At the outset you will soon find yourself involved in chemical and
meteorological questions; as, for instance, when you ask--How is it
that I find one flora on the sea-shore, another on the sandstone,
another on the chalk, and another on the peat-making gravelly
strata? The usual answer would be, I presume--if we could work it
out by twenty years' experiment, such as Mr. Lawes, of Rothampsted,
has been making on the growth of grasses and leguminous plants in
different soils and under different manures--the usual answer, I
say, would be--Because we plants want such and such mineral
constituents in our woody fibre; again, because we want a certain
amount of moisture at a certain period of the year: or, perhaps,
simply because the mechanical arrangement of the particles of a
certain soil happens to suit the shape of our roots and of their
stomata. Sometimes you will get an answer quickly enough; sometimes
not. If you ask, for instance, Asplenium viride how it contrives to
grow plentifully in the Craven of Yorkshire down to 600 or 800 feet
above the sea, while in Snowdon it dislikes growing lower than 2000
feet, and is not plentiful even there?--it will reply--Because in
the Craven I can get as much carbonic acid as I want from the
decomposing limestone; while on the Snowdon Silurian I get very
little; and I have to make it up by clinging to the mountain tops,
for the sake of the greater rainfall.
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