Popular Science Monthly/Volume 9/August 1876/Our Common Moulds
THE following remarks are from personal observations which have been made from time to time as circumstances would permit. For convenience, the term mould will be extended beyond its narrow technical meaning, and include all those forms of vegetable life which are usually designated by that name. No lengthy argument is needed to prove that mould is of common occurrence. It is a fact well known to every person, from the wholesale provision-dealer down to the hungry child who eats his crust upon the street.
Where these plants do not grow it is difficult to say; but to point out the favorable conditions for their growth and some of the forms which they there assume is an easier task, and to this part of the subject the reader's attention is invited.
Moulds belong to that peculiar parasitic group of plants called Fungi, the members of which never have anything like green leaves, the workshops of higher plants, and are therefore unable to build up their tissue from unorganized matter. They must feed upon that which is already organized, either animal or vegetable, living or dead, as the species will decide.
One of the most essential conditions for the development of these minute fungi is the presence of a good degree of moisture. So well known is this, that to many minds moisture and mould bear to each other nothing less than the relation of cause and effect. A warm atmosphere is also required. In winter the housewife exercises fewer precautions to keep these intruders from her viands than during the warm summer weather. Besides organic matter, moisture, and warmth, a free access of oxygen must be added as an essential condition for the perfect development of moulds.
When the season comes and the soil is ready, the farmer knows he must sow the seed, or he cannot hope to reap a harvest. So it is with the moulds: to the conditions for growth there must be added the germs of life, or no mould will be produced. How this sowing is accomplished will be better seen after some of the species are considered more in detail.
Our common bread is a substance which offers special inducements for the growth of various moulds, and, in order to study them, a slice was taken and placed on a zinc rack on a dinner-plate and covered with a glass bell-jar lined with filtering-paper which dipped into some water in the bottom of the plate, producing thus a moist atmosphere by the evaporation from its extensive surface. This culture
(Fig. 1), placed in a warm room, secured all the important conditions for the production of a crop of mould. On the bread thus situated a mould made its appearance in about thirty-six hours, and proved to be one of the most common of the bread-moulds (Mucor stolonifer), shown in Fig. 2. When first noticeable, the surface of the bread is covered with a cobweb-like mass of fine white threads, called mycelium, which run in all directions through the tissue of the bread, and perform the work of absorbing nourishment. Soon other and larger threads begin to rise into the air, their tips enlarge, the protoplasmic contents of the threads passing up into the ends, which finally assume a spherical shape. At first these large round heads are of a white color; but soon they begin to grow darker, their contents shaping into little round bodies, which when ripe are dark-colored and fill the capsule to repletion. These little bodies thus produced in vast numbers in the swollen ends of these vertical threads are termed spores, and answer the same purpose for moulds that seeds do for
flowering plants. From the same base several of these capsules are produced, varying in age from threads with their tips little swollen, to the tall and aged ones which have ripened and scattered their spores. This mould has much the habit of the strawberry-plant, throwing out runners or stolons, which take root and in turn become new plants to increase and continue the species. In the culture we have often 6een this mould hanging from the bread on the rack to the plate below, a distance of four or five inches, with here and there the stolons with their fruit-clusters hanging in mid-air (Fig. 1).
The fruiting which has been described is asexual, and the spores thus formed can be likened to the bulbs in the axils of the leaves of the tiger-lily and other reproductive bodies in flowering plants which do not result from a fertilized ovule. Several trials were made to cultivate the sexual fruit of this plant, but without success; another member of the same genus (Mucor Syzygites), which grows on decaying toadstools, produced them under the bell-jar in large quantities. When this plant reaches the proper stage of development for the formation of its sexual fruit, the tips of various filaments become noticeably swollen. Two of such enlarged ends grow toward each other and finally meet by their extremities, or rather by the blending of the processes which each cell puts out, thus forming at first a small cell between the two united filaments. As maturity is reached, this zygospore acquires a diameter much greater than that of the filaments which have produced it, and is many hundred times the size of the spores formed in the capsules. It is also provided with two coats, the outer one thick, dark-colored, and covered with warty excrescences, except on the two ends where the remnants of the conjugated filaments remain. Successive stages in the development of these spores are shown in Fig. 3. At a the two swollen threads are near each other; b, the process partly completed, with the middle cell plainly seen; and c, the full-formed zygaspore, with the remains of the old cells. The importance of these sexual spores in the economy of the plant is not difficult to
understand. The minute asexual spores have a very thin covering, and under favorable conditions will germinate as soon as formed, but on the other hand are readily destroyed by extreme climatic changes. The large, well-protected zygospores which germinate only after months of ripening, are in every way fitted to carry the species through the unpropitious season of winter, times of drought, and severe exposure. When the zygaspore germinates it produces very soon a large crop of capsules, and their little spores are scattered far and wide ready to develop into plants in a few hours, as circumstances shall decide. This dual form of fruiting is an interesting feature in the life of these little plants, and is as effectual in preserving the species as it is interesting.
After upward of a week from the time the Mucor, of which we have been speaking, made its appearance on the bread, another mould was noticed growing-over its surface, which was of a much finer structure and dingy yellow in color. When placed under the microscope, it was found to be peculiar, both in structure and habit of growth. The filament which bears the spores, as it rises from the matted surface of the Mucor, divides into two branches, each of these into two others, and so on, until ten or more branches are reached. The same angle of divergence being preserved in all the branching, the compound top assumes a very regular, semicircular outline, as shown in Fig. 4, a, where the filaments are represented by single lines; the whole of the branch b is a more highly-magnified view of a dark tip at the end of one of the branches in a and at c is shown, on a still higher scale, one of the ultimate branches in b, with the spores arranged in rows of four around the enlarged end; while d is one of those tips after the spores have fallen away.
The reader will please bear in mind that this figure, and all the others, with the single exception of the first one, represent the object as greatly enlarged—the microscope used for most of the work magnifying 650 diameters. An entire plant of the one in question (Piptocephalis Fresœniana) is scarcely visible to the naked eye when prepared on a glass slide for investigation with the microscope.
That which makes this mould of particular interest is the fact that it is a parasite, and cannot live unless it has some other mould upon which to grow. This easily explains why it does not make its appearance until the Mucor is well established. Here we have a true parasite growing on a saprophyte; or one mould which steals its substance from another which derives its living from the bread. We will not stop to reason upon the matter, or wonder how this strange state of things came about, but will leave the fact as it exists to those who would know the cause of all things both great and small.
There were two or three other members of the Mucor genus which grew on various cultures, but, as they differ only in minor points of structure from the one treated, space will not permit of their being further mentioned.
The bones of a recently-killed dog proved to be very well adapted for the growth of the largest-known species of mould. Those who have a passion for scientific names may call it Phycomyces nitens. It is one of the few moulds which grow on oil or oily substances, and is so filthy in its habits as to flourish in the sewers and cesspools of cities. It is so much like Fig. 2 in structure and manner of fruiting, though many times larger, that it must pass without an illustration.
The pulp of oranges is an especially favorable diet for some of the most delicate moulds. A culture made of it will show decided signs of mouldiness in twenty-four hours, and after thirty-six hours of growth there is a fine crop for study. Those which we have seen on the bread are invariably the first to appear here, though followed in a short time by others, one of the most common of which is given in Fig. 5. At the base a are some mycelial threads which penetrate the tissue of the pulp, and from them, as they come to the surface, arise the fruit-stalks which branch near the top into a loose head with the spore capsules borne on the ends of the branches. At c is one of the Sporangia more highly magnified, showing the spores to be larger and few in number as compared with the Mucor. This species is a member of the same family with those already mentioned, and has its similar zygaspores.
Corn-starch pudding, when placed in a bell-jar, remained unchanged until the fourth week, when its surface became coated with a peculiar yellow-colored substance, and a day or so after black specks began to appear. When viewed with the microscope, this mould exhibited the structure seen in Fig. 6. There arises from the unbranched and imbedded filaments a very much swollen end (a and b) filled with protoplasm, yellow globules of oil, and crystals (d). As this end increases in size, a contraction takes place near the upper end, and soon a distinct spore capsule is formed of the end thus separated. When the plant is ripe a black elastic coat covers the spore-case, which slips partly off when the spores are discharged from below. There are several species growing on various substances, which have the general structure of Fig. 6.
We now come to the most common of all the members of the group of moulds: the blue mould of cheese, bread, and almost every article of food. In its diet, it does not confine itself to those things found in a well-stocked pantry, but will flourish on old boots and other articles of clothing when they are left for a few days in a warm, damp place. No culture was made without its making its appearance, and often to the exclusion of all other forms. It is quite small, never reaching but a very short distance from the substance on which it grows, and under favorable circumstances forms an even blue crust over the surface of the nourishing material whether boots or bread. The structure of this frequent and often unexpected and unwelcome visitor is given in Fig. 7. Two fully-developed fruit-stalks are seen at a and b, branching
irregularly at the top, and bearing the naked spores in chains at the ends of the filaments. At c is a young stalk before the spores have formed from the threads, which is done by a constriction, a familiar but perhaps rude illustration of which is seen in the making of the links of sausage. As the spores fall away, new ones are formed below, and so the process of producing these simplest of reproductive bodies is indefinitely continued. At the base d are the threads which penetrate the nourishing substance, on some of which are formed, as the result of sexual action, spherical bodies which inclose the more enduring sexual spores. From this method of forming subterranean fruit this little mould is a close relative to the truffle so highly prized for food.
With the spores of this mould repeated sowings have been made, with gratifying results. When a slice of fresh bread was placed in the bell-jar and certain marked places were sprinkled with spores by means of a pair of forceps, in the course of twenty-six hours those spots sown were covered with the young mould, while all other places were entirely free from it. In one case a fresh slice of bread was wrapped in a piece of paper, with the exception of a star-shaped figure cut from the paper, which came in the middle of the slice. Over the whole a slice of mouldy bread was shaken so gently that no spores were seen to fall; the paper was then carefully removed, and the bread placed under the bell-jar. After the usual time a fine star of blue mould made its appearance, soon spreading over the whole of the bread. One could as easily write his name in mould on bread as with clover-seed upon the soil, though it would not be as enduring an inscription.
It seems difficult for some people to see how the spores of these various moulds can exist almost everywhere, ready to grow when the first opportunity offers itself. With the hope of making this matter appear clearer, the following calculations have been carefully made: The blue mould (Penicillium crustaceum) is very favorable for the estimation of the number of spores produced, as the heads are quite open, and the spores are naked and distinct.
A piece of decaying apple was selected, because the mould can be removed from that portion covered with the smooth skin without being mixed with foreign matter. When the mould was still young and no spores had fallen away, it was viewed with the high power of the microscope. There were usually twenty filaments to each head, and twenty spores on a filament, or 4,000 spores to a head or single stalk. A small tuft was then carefully taken from the apple, and placed on a glass slide, and its area measured with the micrometer, and the number of fruit-stalks it contained determined. The surface was 1 of an inch square, and the number of stalks 800. As the square inch is a familiar unit of measurement, the estimate made for that small extent of surface gives 3,200,000,000 spores. It is a well-known fact that not only inches, but feet, and even acres of this mould are produced, and the number of spores for daily use must be perfectly appalling. They are light, airy, and invisible to the naked eye and therefore escape our notice; were they female mosquitoes, we would realize their nearness and number. With such multitudes of germs produced we need not wonder that the sowing for moulds should be natural and complete. The light which the microscope throws upon this subject makes it unnecessary to resort to "spontaneous generation" to account for the almost certain growth of mould when the proper conditions are combined.
When a slice of bread which has been thoroughly overrun by the Penicillium is left under the bell-jar, another mould almost invariably comes, covering the blue surface with a rose-colored coat. The asexual spores and their method of formation are given in Fig. 8. It will be observed that the fruit-stalks are unbranched, the spores when mature always double, and arranged on the stalks in whorls. The development is from below upward: a showing an old stalk with the youngest spores nearly ripe, and some of the older ones gone from their attachments; at b is a younger stalk, where the older whorls are complete and the upper ones small and indistinguishable; c is a more enlarged view of a cluster of ripe spores. After this mould, which is of slow growth, has run its course, the bread seldom produces any other forms, and for further study a new culture must be made.
In Fig. 9 is given a mould which makes its home on decaying herbage, and is found to perfection in old waste-heaps where weeds and other green matter have been deposited. So common is it, that a culture is more a matter of convenience than necessity. The fruit-stalks are upright, considerably branched at the top, with the spores borne in bunches at the ends of the filament. At a is a much enlarged view of one of these naked heads of spores, and another where the spores have mostly fallen away. As the fruit-stalks grow old they break down in every way, giving the appearance of a forest over which a tempest has passed.
Of the black moulds Fig. 10 shows a common and very simple representation. It grew as a sooty coating on a culture made of sliced raw potatoes. It is so very simple that any space taken for description seems unnecessary. The black spores are nothing more than portions of single or branched filaments cut off in a very regular manner.
In Fig. 11 is given the general structure of a large number of related moulds which grow wherever they can get a foothold. The drawing was made from one found on some turnip-roots left upon the ground over winter. Like many others, it forms an olive-brown, velvety coating of considerable thickness; and, because of their low habits and inobtrusive nature, they pass readily for dirt or decay, and are seldom noticed. They are the lowly forms which some of the highest of the fungi assume in passing through one stage of their polymorphic existence.
A score of other species of moulds deserve mention here which are found on various substances either forced under the bell-jar, or growing naturally; but we know how unattractive such descriptions would be without accompanying figures, and therefore pass them by.
For the last place in this brief enumeration we have reserved one of the most common of household moulds. It is the prince of moulds, and claims relationship with the most perfect and beautiful of fungi. In diet it is something of an epicure, and may well be called the cake and preserve mould,.and on those substances the best results are always obtained when used for its culture. Though preferring its cake and preserves, it was induced to grow on a tempting dish of stewed prunes, which afterward furnished the substance for its culture.
To the naked eye this mould at first appears as white patches; soon stalks rise from the surface, and on the end of each a small spherical head is borne, which increases in size and turns to a bluish color, so that when this mould is ripe the surface of the nourishing substance does not differ greatly in color from the Penicillium on the bread, though this mould is much taller and more inclined to grow in tufts. Under the microscope the tip of a very young filament which is to form a head of spores is seen to be perfectly smooth and similar to those in the Mucor, but very soon small peg-like projections begin to grow from its surface, which increase in size and finally divide, forming radiating rows of spores.
Various stages in the development of the asexual fruiting of this mould are shown in Fig. 12, a, a, with a view of an imaginary cross-section through one of these heads highly magnified in b, giving the method of attachment of the spore-threads.
Shortly after the heads have formed, the sexual fruit begins to develop on the mycelium at the base of the fruit-stalks. The method is more complicated than in any of the moulds already given, and places this plant among the most highly developed of fungi. The process begins with the coiling of the end of a thread in a corkscrew manner as seen in c and d (Fig. 13). The coils grow closer together as the filament increases in size, until a hollow cylinder is formed. From the same threads below the base of the coil one or more processes grow out which are the male organs, and, where one of these threads reaches the tip of the female coiled filament, that organ is fertilized. From this time on, this body undergoes various and complicated changes, which finally result in a bright-yellow spherical body (Fig. 13, f), which consists of a thin wall inclosing a large number of sacs, g, each of which contains eight spores.
To find all these various stages of development is a matter of some time and patience; but nothing is more satisfactory in the study of moulds than to trace all these steps, from the first bending of the filament to the perfect sphere with its multitude of spores.
For a long time these two forms of fruit in the Aspergillus were considered as belonging to distinct and widely-separated species, but, when the microscope shows that they are produced from the same mycelium, it is time to conclude that they are but two methods of continuing the same species.
Space forbids further details concerning our common moulds; but it is hoped enough has been said to show that among them the species are distinct. The tiny forests which the microscope reveals are made up of forms as decided as those which compose our woodlands and groves. In closing, the reader expects an answer to the question which very naturally arises, viz., "What good do they do?" Though often of great annoyance in domestic and other affairs, yet all in all it is safe to say the good they accomplish far overbalances the harm. They are scavengers which do in their own inobtrusive way a vast amount of sanitary work. Though small in themselves, they are great reducing agents, striving to bring about that equilibrium so necessary to perfect harmony in the organic world. They hasten decay, tear down the accumulating rubbish around us, and allow the elements thus liberated to pass again into the cycle of ceaseless activity and growth.
To the thoughtful mind moulds do not simply excite wonder or disgust, but teach a deeper lesson of adaptation and service of little things, in the perfect and economical scheme of creation.