Fry . — Some Types of Endolithic Limestone Lichens . 559 
to explain the boring action of the fine hyphae into the limestone, but they 
have all been discredited. That put forward by Wallroth (16) may be 
instanced. He stated that the rhizoids excreted an acid fluid which 
dissolved the limestone, the remainder of the thallus being raised in the 
air. Owing to the precipitation of calcium carbonate in the intercellular 
spaces of the thallus, the latter was so highly charged with the carbonate 
that it became as hard as stone. A consideration of the results of this 
investigation has led the writer to form the following hypothesis to explain 
the boring action of these lichens. The boring action is performed by the 
carbon dioxide of respiration, dissolved in the water which soaks down 
from the surface, and is retained by capillarity in the numerous cracks and 
pores in the limestone. This carbonic acid acts on the rock forming the 
soluble bicarbonate. This solution diffuses up, between the hyphae and 
the walls of the cavities made by them, to the surface film of water. From 
the surface it may either be washed away by the next shower of rain, or, 
in the case of drought setting in, it may precipitate the more stable 
carbonate on the surface. The continual precipitation of calcium carbonate 
on the surface of the embedded lichen will account for the permanent 
granular appearance of the surface of the endolithic thallus examined. 
There are numerous small depressions on the flat tables of limestone, and 
in these rain-water tends to collect. If this water is allowed to remain for 
a short time, it is found to contain calcium carbonate in solution. In the 
cases under discussion, since there is no chance of water, already containing 
calcium carbonate, draining into the depression, the carbonate must be 
derived either from the solution of the granules of limestone on the surface 
of the endolithic thalli, or by diffusion into the water of the bicarbonate 
produced by the boring action of the endolithic lichens lining the depression, 
or from both these sources. The rain-water probably contains a small 
amount of atmospheric carbon dioxide in‘ solution, and this, together with 
the carbon dioxide of respiration of the rock lichens, is sufficient to dissolve 
the granules of limestone deposited on the surface as described above. If 
the small collection of water dries up, the dissolved limestone will be 
reprecipitated on the thalli, but in the case of a shower of rain it will be 
washed down into the neighbouring crevices ; since the rainfall in these 
regions is very high, this will frequently take place. This will also explain 
the removal from the apothecial pits of the undissolved particles of lime- 
stone, for the processes which go on on a large scale in the depressions are 
also in progress in the minute apothecial hollows. 
One has already come to the conclusion that the carbon dioxide of 
respiration of lichens on other substrata than limestone, e. g. shale, is 
responsible for a considerable proportion of the decomposition of the rock 
in their immediate vicinity. Miss Mellor (17) also has recently published 
a paper proving that the carbon dioxide of lichen respiration in solution 
