I] INTRODUCTION 17 



emphasized by many of the older plant geographers, notably 

 by Humboldt, A. P. de Candolle, Grisebach, and Thurmann. 

 Thurmann stated (1849: 22) that "la Flore s'entend surtout 

 du nombre des formes ve"g6tales distinctes qu'on y observe, la 

 V^g^tation de leurs proportions et de leur association." From 

 a flora, a knowledge is gained of the occurrence and distribution 

 of the species of a district, of their presence or absence in 

 contiguous districts, of the stations of these species, of the 

 general nature of their habitats, of the altitudes to which 

 they ascend, of their comparative abundance or rarity, of 

 their times of flowering, and of their rank (i.e., whether they 

 are indigenous or not). So much may be expected of any 

 flora which has pretensions to be a scientific work. In some 

 floras, hints are given as to why certain species are confined 

 to certain kinds of habitats : mention" is made of those which 

 are dominant over particular tracts of country : the floristic 

 (not merely the topographical) subdivisions of the district 

 covered by the flora are outlined; and some idea is given of 

 the original migrations of the species into the district in 

 question. Some modern floras rightly furnish details with 

 regard to the very closely allied or " elementary species " 

 which occur in the district, and state how these may be 

 distinguished, whether or not their characters appear to be 

 constant, and whether or not the plants in question are confined 

 to special habitats. 



The flora is composed of the individual species : the vege- 

 tation comprises the groupings of those species into ensembles 

 termed vegetation units or plant communities. 



A botanist who frequently traverses any stretch of unr 

 cultivated land, such as the elevated lands of the Peak District, 

 must recognize sooner or later that the plants have become 

 arranged in definite vegetation groups or plant communities. 

 For instance, in the present district, the gentle slopes of the 

 edges of the peat moors are almost entirely monopolized by 

 heather (Calluna vulgaris), the higher peat moors by cotton- 

 grass (Eriophorum vaginatum), and the highest and most 

 exposed ridges by bilberry ( Vaccinium Myrtilliis). Here then 

 he may distinguish three plant associations which he may 

 term respectively heather moor, cotton-grass moor, and bilberry 

 moor. These associations he finds to be constant both as 



M. 



