More trustworthy evidence of fossil fungi is afforded by the marks of disease in petrified tissue and by the presence of true mycelia. In examining closely the calcareous and siliceous plant-tissues from the Coal-Measures and other geological horizons, one occasionally sees fine thread-like hyphae ramifying through the cells or tracheal cavities; in many cases the hyphae bear no reproductive organs and cannot as a rule be referred to a particular type of fungus. If the hyphal filaments are unseptate, they most likely belong to some Phycomycetous species; or if they are obviously septate the Mesomycetes or the Mycomycetes are the more probable groups. Occasionally there may be found indications of the characteristic clamp-connections in the septate filaments; a small semicircular branch, which is given off from a mycelium immediately above a transverse wall, bends round to fuse with the filament just below the septum, thus serving as a small loop-line connecting the cell-cavity above and below a cross wall. Such clamp-connections are usually confined to the hyphae of Basidiomycetes and thus serve as a useful aid in identification. A good example of a clamp-connection in a fossil mycelium is figured by Conwentz[415] in his monograph on the Baltic amber-trees of Oligocene age. The stout and thick type of hypha found in some fossil woods agrees closely with that of Polyporus, Agaricus melleus and other well-known recent Basidiomycetes.
In a section of a piece of lignified coniferous wood recently brought by Col. Feilden from Kolguev island[416], the brown and stout hyphae of a fungus are clearly seen as distinct dark lines traversing the tracheal tissue. The occurrence of septa and the large diameter of the mycelial branches at once suggest a comparison with such recent forms as Agaricus melleus, Polyporus and other Basidiomycetes. The age of the Kolguev wood is not known with any certainty.
The vesicular swellings such as those represented in fig. 41, A, B, D and E, may easily be misinterpreted. Such spherical expansions in a mycelium, either terminal or intercalary, may be sporangia, oogonia or large resting-spores, or non-fungal cell-contents, and it is usually impossible in the absence of the contents to determine their precise nature. Hartig[417] and others have drawn attention to the occurrence of such bladder-like swellings in the mycelia of recent fungi, which have nothing to do with reproductive purposes; under certain conditions the hyphae of a fungus growing in the cavity of a cell or trachea may form such vesicles, and these, as in fig. 42, D, m may completely fill up the cavity of a large tracheid.
Some good examples of bladder-like swellings, such as occur in the mycelium of Agaricus melleus and other recent fungi, have been figured by Conwentz[418] in fossil wood of Tertiary age from Karlsdorf. The swellings in this fossil fungus might easily be mistaken for oogonia or sporangia; especially as they are few in number and spherical in form.
A similar appearance is presented by a mass of tyloses in the cavity of an old vessel or tracheid; and vesicular cell-contents, as in the cells of fig. 41, A, 2–5, may closely simulate a number of thin-walled fungal spores or sporangia.
A good example of such a vesicular tissue, in addition to that already quoted, is afforded by a specimen of an Eocene fern, Osmundites Dowkeri Carr.[419] described by Carruthers in 1870. The ground-tissue cells contain traces of distinct fungal hyphae (fig. 41, B), and in many of the parenchymatous elements the cavity is completely filled with spherical vesicles; in other cases one finds hyphae in the centre of the cell while vesicles line the wall, as shewn in fig. 41, B. Carruthers refers to these bladders as starch grains, and this may be their true nature; their appearance and abundant occurrence in the parenchyma certainly suggest vesicular cell-contents rather than fungal cells. I could detect no proof of any connection between the hyphae and bladders, and the absence of the latter in the cavities of the tracheids, fig. 41, C, favoured the view of their being either starch-grains or other vacuolated contents similar to that in the cells of the Portland Cycad (fig. 41, A) referred to on p. 88.
PATHOLOGY OF FOSSIL TISSUES.
The vacuolated cell-contents partially filling the cells in fig. 41, D, present a striking resemblance to the contents of the cells 2–5 in fig. 41, A. In fig. D the frothy and contracted substance might be easily mistaken for a parasitic or saprophytic fungus, but this resemblance is entirely misleading. It is by no means uncommon to find the cells of recent plants occupied by such vacuolated contents, especially in diseased tissues in which a pathological effect produces an appearance which has more than once misled the most practised observers.
In the important work recently published by Renault on the Permo-Carboniferous flora of Autun, there is a small spore-like body described as a teleutospore, and classed with the Puccineae[420]. We have as yet no satisfactory evidence of the existence of this section of Fungi in Palaeozoic times, and Renault’s description of Teleutospora Milloti from Autun might be seriously misleading if accepted without reference to his figure. The fragment he describes cannot be accepted as sufficient evidence for the existence of a Palaeozoic Puccinia.
The same author refers another Palaeozoic fungus to the Mucorineae under the name of Mucor Combrensis[421]; this identification is based on a mycelium having a resemblance to the branched thallus of Mucor, but in the absence of reproductive organs such resemblance is hardly adequate as a means of recognition.