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 algaa 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 hyphae 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 murorum, is to 

 be seen on the upper part of the southern face; whilst Am.philoma decipiens 

 occurs on the same face, but only near the ground. On the side with a northern 

 aspect Endocarpon m,iniatum, predominates, and on the north-west face Calopisma 

 citrinum 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 



