The occurrence of hyphal cells in calcareous shells and corals has already been alluded to.[422] In addition to the examples referred to above, there is one which has been described by Etheridge[423] from a Permo-Carboniferous coral. This observer records the occurrence of tubular cavities in the calices of Stenopora crinita Lonsd., and attributes their origin to a fungus which he names Palaeoperone endophytica; he mentions one case in which a tube contains fine spherical spore-like bodies which he compares with the spores of a Saprolegnia. As pointed out above (p. 128), it is almost impossible to decide how far these tubes in shells and corals should be attributed to fungi, and how far to algae.
Fig. 41. A. Cells of Cycadeoidea gigantea Sew. × 355. B and C. Parenchymatous cells and scalariform tracheids of Osmundites Dowkeri Carr. × 230. D. Epidermal cells of Memecylon (Melastomaceae) with vacuolated contents. E. Peronosporites antiquarius Smith, (No. 1923 in the Williamson collection). × 230. F. Zygosporites. × 230. (A, B, C and E drawn from specimens in the British Museum; D from a drawing by Prof. Marshall Ward; F from a specimen in the Botanical Laboratory Collection, Cambridge.)
Fig. 42. A, B, C. Tracheids of coniferous wood attacked by Trametes radiciperda Hart. (Polyporus annosus Fr.) D and E. Tracheids attacked by Agaricus melleus Vahl. A, × 650, B–E, × 360. (After Hartig.)
Passing from the direct evidence obtained from the presence of fungal hyphae in petrified tissues, we must draw attention to the indirect evidence of fungal action afforded by many fossil plants. It is important to be familiar with at least the more striking effects of fungal ravages in recent wood in order that we may escape some of the mistakes to which pathological phenomena may lead us in the case of fossils[424].
The gradual dissociation of the elements in a piece of fossil wood owing to the destruction of the middle lamellae, the occurrence of various forms of slit-like apertures in the walls of tracheids (fig. 42, E) and the production of a system of fine parallel striation on the walls of a vessel are among the results produced by parasitic and saprophytic fungi. With the help of a ferment secreted by its hyphae, a fungus is able to eat away either the thickening cell layers or the middle lamellae or both, and if, as in fig. 42, A, only the middle lamellae are left one might easily regard such tissue in a fossil condition as consisting of delicate thin-walled elements. The oblique striae on the walls of a tracheid may often be due to the action of a ferment which has dissolved the membrane in such a manner as to etch out a system of spiral lines, probably as a consequence of the original structure of the tracheids. In distinguishing between the woods of Conifers the presence of spiral thickening layers in the wood element is an important diagnostic character, and it is necessary to guard against the confusion of purely secondary structures, due to fungal action, with original features which may be of value in determining the generic affinity of a piece of fossil wood.
Oochytrium Lepidodendri, Ren. Fig. 43, 1.
Under this name Renault has recently described a filamentous fungus endophytic in the cavities of the scalariform tracheids of a Lepidodendron[425]. The mycelium has the form of slender branched hyphae with transverse septa. Numerous ovoid and more or less spherical sporangia occur as terminal swellings of the mycelial threads. The long axis of the ovoid forms measures 12–15 µ, and the shorter axis 9–10 µ; the contents may be seen as a slightly contracted mass in the sporangial cavity. In some of the sporangia one sees a short apical prolongation in the form of a small elongated papilla, as shown in fig. 43, 1. Renault refers this fungus to the Chytridineae, and compares it with Cladochytrium, Woronina, Olpidium, and other recent genera.
In the immediate neighbourhood of two of the sporangia shown in the uppermost tracheid of fig. 43, 1, there are seen a few minute dark dots which are described as spores petrified in the act of escaping from a lateral pore. This interpretation strikes one as lacking in scientific caution.