In the remaining four, the result was not so plain, but certainly
whatever difference there was leaned to the side of the upper surface
having suffered most.
It has been stated that some of the leaflets experimented on were fastened
close to the cork, and others at a height of from ? to 3/4 of an inch above
it; and that whenever, after exposure to a frost, any difference could be
detected in their states, the closely pinned ones had suffered most. We
attributed this difference to the air, not cooled by radiation, having been
prevented from circulating freely beneath the closely pinned leaflets. That
there was really a difference in the temperature of leaves treated in these
two different methods, was plainly shown on one occasion; for after the
exposure of a pot with plants of Melilotus dentata for 2 h. to a clear sky
(the temperature on the surrounding grass being -2o C.), it was manifest
that more dew had congealed into hoar-frost on the closely pinned leaflets,
than on those which stood horizontally
[page 296]
a little above the cork. Again, the tips of some few leaflets, which had
been pinned close to the cork, projected a little beyond the edge, so that
the air could circulate freely round them.
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