CHAPTER IX 



ECOLOGY 

 A. GENERAL INTRODUCTION 



ECOLOGY is the science that deals with the habitats of plants and their 

 response to the environment of climate or of substratum. Ecology in the 

 lichen kingdom is habitat "writ large," and though it will not be possible in 

 so wide a field to enter into much detail, even a short examination of lichens 

 in this aspect should yield interesting results, especially as lichens have 

 never, at any time, been described without reference to their habitat. In 

 very early days, medicinal Usneas were supposed to possess peculiar virtues 

 according to the trees on which thy grew and which are therefore carefully 

 recorded, and all down the pages of lichen literature, no diagnosis has been 

 drawn up without definite reference to the nature of the substratum. Not 

 only rocks and trees are recorded, but the kind of rock and the kind of tree 

 are often specified. The important part played by rock lichens in preparing 

 soil for other plants has also received much attention 1 . 



Several comprehensive works on Ecology have been published in recent 

 times and though they deal mainly with the higher vegetation, the general 

 plan of study of land plants is well adapted to lichens. A series of definitions 

 and explanations of the terms used will be of service : 



Thus in a work by Moss 2 we read " The flora is composed of the indi- 

 vidual species: the vegetation comprises the groupings of these species into 

 ensembles termed vegetation units or plant communities." And again : 



1. "A plant formation is the whole of the vegetation which occurs on 

 a definite and essentially uniform habitat." All kinds of plants are included 

 in the formation, so that strictly speaking a lichen formation is one in which 

 lichens are the dominant plants. Cf. p. 394. The term however is very loosely 

 used in the literature. A uniform habitat, as regards lichens, would be that 

 of the different kinds of soil, of rock, of tree, etc. 



2. " A plant association is of lower rank than a formation, and is charac- 

 terized by minor differences within the generally uniform habitat." It 

 represents a more limited community within the formation. 



3. " A plant society is of lower rank than an association, and is marked 

 by still less fundamental differences of the habitat." The last-named term 

 represents chiefly aggregations of single species. Moss adds that: "plant 

 community is a convenient and general term used for a vegetation unit of 

 any rank." 



Climatic conditions and geographical position are included in any con- 

 sideration of habitat, as lichens like other plants are susceptible to external 



influences. 



1 See p. 392. * Moss 1913. 



