552 Fry. — Some Types of Endolithic Limestone Lichens. 
thalli — particularly those of section a in A — and there is, in addition, the 
very poorly developed rhizoidal system. 
I. B. a. Lichen ‘ Y ’ (an indeterminate form , since no f mating bodies 
have been found), 
A detailed description of this species will not be given, since Bachmann 
(loc. cit.) has already described a similar form, but as he did not include any 
detailed drawing of a decalcified section of the thallus as a whole, the 
present writer ventures to insert this form as illustrating a homoiomerous 
thallus, and also because in this type there are several points of interest 
apart from the actual structure of the thallus. 
While still embedded in the rock, lichen ‘ Y ’ has a dark rusty grey 
colour with numerous black spots, but after decalcification it appears golden 
brown. The thallus is very thin, measuring only 140/1-200 //. No rhizoids are 
present. In vertical section (Plate XXI, Fig. 10) there is a discontinuous 
band of dark-coloured hyphae = X,dead Trentepohlia 11 s = Y, and many 
dark bodies = Z, which appear very like fungal spores. These ‘spores’ lie 
on the surface of the thallus and send down into the rock hyphae which wind 
round the gonidial constituent of the lichen. In fact many cases of this kind 
have been seen and many of the hyphae of the thallus originate in this way. 
(Plate XXI, Fig. 13, shows a ‘ spore ’ at the surface giving rise to hyphae 
which encircle the Trentepohlia cell already in the rock ; the dotted region 
represents the limestone.) Dark hyphal cells which lie on the surface put 
out branches into the rock ; these branches are also thick walled and dark 
coloured, but, where in contact with the gonidial cells, their walls are always 
colourless and thin. Below the main masses of dark- coloured hyphae and 
algal cells one usually finds many filaments of Trentepohlia , encircled by 
hyphae, penetrating the rock. Lower in the limestone the gonidia with 
their attendant hyphae are less common, and at about 2co fi below the 
surface they are absent. Sometimes many Trentepohlia filaments lie 
parallel with each other, particularly near the surface of the limestone, yet 
the structure of the rock seems to play no part in the structure of the 
thallus, for the gonidial filaments and the hyphae, when the latter occur 
alone, lie in the limestone at all angles with the surface. 
Generally it is the Trentepohlia which penetrates farthest into the 
rock, either alone or partly surrounded by hyphae ; even in the latter case 
that part of the algal cell farthest in the rock is usually free of hyphae 
(PI. XXI, Figs. 11 and 12). Since hyphal filaments are found in the rock 
without gonidia, one must conclude that they too are capable of the pioneer 
boring of the limestone. This is also clearly indicated by the penetration 
of the rock by the germinating filaments of the ‘ spores ’ at the surface. 
