hrs. 15 m. The fourth leaf, after 3 hrs. 7 m., had only twenty-nine
tentacles more or less inflected. Thus three out of the four leaves
were strongly acted on. It is clear that very sensitive leaves had been
accidentally selected. The day moreover was hot. The four corresponding
leaves in water were likewise acted on rather more than is usual; for
after 3 hrs. one had nine tentacles, another four, and another two, and
the fourth none, inflected. With respect to the leaf of which all the
tentacles, except sixteen, were inflected after 50 m., each gland
(assuming that the leaf bore 160 tentacles) could have absorbed only
1/691200 of a grain (.0000937 mg.), and this appears to be about the
least quantity of the nitrate which suffices to induce the inflection
of a single tentacle.
As negative results are important in confirming the foregoing positive
ones, eight leaves were immersed as before, each in thirty minims of a
solution of one part to 175,000 of water (1 gr. to 400 oz.), so that
each received only 1/6400 of a grain (.0101 mg.). This minute quantity
produced a slight effect on only four of the eight leaves. One had
fifty-six tentacles inflected after 2 hrs. 13 m.; a second, twenty-six
inflected, or sub-inflected, after [page 153] 38 m.; a third, eighteen
inflected, after 1 hr.; and a fourth, ten inflected, after 35 m. The
four other leaves were not in the least affected. Of the eight
corresponding leaves in water, one had, after 2 hrs. 10 m., nine
tentacles, and four others from one to four long-headed tentacles,
inflected; the remaining three being unaffected. Hence, the 1/6400 of a
grain given to a sensitive leaf during warm weather perhaps produces a
slight effect; but we must bear in mind that occasionally water causes
as great an amount of inflection as occurred in this last experiment.]
A Summary of the Results with Nitrate of Ammonia. The glands of the
disc, when excited by a half-minim drop (.0296 ml.), containing 1/2400
of a grain of the nitrate (.027 mg.), transmit a motor impulse to the
exterior tentacles, causing them to bend inwards. A minute drop,
containing 1/28800 of a grain (.00225 mg.), if held for a few seconds
in contact with a gland, causes the tentacle bearing this gland to be
inflected. If a leaf is left immersed for a few hours, and sometimes
for only a few minutes, in a solution of such strength that each gland
can absorb only the (1/691200 of a grain (.0000937 mg.), this small
amount is enough to excite each tentacle into movement, and it becomes
closely inflected.
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