Fry. — Some Types of Endolithic Limestone Lichens . 545 
and less . regular in shape than those nearer the surface. The contents of 
a cell in the lower rhizoid region are small in comparison with those 01 
a hypha of the gonidial zone, but the nucleus is very plainly marked. In 
the upper rhizoidal zone the hyphae resemble very closely those in the 
neighbourhood of the gonidial groups. Commonly the terminal cell or cells 
of a branch or filament swell up into a spherical body or bodies which often 
reach a diameter of 14 /x or more (Text-fig. 2). These are usually thin 
walled, but frequently become thicker and dark coloured, particularly in the 
marginal regions of Verrucaria calciseda. These spherical or inflated hyphae 
are full of a highly refractive greenish oil. Neither the function nor the 
formation of this substance is properly understood, but various suggestions 
Text-fig. i. Verrucaria calciseda (tangential sec- 
tion 30 \x in limestone), showing gonidial groups en- 
circled by hyphae, and hyphae crossing from group to 
group. 
have been made which will be referred to in the general discussion. From 
numerous cases examined in other forms it seems that the oil accumulates 
and the wall enlarges or stretches to keep pace with the accumulation. 
The inflated hyphae which are present immediately below the gonidial 
zone are found commonly all through the rhizoidal region. It has been 
mentioned that at the margin' of the thallus there is a black line. In 
longitudinal section this shows as a mass of dark-coloured, slightly enlarged 
hyphal cells, just above the level of the limestone. Immediately below this 
in the rock the decomposing remains of the neighbouring thallus are seen in 
the meshes of the dark marginal hyphae (PI. XXI, Fig. 2). The presence 
of this dark mass at the surface is not constant. In other places the 
hyphae do not come above the level of the rock, but remain more loosely 
packed in the limestone. They are dark coloured, thick walled, and bear 
numerous terminal inflations. Often they are found in the rock ahead of 
