210 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 
Tt is at least probable that basalt, mica schist, and barren mud- 
stones also have a great effect on the flora in selected cases, For 
example, a distinct difference is traceable between the flora of 
Glasgow and that of Kilmalcolm and Bridge of Weir. 
For all these plants geological boundaries will, at any rate, 
have some effect on distribution. 
Another group is found only under certain conditions of 
what, for want of a better word, we may call climate. The depth 
and regularity of supply of water, and the amount of sunshine 
and frost, are probably the main constituents of this “climate.” 
Now one climate, the climate of extremes, very definitely limits 
a good many plants. ‘This climate is found in many places which 
look at first sight very different, e.g., on the sand and shingle 
of the sea-shore, the cinders of a railway track, the slopes of a 
blaes or ballast heap, and the highest and most exposed part of a 
mountain, where wind and isolation are at a maximum. In all 
these places a climate of extremes prevails, and it is not at all 
remarkable that we find many plants on the mountains and on 
the sea-shore which are totally absent from the intervening country. 
Antiyllis Vulneraria, Linn., and Hieraciwm Pilosella, Linn., are 
found on railway lines in Dumfriesshire. The same two plants I 
found recently at 6,000 feet in the Alps. In both places there 
was this climate of extremes. Hence the range of, e.g., Senecio 
viscosus, Linn., Linaria minor, Desf., Tragopogon, and Melilotus 
has been extended northwards by the artificial effect of man, by 
his producing these bare rocks and blaes where the suitable 
climate of extremes exists. 
Let us next take Alpine plants, which only grow naturally on 
the mountains above 1,400 feet. What assistance would river 
and valley be in the study, e.g., of the range of Saxifraga cernua, 
Linn.? It occurs in one place in Steiermark, in the Himalayas, 
Thibet, Scandinavia, Siebengeburgen, Alps, Wallis, Tyrol, 
Carinthia, and our own country. Ball has shown that 17 per 
cent. of the Alpine species are common to the Arctic regions and 
25 per cent. to the Altai? Most of them can be cultivated in 
gardens, even at sea level, by providing the proper habitat. 

1 Krasan, Botan. Centralblatt, Bd, LXIX., p. 297. 
* Ball, Vrans, Linn. Soc.; Ser: iis, Vol. V., Part iv., 1896. 
