548 Fry . — Some Types * of Endolithic Limestone Lichens . 
apothecia were found to possess numerous spherical oil cells, which usually 
occurred singly. These were readily seen when mature apothecia were 
crushed by slight pressure of the cover-slip. They occurred in the dense 
tissue of the hypothecium and in the region immediately below. As in 
Verrucaria , once the fruiting body has been removed, the walls of the pit 
are occupied afresh by gonidia and hyphae. Spermogonia are also found 
in small hollows, but these penetrate only to the base of the gonidial zone. 
I. A. b. Lichen £ X 5 {not determinable ). 
The surface of the thallus ‘ X * in the limestone shows nothing distinctive. 
There are no apothecia or perithecia and no black spots representing the 
external tips of hyphae. Very occasionally one finds the small blue-green 
algal colonies, but, as they are commonly found on endolithic lichen 
surfaces, they form no distinguishing feature. When decalcified the thallus 
surface appears very much smoother than that of either of the types 
described above, and there is also a greater thickness, the lichen pene- 
trating the rock to a depth of 1,000 /x-i,ioo /x. 
In vertical section the three zones are clearly distinguished. For® 
some time the cortex was thought to be of a very loose texture. It was 
only when the sections had been deeply stained with Congo red, in 
addition to the haematoxylin, that the very fine delicate walls of the 
majority of the hyphae of the cortical zone could be detected. The cortex 
measures 46/X-70/X, which is wide when compared with Lecidea immersa • 
and V. calciseda. It seems to be composed of two kinds of hyphal 
filaments : (i) the smaller number, made up of wide quadrate cells possessing 
dense contents which stain deeply with haematoxylin ; (ii) the larger 
number, whose contents do not stain, 'the presence of these only being 
betrayed by the faint pinkish tinge of their walls (PI. XXI, Fig. 3). The 
hyphae with the dense contents can be traced very easily from the outer 
edge of the thallus to the gonidial zone. In this region they appear to 
pursue a slightly tortuous path, branch but little, and generally preserve 
a direction at right angles to the surface of the rock. Usually the end one 
or two cells are colourless and correspond to the second type of hyphae. 
Filling in nearly all the spaces not occupied by the above hyphae are 
those filaments the cells of which appear to be without contents. At the 
outer limit of the thallus, together with the end cells of the former type, 
they seem to form a fairly continuous layer, which may be slightly brownish 
on the extreme outside. 
The gonidial zone measures on the average 92 jx-140 /x. This great 
width is due to the fact that the narrow gonidial groups are rather elongated 
in the vertical direction. The size of the groups varies considerably 
(compare Text-figs. 7 and 8 with PL XXI, Fig. 3). The gonidia, both as 
clusters and individuals, are spun round by both kinds of hyphae, but more 
