The Nature of Fungi.
The amount of evidence possessed at the present day favours the idea that the Fungi have descended in a direct line from the Algæ, and, by gradual differentiation and adaptation to a life on dry land, have at some distance from the point of departure from the parent stock gradually acquired a set of characteristics which collectively give an individuality to the group. On the other hand, the Fungi are strictly a terminal group—in other words, there are no indications of a departure, either morphologically or physiologically, from the Fungi that suggest the starting-point of a new order of things. The mutualism between Fungi and Algae that has resulted in the production of the group of plants collectively known as
Lichenes cannot be considered in the sense of being a branch from the fungal stock, and must have come into existence long after the complete differentiation of the Fungi was effected, because the fungal element in lichens corresponds to the ascigerous Fungi, Sphæriaceæ and Discomycetes, which do not belong by any means to the earliest differentiated groups of Fungi.
The bulk of Fungi with which people generally are most familiar are truly terrestrial, belonging to sections that have ages ago forsaken the aquatic home of their ancestors; nevertheless, numerous truly aquatic Fungi do still exist, and such, as would be expected, are morphologically most in touch with the Algæ. As an example of such algal-like Fungi, as they are termed by Brefeld, may be mentioned the Saprolegniæ, which morphologically, and in the sexual mode of reproduction, closely agree with such Algæ as Vaucheria. These primitive Fungi also agree with some of the Algæ in possessing a unisexual, as well as a sexual, mode of reproduction, and one feature that has been constantly kept in view in the evolution of the Fungi, and to which they owe to a great extent their individuality as a distinct group of organisms at the present day, is the gradual suppression of the sexual mode of reproduction, and the proportional elaboration of the asexual method, until finally, in the most highly evolved and at the same time the most modern section of the Fungi, the Basidiomycetes, the sexual mode of reproduction has completely disappeared, and so dissimilar are the components of this group, including the numerous forms popularly known as mushrooms, toadstools, puffballs, & c., to the Algæ that, but for the connecting-links still existing in an almost unbroken chain, their origin would certainly never have been suggested.
The only observable difference between the Algæ and those Fungi most closely related consists in the suppression of chlorophyll in the latter—a condition which necessitated a change in the mode of life: the Fungi, being unable to assimilate inorganic food, became parasites, obtaining their food from living organisms—plants or animals; or saprophytes, obtaining their food from dead and decaying organic matter.
The early groups of Fungi include numerous parasitic species, whereas in the later, or more modern, groups saprophytic species are most abundant. Numerous species can only live, through their entire life-cycle, as parasites, and are termed “obligate parasites,” as the members of the Uredineæ, commonly known as “rusts,” the destructive parasites of cereals, & c.; others possess the power of living as saprophytes and parasites respectively at different periods of their existence, and are called “facultative parasites.”
The supposed rapidity with which Fungi spring up, mature their spores or reproductive bodies, and disappear has become proverbial—at least in the Northern Hemisphere. Nevertheless, such an idea is a mistake. The part popularly considered as constituting the entire fungus—that is, the part appearing outside the matrix on which the fungus is growing—is only a part of the organism, corresponding functionally to the fruit of a flowering-plant, and is solely concerned with the continuation of the species; whereas the vegetative portion of the fungus—the portion upon which the continuation of the individual depends—is always buried in the substance upon which the fungus is growing. If we take as an example any ordinary mushroom or toadstool, we find a weft of slender threads forming the vegetative portion, known as mycelium, or spawn, permeating the matrix or substance on which the fungus is growing. On this mycelium the sporophores, or spore-producing portions, first appear as minute white lumps, not a millimetre in diameter. These continues to increase in size, and become differentiated into pileus, or cap, stem, gills, & c., while yet underground; and finally, when the structure is completely elaborated, it pushes up above the surface of the matrix, for the purpose of having its spores dispersed by wind or other agents.
In the majority of Fungi the very minute spores are dispersed by currents of air; in many of the subterranean Fungi animals, especially rodents, scratch up the Fungi, which they eat, and probably the spores pass through the alimentary canal uninjured, and are thus dispersed. Finally, in one group of Fungi—the Phalloideæ—brilliant colours, combined with a powerful smell, attract insects, which readily feed on a sweet, semi-liquid substance, containing the minute spores in suspension, which are thus disseminated. It is interesting to note that colour and smell, the agents used by many flowering-plants for the purpose of advertising their whereabouts to insects, which, in return for a supply of nectar, unconsciously effect cross-fertilisation, should be utilised by some Fungi for the purpose of securing spore dissemination.
As previously stated, many Fungi have two or more different modes of reproduction. This is especially observable in the instance of destructive parasites, or, in other words, this arrangement enables certain species to act as wide-spreading and injurious pests to cultivated crops. The process is as follows: In species parasitic on annual plants, as cereals, the fungus continues to produce, in rapid succession, innumerable asexually formed conidia, or reproductive bodies, which possess the power of germinating the moment they are mature. These conidia are washed by rain or carried by wind on to the surface of healthy leaves, where they germinate at once, enter
the tissues, and within a very short time form a new centre of disease, producing conidia, which in turn are dispersed and extend the disease.
From the above account it may be seen how quickly a disease can spread when it has once gained a foothold in places where the host-plant is in considerable numbers and close together, as in a field of corn, in an orchard, forest, & c. These conidia are known as summer-spores, their special function being to enable the fungus to extend its geographical area.
Towards the end of the season, when the vitality of the host-plant is on the wane, the same mycelium which produced conidia, or summer-spores, now gives origin to a different form of fruit, known as resting-spores or winter-spores. These spores remain in a passive or resting state until the following season, and germinate at the time when the host-plant is pushing into active life. The function of the resting-spore condition is to secure the continuation of the species, by tiding it over that portion of the year when the host-plant is not in active growth.
In addition to the production of resting-spores, the mycelium of many species of Fungi becomes concentrated into a number of compact masses or nodules, called “sclerotia,” usually of a black colour externally. These sclerotia, which vary in size in different species from that of a pin's head to a cricket ball, also remain in a quiescent state during the winter, in the tissues of the leaves or stem where they are formed, or in the ground, and in the following spring produce reproductive organs, which infect the young leaves of the host-plant, and commence anew the cycle of development.
From the above account it will have been gathered that resting-spores, or sclerotia, give origin to the disease in the first instance by inoculating the host-plant, the spread of the pest being afterwards secured by the rapid production of summer-spores. Although the leaves or stems bearing resting-spores or sclerotia may completely decay during the winter, the reproductive parts of the fungus do not perish, but remain lying on the ground until the host again makes its appearance, and then act as already described. The resting-spores on the straw of cereals do not fall away readily. The corn may be harvested, the straw used for fodder, pass on to the manure-heap, be returned to the land as manure, and yet the resting-spores may be found attached to fragments of the decaying straw, and quite capable of germination. Finally, resting-spores and sclerotia often possess the power of remaining in a dormant condition for several years, thus extending the means for continuing the struggle for the survival of the fittest, often much to the disadvantage of the farmer and horticulturist.
It has been estimated that damage to the extent of fifty millions of pounds annually is caused by parasitic Fungi alone to cultivated crops, and it is also equally certain that the greater part of this damage could be prevented if proper precaution was exercised. This precaution can only be carried into effect when a sufficient number of experts are available for the purpose of imparting information directly to those most immediately concerned.