1] 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 végétales distinctes qu’on y observe, la 
Végétation 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 (1e., 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 un- 
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 (Hriophorum vaginatum), and the highest and most 
exposed ridges by bilberry (Vacciniwm Myrtillus). 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. 2 
