Fry. — Some Types of Endolithic Limestone Lichens. 557 
the thalli exhibit the structure of ordinary epilithic forms while embedded 
in the rock, it may be assumed that the ordinary life of the organisms is 
carried on unhindered by the enclosing limestone. From the nature of the 
habitat it would seem that neither the cortex nor the rhizoidal zone is 
essential for the performance of the simple and obvious functions of pro- 
tecting the gonidial layer and of fixing the plant to the substratum. In 
view of this, one may be permitted to conclude that both in epilithic and 
endolithic forms, unless it is a matter of inherent tendency on the part of 
the lichen to produce them, both cortical and rhizoidal zones have definite 
functions to perform, apart from the simpler ones mentioned above. 
A feature which is present in most, but not all, endolithic lichens is 
the development, more particularly in the rhizoidal region, of spherical oil 
cells. Oil globules, however, are found in the rhizoids of all the endolithic 
species examined, whether they possess the inflated cells or not. The oil 
cells occur singly or in pairs, as in the terminal inflations of V . calciseda , in 
simple chains, as in lichen ‘X ’, or in large complex cell-masses such as are 
found in the immersed part of A. calcar ea. Similar oil hyphae have been 
found by Rosendahl (9) and Bachmann (10 and 14) in a few lichens which 
do not grow on a calcareous substratum, but these may be regarded as 
exceptional cases. It may be helpful at this point to summarize the cases 
where these inflated cells occur in the thalli described, and also their 
relative abundance. 
Verrucaria calciseda. The rhizoidal and marginal regions, where they 
are very common. The young perithecium (lid and basal part). 
Lecidea immersa.' Apothecial tissue — common. 
Lichen ‘X’. Rhizoidal region — common. 
Aspicilia calcar ea . Rhizoidal region, more particularly in the neighbour- 
hood of the apothecia. 
Placodium rnpestre , var. calvum , f. incrustans. Absent. 
Lichen ‘ Y \ Absent. 
Since they are not present in all forms and in all parts of the thallus 
they cannot be considered essential for the endolithic habit nor a 
characteristic of endolithic thalli, though the oil itself, which is common 
to the hyphae — more particularly the rhizoidal hyphae — may be an essential 
constituent of the thallus tissue. If .one considers the position of the inflated 
oil cells in the types described, one finds that they occur in regions of 
special growth. The thallus of V. calciseda is one of the more rapidly 
growing species. It has been observed during the present investigation to 
oust from their positions P. rupestre , var. calvum , forma incrustans , and 
L. immersa , both of which are found to be without inflated cells in their 
thallus tissues. Funfsttick (loc. cit.) found inflated oil cells of L. immersa 
8 mm. below the limestone surface, but in the thalli examined in the present 
