216 THALLOPHYTA. [CH. 



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'^. 

 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//,, and the 

 shorter axis 9 — 10 /t ; 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 

 Cladochytrinm, 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 



1 Eenaint (96) p. 425, fig. 78. 



