556 Fry. — Some Types of Endolithic Limestone Lichens . 
limestone does not seem to hinder in any way the general arrangement and 
growth of the endolithic lichens. In epilithic thalli it is difficult to find 
a parallel to that presented by Placodium rnpestre , var. calvum , forma 
incrustans, unless one could compare with it the small isolated squamules 
of the primary thallus of a Cladonia. The structure of certain species of 
epilithic and endolithic lichens, which have the filamentous Trentepohlia 
as their algal constituent, may also be compared. For example, Coenogonium 
ebeneu'm has a black-coloured, filamentous epilithic thallus, the hyphae 
encircling the erect Trentepohlia filaments. In the endolithic form termed 
£ Y ’, the algal filaments penetrate the rock, where they are subsequently 
surrounded by fungal hyphae in a manner exactly similar to that which 
obtains in Coenogonium. Consequently £ Y ’ could be regarded almost as 
an endolithic filamentous form, the filaments of which, instead of growing 
upwards into the air, penetrate downwards into the limestone, growing in 
length by an apical cell (PI. XXI, Figs. 11 and 12) similar to that of the 
aerial form. 
The collective mass of gonidia and hyphae in an ordinary lichen 
thallus is not usually formed from a single spore and a single algal cell, 
but from several masses of tissue, each starting from a separate growth 
centre. A similar development of the thallus obtains in endolithic forms. 
Spores falling on the surface of the rock may send out germinating tubes 
into the limestone. If the spore belongs to the same species as the endo- 
lithic thallus already present, then the hyphae germinating from the former 
will contribute to the formation of that thallus. This has been observed in 
several cases in lichen ‘Y’ (PI. XXI, Figs. 10 and 13). In one or two old 
perithecial pits of Lecidea immersa , where re-colonization of the limestone 
lining was going on, dark-coloured rounded spores have been observed 
germinating and sending their hyphae into the cortical tissues of Lecidea. 
Though these hyphae resembled very closely those forming the cortex, yet 
the spores were not those of Lecidea immersa , which are oval in shape and 
colourless. The fate of such foreign hyphae in a thallus is not known, but 
the idea suggests itself that they may be the starting-point of a more 
vigorous species which will oust the slower-growing Lecidea . Foreign 
spores are often found on the surface of the endolithic thalli, where, as a rule, 
they do not germinate, but merely contribute to the speckled appearance of 
the surface. 
Observations on the infection of the marginal regions by gonidia 
belonging to the Chlorophyceae have not been made. Since the gonidia 
are present in little clusters in pockets in the rock, the usual method of 
spreading the gonidia in the lateral direction, in which the hyphae first 
separate and then push the algal cells apart, would scarcely seem to hold. 
The endolithic lichens undoubtedly obtain protection both from 
excessive drought and extremes of temperature ; yet, from the fact that 
