But in some instances they were fastened down by narrow strips of
card, and in others by their petioles being passed through slits in the
cork. The leaves were at first fastened close to the cork, for as this is a
bad conductor, and as the leaves were not exposed for long periods, we
thought that the cork, which had been kept in the house, would very
slightly warm them; so that if they were injured by the frost in a greater
degree than the free vertical leaves, the evidence would be so much the
stronger that the horizontal position was injurious. But we found that when
there was any slight difference in the result, which could be detected only
occasionally, the leaves which had been fastened closely down suffered
rather more than those fastened with very long and
[page 287]
thin pins, so as to stand from ? to 3/4 inch above the cork. This
difference in the result, which is in itself curious as showing what a very
slight difference in the conditions influences the amount of injury
inflicted, may be attributed, as we believe, to the surrounding warmer air
not circulating freely beneath the closely pinned leaves and thus slightly
warming them.
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