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 that the 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 the root-hairs of the tree. Filaments (mycelia) are again given off which terminate in asci or sacs filled with minute spores of about 1⁄2500th 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 many inconsistencies 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.