SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 873 
however, against interpreting the relation as mycorrhizal. The evidence in this case 
is on the whole more consistent with a saprophytic fungus selecting for its growth 
certain tissues or regions of a plant. 
The fungus that has been named Palseomyces Asteroxyli is very regularly found 
in the inner cortex of rhizomes of Asteroxylon and of the basal regions of stems with 
the transition structure. Some rhizomes, especially finer branches, are, however, met 
with apparently free from the fungus. The localisation of the fungus to the inter- 
cellular spaces of the broad inner cortex, leaving both the outer cortex and the 
phloem free, is in support of the view that it was mycorrhizal. It must be borne in 
mind, however, that a similar regional preference has been seen in the case of other 
fungi in the inner cortex of stems of Rhynia Gwynne-Vaughani and R. major, 
where these occasional inhabitants were clearly saprophytic. The regularity of the 
association in Asteroxylon and its occurrence in the subterranean regions of the plant 
appear to justify us in keeping open the question of its having been normally asso- 
ciated with the plant when alive. In this case also, although the evidence of a 
mycorrhizal relation is considerably stronger, it does not amount to proof, and it is 
clear that the fungus is often associated with the breaking-down of the inner cortex 
of the rhizomes ; its original intercellular distribution is then, of course, lost. 
The case of the two species of Rhynia is of a rather different nature.* There is 
no doubt that the active growth of various fungi with evident mycelium bearing 
vesicles or resting-sphores was here saprophytic. The distribution and appearance 
of the layer of cells with very persistent dark contents immediately below the outer 
cortex suggests, however, the possibility that this region might have contained a 
symbiotic organism. The distribution of this layer is shown for Rhynia major in 
Part I, figs. 21 and 22, and for R. Gwynne-Vaughani in Part I, fig. 59. Against 
such an interpretation of the contents is the fact that this layer is marked in the 
stouter stems, and not in the rhizomes. The contents are not evidently mycelial 
when carefully examined. In some cases, however, appearances have been noted 
that would be consistent with the presence of a very fine and closely packed 
mycelium. Figs. 82-84 are of such a specimen, and to some extent indicate the 
appearance and the difficulty of coming to a definite decision on the question. It is 
suggestive that such fungal growths as that shown in fig. 43 often, though not 
always, originate in the position of this layer of cells (cf p. 862) ; this would be equally 
consistent with the evident fungus being a further development of a mycorrhizal 
organism, or being a purely saprophytic fungus selecting for its growth this peculiar 
layer of cells. Thus in the case of Rhynia also the only conclusion at present seems 
to be that proof of the existence of mycorrhiza is wanting, though there are grounds 
for further enquiry into the question. 
As already mentioned, the inference that most of the fungi in the deposit were 
living as saprophytes seems to be well justified. It is difficult to draw any sharp line 
* Hornea showed corresponding structures to those to be described, but the preservation was poor. 
