45 
Fundamental Units of Vegetation. 
their formations or associations into groups which followed the 
ancient division of the vegetable kingdom into trees, shrubs and 
herbs ; but it is scarcely possible that such a basis can ever yield a 
scientific or natural classification of plant communities; although 
Warming (1909: 140-141) makes this “ the prime basis of classifi¬ 
cation ” in subdividing the groups of hydrophilous, xerophilous, and 
mesophilous plants. One finds too that authors have adopted points 
of view which differ according to the size and extent of the earth’s 
surface which they happened to be investigating ; and this perhaps 
is inevitable. It is the same in taxonomy. Those systematists who 
have dealt with continental areas or with whole phyla of plants 
have invariably given a wide significance to the species; and those 
who have studied intensively the flora of small districts, or who 
have monographed special genera, have generally used the term 
species in a much narrower sense. 
Many of the physiognomical “formations,” e.g., tundra, of 
Grisebach and others, are to be regarded either as groups of 
formations or groups of associations ; and these may or may not 
be natural groups. Kerner (1863) arranged his minor “formations” 
into groups (Gruppen von Genossenschaften). Drude (1890) in his 
Handbucli, in which he dealt with the whole world, adopted a 
physiognomical classification on the whole : in his Deutschlands 
Pflnnzetigeographic (1896), his “formations” (chief associations) 
were arranged into groups some of which depend largely on 
physiognomy, e.g., wood “ formations” and scrub “formations,” and 
others largely on habitat, e.g., moor “formations” and aquatic 
“ formations ; ” whilst in his still more detailed study of the forma¬ 
tions of the Hercynian region (1899), he adopted a more natural 
classification and one based more completely on habitat. 
Warming’s (1895) classification of plant communities, to some 
extent based on the work of Schouw (1822, 1823), was undoubtedly 
the most fundamental which had at that time been put forward. 
Warming based his major divisions, hydrophyte communities, 
xerophyte communities, halophyte communities, and mesophyte 
communities, on the nature of the habitat, partly on the water content 
and partly on the mineral content of the soil. This classification, 
however, met with some adverse criticism, particularly with regard to 
the inclusion of “ bog xerophytes ” among hydrophytes, with regard 
to the separation of halophytes from xerophytes, with regard to the 
inclusion of all conifers among xerophytes and of all deciduous trees 
among mesophytes (cf. Stopes, 1907; Moss, 1907b: 6), and with 
