grain [page 271] (.00024 mg.) is sufficient to excite a single tentacle
into movement.
The nitrate of ammonia induces aggregation of the protoplasm much less
quickly than the carbonate, but is more potent in causing inflection. A
drop containing 1/2400 of a grain (.027 mg.) placed on the disc acts
powerfully on all the exterior tentacles, which have not themselves
received any of the solution; whereas a drop with 1/2800 of a grain
caused only a few of these tentacles to bend, but affected rather more
plainly the blade. A minute drop applied as before, and containing
1/28800 of a grain (.0025 mg.), caused the tentacle bearing this gland
to bend. By the immersion of whole leaves, it was proved that the
absorption by a single gland of 1/691200 of a grain (.0000937 mg.) was
sufficient to set the same tentacle into movement.
The phosphate of ammonia is much more powerful than the nitrate. A drop
containing 1/3840 of a grain (.0169 mg.) placed on the disc of a
sensitive leaf causes most of the exterior tentacles to be inflected,
as well as the blade of the leaf. A minute drop containing 1/153600 of
a grain (.000423 mg.), applied for a few seconds to a gland, acts, as
shown by the movement of the tentacle. When a leaf is immersed in
thirty minims (1.7748 ml.) of a solution of one part by weight of the
salt to 21,875,000 of water, the absorption by a gland of only the
1/19760000 of a grain (.00000328 mg.), that is, about the
one-twenty-millionth of a grain, is sufficient to cause the tentacle
bearing this gland to bend to the centre of the leaf. In this
experiment, owing to the presence of the water of crystallisation, less
than the one-thirty-millionth of a grain of the efficient elements
could have been absorbed. There is nothing remarkable in such minute
quantities being absorbed by the glands, [page 272] for all
physiologists admit that the salts of ammonia, which must be brought in
still smaller quantity by a single shower of rain to the roots, are
absorbed by them. Nor is it surprising that Drosera should be enabled
to profit by the absorption of these salts, for yeast and other low
fungoid forms flourish in solutions of ammonia, if the other necessary
elements are present. But it is an astonishing fact, on which I will
not here again enlarge, that so inconceivably minute a quantity as the
one-twenty-millionth of a grain of phosphate of ammonia should induce
some change in a gland of Drosera, sufficient to cause a motor impulse
to be sent down the whole length of the tentacle; this impulse exciting
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