig. 279.—Saccharomyces and Moulds.
1. Section from a tomato, showing spores growing from cuticle; 2. Portion detached to show budding-out process; 3. Lateral view of spore sac with oospores issuing forth; 4. Apiculated ferment spores; 6 and 7.Mycoderma cerivisiæin different stages of growth, as seen on wine bottles; 8 and 9.Torulæ diabeticæ, torulæ and fission spores.
“This view has been entirely confirmed by an inquiry into the mode of brewingsakéby the Japanese, by the aid of the Aspergillus fungus. Further researches established the fact that other forms of fungi,e.g., those on the surface of fruits, developed endogenous spores, which cause alcoholic fermentation. More recently, and by further experimental inquiry, partly by pure cultures of separate forms, and partly by well-devised cultures on ripening fruits still attached to the plant but imprisoned in sterilised glass vessels, it has been found that yeast and moulds are separate forms, not genetically connected, but merely associated in nature, as are so many other forms of yeasts, bacteria, and moulds. Further, Hansen has discovered that several yeasts furnish quite distinct races or varieties in different breweries in various parts of the world, so that we cannot avoid the conclusion that their race characteristics have been impressed on the cells by the continued action of the conditions of culture to which they have so long been exposed—they are, in fact, domesticated races.”
The environments of yeasts are peculiar. Sauer found that a given variety of yeast, whose activity is normally inhibited when the alcohol attains a certain degree of concentration in the liquid, can be induced to go on fermenting until a higher degree is attained by the addition of a certain lactic acid bacterium. The latter, indeed,appears to prepare the way for the yeast. It has been shown, also, that damage may be done to beers and wines by allowing plant germs to gain access with the yeast; there are, too, several forms of yeast that are inimical to the action of the required fermentation. Other researches show that associated yeasts may ferment better than any single yeast, and such symbiotic action of two yeasts of high fermenting power has given better results than either alone. English ginger-beer furnishes a curious symbiotic association of two organisms—a true yeast and a true bacterium—so closely united that the yeast cells become imprisoned in the gelatinous meshes of the bacterium; and it is a curious fact that this symbiotic union of yeast and bacterium ferments is far more energetic than either when used alone, and the product is different, large quantities of lactic and carbonic acids being formed, and little or no alcohol.
Many years ago I gave an account of similar curious symbiotic results obtained by introducing into a wort-infusion a small proportion ofGerman yeast, an artificial product composed of honey, malt, and a certain proportion of spontaneously-fermented wheat flour. This, to my astonishment, produced ten per cent. more alcohol than any of its congeners, and did not so soon exhaust itself as brewer’s yeast.54
In the hephir used in Europe for fermenting milk, another symbiotic association of yeast and a bacterium, it is seen that in this process no less than four distinct organisms are concerned. I have already instanced the fermentation of rice to produce saké, which is first acted upon by an Aspergillus that converts the starch into sugar and an associated yeast, and this is also shown to be a distinct fungus, symbiotically associated in the conversion. “Starting, then, from the fact that the constitution of the medium profoundly affects the physiological action of the fungus, there can be nothing surprising in the discovery that the fungus is more active in a medium which has been favourably altered by an associated organism, whether the latter aids the fungus by directly altering the medium, or by ridding it of products of excretion, or by adding gaseous or other body. It is not difficult to see, then, that natural selection will aid in the perpetuation of the symbiosis, and in caseslike that of the ginger-beer plant it is extremely difficult to get the two organisms apart, a difficulty similar to that in the case of the soredia of lichens.”
Buchner discovered that by means of extreme pressure a something can be extracted from yeast which at once decomposes sugar into alcohol and carbon-dioxide. This something is regarded as a kind of incomplete protoplasm—a body, as we have already seen, composed of proteid—and in a structural condition somewhere between that of true soluble enzymes like invertin and a complete living protoplasm. This reminds me of an older experiment of mine, the immediate conversion of cane-sugar into grape-sugar. If we take two parts of white sugar and rub it up in a mortar with one part of a perfectly dry solid, the German yeast before spoken of, it is immediately transformed as if by magic into a flowing liquid mass—a syrup. This process of forming “invert sugar” can be watched under the microscope; the liberation of carbonic acid gas in large bubbles is seen to go on simultaneously with the assimilation of the dextrose, and the breaking up of the crystals of sugar; the cell at the same time increasing in size as well as in refractive power; a curious state of activity appears to be going on in the small mass, which is very interesting to watch throughout.
However, the enzymes of Buchner are probably bits off the protoplasm, as it were, and so the essentials of the theory of fermentation remain, the immediate agent being not that of protoplasm itself, but of something made by or broken off from it. Enzymes, or similar bodies, are known to be very common in plants, and the suspicion that fungi do much work with their aid is abundantly confirmed. It seems, indeed, that there are a whole series of these bodies which have the power of carrying over oxygen to other bodies, and so bringing about oxidations of a peculiar character. These curious enzymes were first observed owing to studies on the changes which wine and plant juice undergo when exposed to the action of the oxygen of the air.
The browning of cut apples is known to be due to the action of an oxydase, that is, an oxygen carrying ferment, and the same is claimed for the deep colouring of certain lacs obtained from the juice of plants, such as Anacardiaceæ, which are pale and transparent when fresh drawn, but which gradually darken in colour on exposureto the air. Oxydases have been isolated from beets, dahlia, potato-tubers, and several other plants. This fact explains a phenomenon known to botanists, and partly explained by Schönbein as far back as 1868, that if certain fungi (e.g.,Boletus beridies) are broken or bruised, the yellow or white flesh at once turns blue; this action is now traced to the presence in the cell sap of an oxydase.
It is the diastatic activity of Aspergillus which is utilised in the making of saké from rice, and in the preparation of soy from the soja bean in Japan. Katz has recently tested the diastatic activity of Aspergillus, of Penicillium, and ofBacterium megatherium, in the presence of large and small quantities of sugar, and found all are able to produce not only diastase, but also other enzymes; as the sugar accumulates the diastase formed diminishes, whereas the accumulation of other carbo-hydrates produces no such effect. Harting’s investigation on the destruction of timber by fungi derives new interest from the discovery of an emulsion-like enzyme in many such wood-destroying forms, which splits up glucosides, amygdalin, and other substances into sugar, and that hyphæ feed on other carbo-hydrates. The fact, also, that Aspergillus can form inverts of the sucrase and maltase types, as well as emulsin, inulate, and diastase, according to circumstances of nutrition, will explain why this fungus can grow on almost any organic substance it may happen to alight upon. The secretion of special enzymes by fungi has a further interest just now, for recent investigations promise to bring us much nearer to an understanding of the phenomena of parasitism than it was possible when I was at work upon them some forty or fifty years ago.
It was De Bary who impelled botanists to abandon older methods, and he who laid the foundation of modern mycology. Later on he pointed out that when the infecting germinal tube of a fungus enters a plant-cell, two phenomena must be taken into account, the penetration of the cell-walls and tissues, and the attraction which causes the tips of the growing hypha to face and penetrate these obstacles, instead of gliding over them in the lines of apparent least resistance. The further development of these two factors shows that in the successful attack of a parasitic plant on its victim or host these fungi can excrete cellulose-dissolving enzymes, and that they have the power of destroying lignine. Zopf has also furnishedexamples of fungi which can consume fats. There is, however, one other connection in which these observations on enzymes in the plant-cell promise to be of considerable importance, viz., the remarkable action of certain rays of the solar light on bacteria. It has been known for some time past that if bacteria in a nutrient liquid are exposed to sunlight they quickly die. The further researches of Professor Marshall Ward and other workers in the same direction have brought out the fact that it is really the light rays, and not high temperatures, that it is especially the blue-violet and ultra-violet rays, which exert the most effective bactericidal action. This proof depended upon the production of actual photographs in bacteria of the spectrum itself. Apart from this, the Professor demonstrated that just such spores as those of anthrax, at the same time pathogenic and highly resistant to heat, succumb soonest to the action of these cold light-rays, and that under conditions which preclude their being poisoned by a liquid bath. It is in all probability the action of these rays of light upon the enzymes, which abound in the bacterial cells, that bring about their death.
The sun, then, is seen to be our most powerful scavenger, and this apparently receives confirmation in connection with Martinaud’s observations, that the yeasts necessary for wine-making are deficient in numbers and power on grapes exposed to intense light, and to this is due that better results are obtained in central France as contrasted with those in the south. “When we reflect, then, that the nature of parasitic fungi, the actual demonstration of infection by a fungus spore, the transmission of germs by water and air, the meaning and significance of polymorphism, heteræcism, symbiosis, had already been rendered clear in the case of fungi, and that it was by these studies in fermentation, and in the life-history of the fungus Saccharomyces, that the way was prepared for the ætiology of bacterial diseases in animals, there should be no doubt as to the mutual bearings of these matters.”
There are many industrial processes which are more or less dependent for success on bacterial fermentations. The subject is young, but the results already obtained are seen to be of immenseimportance from a scientific point of view, and to open up vistas of practical application already being taken advantage of in commerce, while problems are continually being raised by the forester, the agriculturist, the gardener, the dairyman, the brewer, dyer, tanner, and with regard to various industries, which will eventually confer great advantages in their economic application.
The remarkable discovery made by Alvarez of the bacillus, which converts a sterilised decoction of the indigo plant into indigo sugar and indigo white, the latter then oxidising to form the valuable blue dye, whereas the sterile decoction itself, even in presence of oxygen, forms no indigo, plainly proves how these minute organisms may be turned to a good account. There are, however, important points to be determined as to the action of the fermentation brought about by these enzymes, and the appearance of certain mysterious diseases in the indigo vats. Again, certain stages in the preparation of tea and tobacco leaves are found to depend upon very carefully regulated fermentations, which must be stopped at the right moment, or the product will be spoilt. Regarding the possiblerôleof bacteria, the West Indian tobacco has a special bacterium, which has been isolated and found to play a very important part in its flavour. Every botanist knows that flax and hemp are the best fibres of Linum and Cannabis respectively, separated by steeping in water until the middle lamella is destroyed and the fibres isolated; but it is not so well known thatnot every wateris suitable for this “retting” or steeping process; and for a long time this was as much a mystery as why some waters are so much better than others for brewing. Quite recently Fribes has succeeded in isolating the bacillus upon which the dissolution of the middle lamella depends. This investigation brought out other interesting details as to the reaction produced by living micro-organisms, and which can be utilised in deciding questions of plant chemistry too subtile for testing with ordinary re-agents. One other important fact connected with these researches is that botanists have now discarded the view that the middle lamella of the plants referred to is composed of cellulose, and know that it consists of pectin compounds. Fribes’ anærobic bacillus is found to dissolve and destroy pectins and pectinates, but does not touch cellulose or gum. It is well known that the steeping of skins in water in preparation for tanning involves bacterialaction, owing to which the hair and epidermal coverings are removed, but it also appears that in the process of swelling the limed skins, the gases evolved in the substance of the tissues, and the evolution of which causes the swelling and loosens the fibre so that the tanning solutions may penetrate, are due to a particular fermentation caused by a bacterium, which, according to some investigators, is identical with a lactic ferment introduced by the pine bark, and which is responsible for the advantageous acidification of the tanning solutions.
Hay is made in different ways, and in those where a “spontaneous” heating process is resorted to the fermentation is no doubt dependent upon the presence of thermogenic bacteria. But probably no other subject has attained to so much importance as the bacteriology of the dairy: the study of the bacteria found in milk, butter, and cheese in their various forms.
Of milk, especially, much has been written and said as a disease-transmitting medium, and with every good reason, and, if the statement of a Continental authority may be accepted that each time we eat a slice of bread and butter we devour a number of bacteria equal to the population of Europe, we have sure grounds for seeking for further information as to what these bacteria are and what they are doing. And similarly so with cheese, which teems with millions of these minute organisms.
“Some few years ago it was found that the peculiar aroma of butter was due to a bacterium. There are two species of bacteria, one of which develops an exquisite flavour and aroma, but the butter keeps badly, while the other develops less aroma, but the butter keeps better. In America, however, they have isolated and distributed pure cultures of a particular butter bacillus which develops the famous ‘June’ flavour, hitherto only met with in the butter made in a certain district during a short season of the year. This fine-flavoured butter is now constantly manufactured in a hundred American dairies; and the manufacture of pure butter with a constant flavour has become a matter of certainty.
“Properly considered, the manufacture of cheese is a form of ‘microscopic gardening’ even more complex and more horticultural in nature than the brewing of beer. From the first moment, when the cheesemaker guards and cools his milk, till his stock is readyhe is doing his best to keep down the growth of micro-organisms rushing about to take possession of his milk. He therefore coagulates it with rennet—an enzyme of animals, but also, as we have seen, common in plants—and the curd thus prepared is simply treated as a medium, on which he grows certain fungi and bacteria, with every needed precaution for favouring their development, and protecting them against the inroads of other pests and against unsuitable temperature, moisture, and access of light. Having succeeded in growing the right kind of plants on his curd, his art then demands that he shall stop their growth at the critical moment, and his cheese is ready for market.
“Furthermore, the particular flavour and peculiar odours of cheeses, as Camembert, Stilton, and Roquefort, have to be obtained, and this is secured, for instance, by cultivating a certain fungus, Penicillium, on bread, and purposely adding it to Roquefort. This is found to destroy the lactic and other acids, and so enables certain bacteria in the cheese to set to work and further change the medium; whereas in another kind of cheese the object is to prevent this fungus paving the way for these bacteria. Another kind of bacillus has been discovered which gives a peculiar clover aroma to certain cheeses.
“It is thought that more definite results will be obtained by the investigation of the manufacture of the vegetable cheeses of China and Japan, which are made by exposing the beans of the leguminous plant,Glycine—termed soja-beans—to bacterial fermentations in warm cellars with or without certain mould-fungi. Several kinds of bean-cheeses are made in this way, known by special names. They all depend upon the peculiar decompositions of the tissues of the cotyledons of the soja-beans, which contain 35 to 40 per cent. of proteids and quantities of fatty matter. The softened beans are first rendered mouldy, and the interpenetrating hyphæ render the contents accessible to certain bacteria, which peptonise and otherwise alter them. There is the further question of the manufacture of vinegar by fermentation, of the preparation of soy from a brine extract of mouldy and fermented soja-beans, of bread-making, and other equally interesting manufactures.”
“When the idea of parasitism was rendered definite by the fundamental distinction drawn by De Bary between aparasiteand asaprophyte, it soon became evident that some further distinction must be made betweenobligate facultativeparasites and saprophytes respectively. De Bary, when he proposed these terms for adoption, was clearly alive to the existence of transitions which we now know to be numerous and so gradual in character that we can no longer define any such physiological groups. Twenty years ago penicillium and mucor would have been regarded assaprophytesof the most obligate type, but we now know that under certain circumstances these fungi can become parasites, and the borderland between facultative parasites and saprophytes on the one hand, and between the former and true parasites on the other, can no longer be recognised.”
In 1866 the germ of an idea was sown which has taken root and extended. De Bary pointed out that in the case of lichens we have either a fungus parasite on an algæ, or else certain organisms hitherto accepted as algæ are merely incomplete forms.
“In 1879 the same observer definitely launched the new hypothesis ofsymbiosis. The word itself is due to Frank, who, in a valuable paper on the biology of the thallus of certain lichens, very clearly set forth the existence of various stages of life in common among all the lower forms of plants. The details of these matters are now principally of historical interest. We now know that lichens are dual organisms, composed of various algæ, symbiotic with Ascomycetes, with Basidiomycetes, and, as Massee has shown, even with Gastromycetes. The soil contains also bacterio-lichens. Hence arose a new biological idea—that a fungus may be in such nicely-balanced relationship with the host from which it derives its sustenance, that it may be attended with nearly equal advantage to both.
“In the humus of forests we find the roots of beeches and other Cupuliferæ (willows, pines, and so forth) clothed with a dense mantle of hyphæ, and swollen into fleshlike masses of mycorhiza. In similar soils, and in moorlands, which abound in the slowly decomposing root-fibres and other vegetable remains so characteristic ofthese soils, the roots of orchids, heaths, gentians, &c., are similarly provided with fungi, the hyphæ of which penetrate further into the tissues, and even send haustoria into the living cells, but without injuring them. As observations multiplied it became clear that the mycorhiza, or fungus-root, was not to be dismissed as a mere case of roots affected by parasites, but that a symbiotic union, comparable to that of the lichens, exists, and we must assume that both tree and fungus derive benefit from the connection.
Fig. 280.—Fine Section through Truffle.a.Asci filled with spores;b.Mycelia, × 250.
Fig. 280.—Fine Section through Truffle.
a.Asci filled with spores;b.Mycelia, × 250.
“Frank stated, as the result of his experimental research, that seedling forest-trees cannot be grown in sterilised soil, where their roots are prevented from forming mycorhiza; and he concluded that the fungus conveys organic materials to the roots, which it obtains by breaking down the leaf-mould and decaying plant remains, together with water and minerals from the soil, and plays the especial part of a nitrogen-catching apparatus. In return for this import service the root pays a tax to the fungus by sparing it certain of its tissue contents. It is a curious fact then thatthe mycorhiza is only formed where humus or vegetable mould abounds.”
These instructive investigations offer an intelligible explanation of the growth of that well-known subterranean fungus, the truffle (Tuber cibarium), the microscopic appearances of a section of which formed the subject of a paper I contributed to “The Popular Science Review” some years ago (1862). The fungus, as will be seen by the fine section cut through a truffle,Fig. 280, consists of flocculent filaments, which in the first instance cover the ground at the fall of the leaf in autumn, under oak or beech trees, the hyphæ of which penetrate the ground, through the humid soil to theroot-hairsof the tree. Filaments (mycelia) are again given off which terminate in asci or sacs filled with minute spores of about1⁄2500th of an inch in size, while the interspaces are filled up by mycelia, that become consolidated into a firm nut-like body.
What happens, then, is this: Trees and plants with normal roots and root-hairs, when growing in ordinary soil, can adapt their roots to life in a soil heavily charged with humus only by contracting symbiotic association with the fungus and paying the tax demanded by the latter in return for its supplies and services. If this adaptation is impossible, and no other suitable variation is evolved, such trees cannot grow in such soils. The physiological relations of the root to the fungus must be different in details in the case of non-green, purely saprophytic, plants, Neottia, Monotropa, &c., and in that of green plants like Erica, Fagus, and Pinus. It is, however, a well-known fact that ordinary green plants cannot utilize vegetable débris directly, and forest trees do so in appearance only, for the fungi, yeasts and bacteria there are actively decomposing the leaves and other remains. A class of pseudo-symbiotic organisms are, however, being brought into the foreground, where the combined action of two symbionts results in the death of or injury to a third plant, each symbiont alone proving harmless. Some time ago Vuillemin showed that a disease in olives results from the invasion of a bacillus (B. oleæ), which can, however, only obtain its way into the tissues through the passages driven by the hyphæ of a fungus (Chætophoma). The resulting injury is a sort of burr. This observer also observed the same bacillus and fungus in the canker burrs of the ash.
Among many similar cases well worth further attention are the invasion of potato-tubers by bacteria, these making their way down the decaying hyphæ of pioneer fungi. Professor Marshall Ward has seen tomatoes infected by similar means, and other facts show that many bacteria which quicken the rotting of wood are thus led into the tissues by fungi.
Probably no subject in the whole domain of cryptogamic botany has wider bearings on agricultural science than the study of the flora and changes on and in manure and soil. Nitrifying bacteria play a very important part by providing plant life with a most necessary food. They occur in the soil, and two kinds have been described—the one kind converting ammonia into nitrous acid, and the other changing nitrous into nitric acid. We are principally indebted to Winogradsky for our knowledge of these bacteria; he furnishes instances of the bearing of bacteriological work on this department of science, and explains, not only the origin of nitre-beds and deposits, but also the way the ammonia compounds fixed by the soil in the neighbourhood of the root-hairs are nitrified, and so rendered directly available to plant life. The investigations of other observers show that the nitrifying organism is a much more highly-developed and complex form than had been suspected; that it can be grown on various media, and that it exhibits considerable polymorphism—i.e., it can be made to branch out and show other characteristics of a true fungus. “I have,” writes Professor Ward, “for some time insisted on the fact that river water contains reduced forms of bacteria—i.e., forms so altered by exposure to light, changes of temperature, and the low nutritive value of the water, that it is only after prolonged culture in richer food media that their true nature becomes apparent.” Strutzer and Hartleb show that the morphological form of the nitrifying organism can be profoundly altered by just such variations of the conditions described by Ward, and that it occurs as a branched mycelial form; as bacilli or bacteria; or as cocci of various dimensions, according to the conditions.
“These observations, and others made on variations in form (polymorphism) in other fungi and bacteria, open out a vast field for further work, and must lead to advancement in our knowledge of these puzzling organisms; they also help us to explain manyinconsistencies in the existing systems of classification of the so-called ‘species’ of bacteria as determined by test-tube culture.”
Algæ.—The algals have a special charm for microscopists. I am free to confess my interest in these organisms, and for several reasons. In this humid climate of ours they are accessible during the greater part of the year; they can be found in any damp soil, in bog, moss, and in water—indeed, wherever the conditions for their existence seem to be at all favourable for development. Should the soil dry up for a time, when the rain returns algæ are seen to spring into life and give forth their dormant spores, which once more resume the circle of formation and propagation. In the earliest stage of development the spore or spore cell is so very small when in a desiccated state, that any number may be carried about by the slightest breath of air and borne away to a great distance. To all such organisms I originally gave the name of Ærozoa; now recognised as ærobic and anærobic organisms (Fig. 281).
Fig. 281.—Ærobic Spores × 200.1. Ærobic fungi caught over a sewer; 2. Fragments of Penicillium spores; 3. Ærobic fungi taken in the time of the cholera visitation, 1854.
Fig. 281.—Ærobic Spores × 200.
1. Ærobic fungi caught over a sewer; 2. Fragments of Penicillium spores; 3. Ærobic fungi taken in the time of the cholera visitation, 1854.
With reference to the ærobic bacteria I have only to add that in addition to the simple mode of taking them on glass slides smeared over with glycerine, special forms of æroscopes are now in use for the purpose, consisting of a small cylinder in which a current of air is produced by an aspirator and diffused through a glass vessel containing a sterilised fluid. These are in constant use in all bacteriological laboratories. The results obtained are transferred to sterilised flasks or tubes as those shown in a former chapter.
Miquel, who has given considerable attention to the subject of ærobic and anærobic bacteria, reckons that the number of spores that find their way into the human system by respiration, even should health be perfectly sound, may be estimated at 300,000 a day.
One of the most commonly met with forms of micro-organisms isLeptothrix buccalis. It chiefly finds its nutritive material in the interstices of the teeth, and is composed of short rods and tufted stems of vigorous growth, to which the name ofBacillus subtilishas been given (Fig. 282). Among numerous other fungoid bodies discovered in the mouth, Sarcinæ have been found. SeePlate IX., No. 7.
Fig. 282.—Section of the Mucous Membrane of the Mouth, × 350.Showing:a.The denser connective tissue;b.Teased out tissue;c.Muscular fibre;d.Leptothrix buccalis, together with minute forms of bacteria and micrococci;e.Ascomycetes and starch granules.
Fig. 282.—Section of the Mucous Membrane of the Mouth, × 350.
Showing:a.The denser connective tissue;b.Teased out tissue;c.Muscular fibre;d.Leptothrix buccalis, together with minute forms of bacteria and micrococci;e.Ascomycetes and starch granules.
The Beggiatoa, a sewage fungus, found by me in the river Lea water of 1884 growing in great profusion, consists chiefly of mycelial threads and a number of globular, highly refractive bodies, and may be regarded as evidence of the presence in the water of an abnormal amount of sulphates which set free a gas, sulphuretted hydrogen, of a dangerous and offensive character. Another curious body closely allied toBeggiatoa albais Cladothrix; this assumes a whitish pellicle on the surface of putrefying liquids.
These saprophytes obtain nourishment from organic matter; nevertheless they are not true parasites in the first stage of their existence, during which they live freely in the water or in dampsoil; they, however, become pathogenic parasites when they penetrate into the tissues of animals, and necessarily live at the expense of their host.
Fungi, Algæ, Lichens, etc.Tuffen West, del.Edmund Evans.Plate I.
Fungi, Algæ, Lichens, etc.
Tuffen West, del.Edmund Evans.
Plate I.
Bacteria, as I have said, were for a long time classed with fungi under the name of Schizomycetes. But the more recent researches into their organisation, and more especially into their mode of reproduction, show that they rather more resemble a group of algæ devoid of chlorophyll. Zopf asserts that the same species of algals may at one time be presented in the form of a plant living freely in water, or in damp ground, in association with chlorophyllaceous protoplasm, and at another in the form of a bacterium devoid of green colouring matter, and receiving nourishment from organic substances previously elaborated by plants or animals, thus accommodating itself, according to circumstances, to two very different modes of existence.
That widely-distributed single-cell plant, thePalmoglœa macrococcaof Kützing, that spreads itself as a green slime over damp stones, walls, and other bodies, affords an example. If a small portion be scraped off and placed on a slip of glass, and examined with a half or a quarter-inch power, it will be seen to consist of a number of ovoid cells, having a transparent structureless envelope, nearly filled by granular matter of a greenish colour. At certain periods this mass divides into two parts, and ultimately the cell becomes two. Sometimes the cells are united end to end, just as we see them united in the actively-growing yeast plant; but in this case the growth is accelerated, apparently, by cold and damp. Another plant belonging to the same species, theProtococcus pluvialis, is found in every pool of water, the spores of which must be always floating in the air, since they appear after every shower of rain.
Protococcus pluvialisis furnished with motile organs—two or more vibratile flagella passing through perforations in the cell-wall—whereby, at certain stages, they move rapidly about. The flagella are distinctly seen on the application of the smallest drop of iodine. The more remarkable of the several forms presented by the plant is that of naked spores, termed by FlotowHæmatococcus porphyrocephalus. These minute bodies are usually seen to consist of green, red, and colourless granules in equal proportions, and occupying different portions of the cell. They seem to have some share in the after subdivision of the cell (Fig. 283). There are alsostill-cells, which sub-divide into two, while the motile cells divide into four or eight. It is not quite clear what becomes of the motile zoospores,B, but as they have been seen to become encysted, they doubtless have a special function, or becomestill-cells under certain circumstances.
It appears that both longitudinal and transverse division of the primordial cell takes place; and that the vibratile flagella of the parent cell retain to the last their function and their motion after the primordial cell has become detached and transformed into an independent secondary cell (Fig. 283,G).
Fig. 283.—Cell Development. (Protococcus pluvialis.)Protococcus pluvialis, Kützing.Hæmatococcus pluvialis, Flotow.Chlamidococcus versatilis, A. Braun.Chlamidococcus pluvialis, Flotow and Braun.A.Division of a simple cell into two, each primordial vesicle having developed a cellulose envelope;B.Zoospores, having escaped from a cell;C.Division of an encysted cell into segments;D.Division of another cell, with vibratile flagella projecting through cell-wall;E.An encysted flagellate cell;F.Division of an encysted nucleated cell into four parts, with vibratile filaments projecting;G.Fission of a young cell.
Fig. 283.—Cell Development. (Protococcus pluvialis.)
Protococcus pluvialis, Kützing.Hæmatococcus pluvialis, Flotow.Chlamidococcus versatilis, A. Braun.Chlamidococcus pluvialis, Flotow and Braun.
A.Division of a simple cell into two, each primordial vesicle having developed a cellulose envelope;B.Zoospores, having escaped from a cell;C.Division of an encysted cell into segments;D.Division of another cell, with vibratile flagella projecting through cell-wall;E.An encysted flagellate cell;F.Division of an encysted nucleated cell into four parts, with vibratile filaments projecting;G.Fission of a young cell.
The most striking of the vital phenomena presented by Protococcus is that of periodicity. Certain forms—for instance, encysted zoospores, of a certain colour, appear in a given infusion, at first exclusively, then they gradually diminish, become more and more rare, and finally disappear altogether. After some time their number again increases, and this may be repeated. Thus, a cell which at one time presented only still forms at another contained only motile ones. The same may be said with respect to segmentation. If a number of motile cells be transferred from a larger vessel into a smaller one, in the course of a few hours most of them will havesubsided to the bottom, and in the course of the day observed to be on the point of sub-division. On the following morning division will have become completed; on the next day the bottom of the vessel will be found covered with a new generation of self-dividing cells, which, again, will produce another generation. This regularity, however, is not always observed. The influence of every change in the external conditions of life upon the plant is very remarkable. It is only necessary to pour water from a smaller into a larger or shallower vessel to at once induce segmentation of cells. The same phenomenon occurs in other algals; thus Vaucheria almost always develops zoospores at whatever time of year they may be brought from their natural habitat into a warm room. Light is conducive to the manifestation of vital action in the motile spores; they usually collect in great numbers on the surface of the water, and at that part exposed to the strongest light.
But in the act of propagation, on the contrary, and when about to pass into the still condition, the motile Protococcus cell seems to shun light, and falls to the bottom of the vessel. Too strong sunlight, as when concentrated by a lens, quickly kills the young zoospores. A temperature of undue elevation is injurious to the development of their vital activity and the formation of the zoospores. Frost destroys motile, but not still zoospores.55
Stephanosphæra pluvialisis a conspicuous variety of the fresh-water algals, described by Cohn. It consists of a cell containing eight primordial cells filled with chlorophyll, uniformly arranged (seePlate I., No. 24d). The globular mother-cell rotates, somewhat in the same way as the volvox, by vibratile flagella, two of which are seen projecting from each cell and piercing the transparent outer cell wall. Every cell divides first into two, then four, and lastly eight cells, each one of which again divides into a number of micro-gonidia, which have a motion within the mother-cell, and ultimately escape from it. Under certain circumstances each of the eight young cells is observed to change places in the interior of the cell; eventually they escape, lose their flagella, form a thicker membrane as atb, and for a time remain motionless, and sink to the bottom of the vessel in which they are contained. If the vessel is permitted to become thoroughly dry, and then again has water pouredinto it, motile cells reappear; from which circumstance it is probable that these represent the resting spores of the plant. When in the condition of greatest activity its division into eight is perfected during the night, and early in the morning light the young cells escape and pass through similar changes. It is calculated that in eight days, under favourable circumstances, 16,777,216 families may be formed from one resting-cell of Stephanosphæra. In certain of the cells, and at particular periods, remarkable amœboid bodies (Plate I., No. 24c) make their appearance. There is a marked difference between Stephanosphæra and Chlamydococcus, for while in the latter the individual portions of a primordial cell separate entirely from one another, each developing its own enveloping membrane, and ultimately escaping as a unicellular individual; in the former, on the other hand, the eight portions remain for a time living in companionship.
Volvocineæ.—A fresh-water unicellular plant of singular beauty and interest to the microscopist is theVolvox globator(Plate I., No. 15). No. 16 represents a portion of another cell, with brownish amœboid bodies enclosed in the protoplasmic web. It is common to our fresh-water pools, and attains a diameter of about1⁄20th or1⁄30th of an inch. Its movement is peculiar, a continued roll onwards, or a rotation like that of a top; at other times it glides along smoothly. When examined under a sufficiently high power, it is seen to be a hollow sphere, studded with green spots, and traversed by green threads connecting each of the spots or spores with the maternal cell. From each of the spores proceed two long flagella, lashing filaments, which keep the globular body on the move. After a time the sphere bursts, and the contained sporules issue forth and speedily pass through a similar stage of development. These interesting cells were long taken to be animal bodies. Ehrenberg described them asMonads, possessing a mouth, stomach, and an eye.
The setting free of the young volvox is essentially a process of cell division, occurring during the warmer periods of the year, and, as Professor Cohn shows, is a considerable advance upon the simpler conjugation of two smaller cells in desmids; it more closely resembles that which prevails among the higher algæ and a large number of cryptogams. As autumn advances the volvox spherulesusually cease to multiply by the formation of zoosporanges, and certain of their ordinary cells begin to undergo changes by which they are converted, some into male or sperm-cells, others into germ-cells, but the greater number appear to remain sterile. Both kinds of cells at first so nearly resemble each other that it is only when the sperm cells begin to undergo sub-division that they are seen to be about three times the size of the sterile cells. Then the primary cell resolves itself into a cluster of peculiar secondary cells, each consisting of an elongated body containing an orange-coloured endochrome and a pair of long flagella, as seen in the antherozoids of the higher cryptogams. As the sperm-cells approach maturity the clusters may be seen to move within them; the bundles then separate and show an independent active movement while still within the cavity of the primary cell, and finally escape through a rupture in the cell-wall, rapidly diffusing themselves as they pass through the cavity. The germ-cells continue to increase in size without undergoing sub-division, at first showing large vacuoles in their protoplasm, but subsequently becoming filled with a darker coloured endochrome. The form of the cell also changes from its flask-like shape to the globular, and at the same time seems to acquire a firmer envelope. Over this the swarming antherozoids diffuse themselves and penetrate the substance to the interior, and are then lost to view. The product of this fusion, Cohn tells us, is a reproductive cell, or “oospore,” which speedily becomes enveloped in another membrane with a thicker external coat, beset with conical-pointed processes; and now the chlorophyll of the young cell gives place, as in Palmoglæ, to starch and reddish or orange-coloured, and a more highly refractive, fluid. As many as forty of such oospores have been counted in a single sphere of volvox, which then acquires the peculiar appearance observed by Ehrenberg, and described by him under the name ofVolvox stellatus. The further history of this wonderful spheroid unicellular plant has been traced out by Kirchner, who found that their germination commences in the early months of the year—in February—with the liberation of the spherical endospore from its envelope and its division into four cells. A remarkable phenomenon has been observed by Dr. Braxton Hicks—the conversion of an ordinary volvox cell into a moving mass of protoplasm that bears a striking resemblance to the well-knownamœba. “Towards the end of the autumn the endochrome mass of the volvox increases to nearly double its ordinary size, but instead of undergoing the usual sub-division so as to produce a macrogonidium, it loses its colour and regularity of form, and becomes an irregular mass of colourless protoplasm, containing a number of brownish granules.” The final change and the ultimate destination of these curious amœboid bodies have not been satisfactorily made out, but from other observations on the protoplasmic contents of the cells of the roots of mosses, which in the course of two hours become changed into ciliated bodies, it is believed that this is the mode in which these fragile structures are enabled to retain life and to resist all the external conditions, such as damp, dryness, and the alternations of heat and cold.
It would be quite impossible to deny the great similarity there is between the structure of volvox and that of the motile cell ofProtococcus pluvialis. The influence of reagents will sometimes cause the connecting processes of the young cells, as in Protococcus, to be drawn back into the central mass, and the connecting threads are sometimes seen as double lines, or tubular prolongations of the membrane. At other times they appear to be connected by star-like prolongations to the parent cell (Plate I., No. 15), presenting an almost identical appearance withPediastrum pertusum. Another body designated by EhrenbergSphærosira volvoxis an ordinary volvox in a different stage of development; its only features of dissimilarity being that a large proportion of the green cells, instead of being single, are double or quadruple, and that the groups of flagellate cells form by their aggregation discoid bodies, each furnished with a single flagellum. These clusters separate themselves from the parent cell, and swim off freely under the forms which have been designated Uvella and Syncrypta by Ehrenberg. Mr. Henry Carter, F.R.S., who made a careful investigation of unicellular plants, described Sphærosira as the male, or spermatic form ofvolvox.
Among other organisms closely allied to volvox and included in Volvocineæ, affording the microscopist many interesting transitional forms in their various modes of fructification, are the Eudorina, still-water organisms that pass through a similar process of reproduction as the volvox. In thePandorina morum, its reproduction is curiously intermediate between the lower and the higher types; aswithin each cell is a mulberry-like mass, composed of cells possessing a definite number of swarm spores, sixteen usually, which rupture the mother cell, and swim off furnished with a pair of flagella. A similar process takes place in some of the Confervaceæ and other fresh-water algæ. The Palmella, again, consist of (Plate I., No. 21) minute organisms of very simple structure, which grow either on damp surfaces or in fresh water. The stonework of some of our churches is often seen to be covered with a species of Palmella, that take the form of an indefinite slimy film. The “red snow” of Arctic or Alpine regions, considered to be a species of Protococcus, is frequently placed among Palmella. A more characteristic form of theP. cruentais theHæmatococcus sanguinis, the whole mass of which is sub-divided by partitions enclosing a larger or smaller number of cells, which diffuse their granular contents through the gelatinous mass in which their several changes take place. The albuminoid envelope of these masses is seen to contain parasitic growths, which have given rise to some discussion, especially when their filaments are observed to radiate in various directions.
TheOscillariaceæconstitute a genus of Confervaceæ which have always had very great interest for the microscopist in consequence of their very remarkable animal-like movements, and from which they derive their generic name. For more than a century these Bacillaria have excited the curiosity of all observers without any one having derived more than an approximate idea of their remarkable rhythmical movements. The frustule consists of a number of very fine short threads attached together by a gelatinous sheath, in one species all of equal length. Their backward and forward movement is of a most singular character; the only other conferva in which I have seen a motion of a similar kind is the Schizonema. In this species the frustules are packed together in regular series, the front and side views being always in the same direction. These several bodies move along within the filamentous sheath without leaving their respective places. On carefully following the movement, it is seen at first much extended, and then more compressed, while the frustules become more linear in their arrangement, and present a closer resemblance toBacillaria paradoxa, augmented by the circumstance that the frustules are seen to move in both directions. A frustule of Schizonema can move independently of thesheath, and so will a detached frustule of bacillaria. This peculiar and exceptionally anomalous phenomenon as that of the movements of bacillaria can hardly be confined to a solitary species. The movements of the frustules are much accelerated by warmth and light. The longer filaments of other minute species only slightly exhibit any motion of the kind, but have peculiar undulating motions.