LICHENS. 247 
other potential partner, the latter embrace and enmesh them, as is shown in the 
above figures, and thus is produced the confederacy called a Lichen. The member 
destitute of chlorophyll takes up nutriment from the external environment; it 
possesses, in particular, the property of condensing aqueous vapour, and has, besides, 
the power of bringing the solid substratum partially into solution by means of 
excreted substances; it effects adhesion to the substratum, and, in a majority of 
cases, determines the form and colour of the lichen-thallus as a whole. The second 
member, whose cells contain chlorophyll, undertakes the task of producing organic 
matter, under the influence of sunlight, from the materials conveyed to it; by this 
means it multiplies the number of its cells and increases in volume, whilst, at the 
same time, it yields to its mate so much as is necessary in order to enable the latter 
to keep pace with it in growth. 
The number of alge which enters into a partnership of this kind is, in any 
case, much less considerable than that of the fungi, and it must be assumed that 
one species of alga may unite with the hyphe of different lichen-fungi. The 
extreme variety, moreover, in the combinations of the two sorts of confederate 
occurring on a very small area is obvious from the circumstance that it is not 
rare for half a dozen different species of lichen to spring up side by side on a patch 
of rock no bigger than one’s hand. Whether they all achieve an equally hardy 
development, or whether some perchance are not crowded out and overgrown 
by others depends on various external conditions—on the chemical composition 
of the substratum, and particularly on the conditions of moisture and illumination 
of the site in question. Lichens are very sensitive in this respect, and the different 
sides of a single rock often exhibit quite different growths of lichens. A very 
instructive example of this is afforded by a marble column near the famous castle 
of Ambras in Tyrol. This column is octagonal, and has been standing in its place 
for more than two hundred years, with all its sides exposed to wind and weather. 
Lichens have settled on all the eight faces, and, indeed, are present in such abund- 
ance that the stone is quite covered by patches the size of a man’s hand. Many 
of these growths are but poorly developed, and not susceptible of being identified 
with certainty; but altogether on this column there must be over a dozen different 
species, the germs of which can only have been brought by winds. ‘These species 
are, however, by no means uniformly disposed; some prevail on one side, some 
on another, and a few are confined exclusively to one of the eight faces. Of three 
species of Amphiloma, the one named A. elegans is restricted to the warmest side, 
i.e. the face exposed to the south-west; a second, Amphiloma mwrorum, is to 
be seen on the upper part of the southern face; whilst Amphiloma decipiens 
occurs on the same face, but only near the ground. On the side with a northern 
aspect Endocarpon miniatwm predominates, and on the north-west face Calopisma 
citrinwm and Lecidea are the prevailing forms. 
What thousands of spores and algal cells must have been blown on to this 
pillar to enable all these combinations to arise! What complex processes must 
have gone on before the selection of lichens best adapted to each different quarter 
