CHAPTER XXXVII. PARASITIC FUNGI AS FOUND IN A FOSSIL STATE. IT is a curious fact that representatives of some of our common parasitic fungi are found in a silicified state in fossil plant stems and roots of great antiquity. Some of these parasites were in existence in company with the higher cryptogams in Palæozoic times. Indeed it is probable that some fungi, not dissimilar in structure from fungi which are now parasitic, led in remote geological times a non-parasitic life upon the ground. We still have a Botrytis named B. terrestris, Pers., which is frequent on the naked ground. The species belonging to Botrytis are very similar with the species described under Peronospora, and they were till quite recently all grouped together. The late Mr. Charles Darwin informed us that more than forty years ago, Mr. Robert Brown, then Keeper of the Department of Botany at the British Museum, showed him silicified fungus mycelium in slices of fossil wood. Mr. William Carruthers, F.R.S., the present Keeper of Botany at the British Museum, South Kensington, has described silicified fungus mycelium resembling that of a Peronospora found in the tissues of a fossil fern named Osmundites Dowkeri, Carr., from the lower Eocene strata of Herne Bay. The same gentleman has also detected a fungus in a fossil Lepidodendron from the coal measures ; and Mr. Butterworth, of Oldham, has also met with a fungus in the vascular axis of Lepidodendron. A portion of the latter example was drawn by us, and the drawings are now in the Museum of Practical Geology in Jermyn Street London. Engravings from the same transparent slice were published by us, with a description, in the Gardeners' Chronicle for 20th October 1877. Mr. Carruthers also published an extremely small engraving and a brief description of the fungus in his printed address, read before the Geologists' Association in 1876. We have named this parasite Peronosporites antiquarius, W.Sm. the mycelium appears to be distinctly septate, and the large globular oogonia or zoosporangia appear to show clear traces of zoospores within their walls. To us Mr. J. T. Young, F.G.S., the owner of the transparent slice of fossil Lepidodendron, has recently replaced the example in our hands for a new illustration; and our engraving at Fig. 139, enlarged 400 diameters, has been made direct from the microscope. Notwithstanding criticisms to a contrary effect, we have no hesitation in repeating, after a renewed and prolonged examination of the preparation, that traces of zoospores are distinctly visible in many of the oogonia; there is no reason why they should not exist, but good reason why they should; the mycelium is septate; and the oogonia, as in all Peronosporeæ and Saprolegnieæ, are cut off from the supporting threads by distinct septa. The slice of Lepidodendron from which our illustration is taken has a large number of free oogonia in different parts of the silicified tissue; such free oogonia or zoosporangia are very commonly seen in Peronospora, as in P. ganglioniformis, B. The zoospores in some of these free isolated examples are much more distinct than in the characteristic group, engraved to show the oogonia only, in Fig. 139. The traces of zoospores, seen in Peronosporites, exactly agree in size with the zoospores of Peronospora infestans, Mont., Figs. 128 and 136. The genus Peronospora is in close and obvious relationship to the Saprolegnieæ, one member of which, Saprolegnia ferax, Kutz., is the cause of the salmon disease. Professor de Bary has even said that facts do not exclude the possibility of the fungus of the FIG. 139.-FOSSIL PARASITIC FUNGUS. Peronosporites antiquarius, W.Sm., in a slice of fossil Lepidodendron from the Coal Measures. Enlarged 400 diameters. potato disease being one of the Saprolegniea (Journal of the Royal Agricultural Society, p. 249, 1876). It is some times (as when the plants are not fully developed) impossible to distinguish between one genus and the other. Peronosporites, therefore, has without doubt relations with the Saprolegnieæ, as correctly pointed out by Professor W. FIG. 140.-SPORANGIUM OF A FOSSIL FUNGUS. C. Williamson, F.R.S., in the Philosophical Transactions of the Royal Society, 1881. We have a second representative of fungi of enormous antiquity in a transparent silicified slice of a rootlet of Lepidodendron from the coal measures, now in the British Museum at South Kensington. This slice exhibits numerous unusually large sporangia of a fungus not to be distinguished from Protomyces. Very little mycelium can be detected; and many of the sporangia of the fungus are situated in positions where the tissues of the host plant have apparently, but perhaps not really, decayed. We have illustrated one sporangium of this fungus, which may be named Protomycites protogenes, W.Sm., at Fig. 140, enlarged 400 diameters (protogenes, first produced or primæval). In most of the silicified examples an outer or exospore, and inner or endospore are distinctly visible. This fungus presents some analogy with the alga named Chlorochytrium Lemnæ, Cohn., which grows within the fronds of duckweed, the spores from the zoosporangium О8880 A B C FIG. 141. Diagram showing development of simple fungi by cell-division. of which conjugate or fuse in the style of the zoospores (or zygozoospores) of Protomyces, and so produce zygospores. The observer should, however, be ready to distinguish between mere fusing, which is very common in fungi, and true conjugation, which is by no means common. Although these two fungi have been detected in Palæozoic rocks, it must not be concluded that they are the simplest known forms of primal fungi. In Peronospora and Protomyces alike, sexual organs occur; and the fact of a separation of sexes shows a great advance upon a primordial form. Besides, the members belonging to the two genera are parasites, and doubtlessly lived in Palæozoic times, as their representatives do now, upon the living tissues of more highly-organised plants. |