220 OLAI- GAI.L0K 



Pendulous fruticose lichens can he found on rocks of all 

 degrees of inclination: horizontal surfaces, vertical and sloping rock- 

 faces, etc. 



The associations may be - - as in the case of the phanerogams 

 divided into formations, facies or whatever we may choose to 

 call them, and they may he named after the one or more species 

 which dominate the community. 



In addition to the chemical and physical qualities of the rock 

 and the degree of inclination of the substratum, there are other 

 conditions which play a part as regards the physiognomy of the 

 vegetation, primarily conditions pertaining to moisture, and the 

 competitive relations between the species themselves. 



Thus the same vegetations are not found on rocks wetted with 

 spray, on submerged rocks and on dry emergent rocks. The quality 

 of the water also - - salt, fresh or distilled (rain) water - - plays an 

 essential role here. Moreover, it is of no slight importance, whether 

 the rocks are frequently manured by birds or whether this does 

 not take place. 



We can, as already mentioned, divide the associations, which 

 are produced by the action of each of these complexes of life-con- 

 ditions, in very different ways: we may speak of "nitrophilous 

 associations" (Sernander), of halophilous associations, associations 

 of hollows, associations of horizontal surfaces, etc., according to our 

 knowledge of the factors which determine the association. But this 

 mode of naming them appears to me to be extremely unpractical, 

 because we may very often be at a loss with regard to the group 

 to which we are to refer the association in question. It is in reality 

 not at all possible to draw a decided line between a nitrophilous 

 and a non-nitrophilous association: all lichens are in fact nitro- 

 philous to some degree. 



It is the same difficulty with which the ecologists have had to 

 contend as regards the soil-associations, but in this department order 

 is appearing owing to the fact that the association is not named 

 after Factors as a rule imperfectly known - - which condition its 

 well-being ("sand-vegetation," "rock-vegetation," "xerophilous cop- 

 pice," etc.), but after the plants themselves (phanerophyte-vegetation, 

 chamaephyte-vegetation, etc.). 



Whether one choose the one or the other mode of procedure 

 is by no means a matter of indifference. The associations living in 

 nature are naturally the same, whether we give them the one or 



