The Distribution of Plants. 
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BY 
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H. N. RIDLEY, C.M.G., F.R.S. 
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I N examining the constituents of the flora of any country one is struck by 
the great variety of species, genera, and orders represented in it, and 
secondly, one notices that the species are not altogether those occurring in 
the nearest lands, though in most countries the greater number of the species 
have affinities or are identical with those of adjacent countries, even if these 
countries are separated now by considerable stretches of sea ; there are 
besides a certain number—usually very local and generally consisting of one, 
or perhaps two, species of a genus—which have no affinity at present with 
anything else in the area, but with plants of far distant regions, and which 
are the relics of a long-lost flora. Besides these we have now in all parts of 
the world, wherever man has trod, a larger or smaller number of plants 
introduced accidentally or intentionally by man. 
Ground bare of plants fills up very rapidly with vegetation brought to 
it in the form of seeds from the nearest land by various means of dispersal, 
and eventually becomes so densely covered that there is no room for 
additional species, and it remains in a state of equilibrium until one of the 
great factors of change comes again into play. 
As a rule, the greatest variation is to be found where several distinct 
floras are, or have been in the past, sufficiently near to supply any given 
country with its flora. 
In the British Isles we have a central European element, an Arctic 
element, a Portuguese element, and a North American element. In the 
Malay Peninsula a large percentage of the species is common to Sumatra 
or has close affinities with those of Sumatra ; a number are represented in 
Borneo only ; others are Javanese, Burmese, Indian, Cochin Chinese, and 
Siamese ; while both in the British Isles and in the Malay Peninsula we have 
a number of established weeds, in the latter country chiefly from South 
America and the West Indies, which go to make the mixed floras as we find 
them to-day. 
This mixture of plants in a country is due to the three great factors of 
change, which are—(i) change of climate, (2) change of the land surface, 
(3) change due to human agency. 
[Annals of Botany, Vol. XXXVII. No. CXLV. January, 1923.] 
