Leaf immersed, with time allowed for each gland to absorb all that it
can : 1/268800 of a grain, or .00024 mg. : 1/691200 of a grain, or
.0000937 mg. : 1/19760000 of a grain, or .00000328 mg.
Amount absorbed by a gland which suffices to cause the aggregation of
the protoplasm in the adjoining cells of the tentacles. 1/134400 of a
grain, or .00048 mg.
From the experiments tried in these three different ways, we see that
the carbonate, which contains 23.7 per cent. of nitrogen, is less
efficient than the nitrate, which contains 35 per cent. The phosphate
contains less nitrogen than either of these salts, namely, only 21.2
per cent., and yet is far more [page 169] efficient; its power no doubt
depending quite as much on the phosphorus as on the nitrogen which it
contains. We may infer that this is the case, from the energetic manner
in which bits of bone and phosphate of lime affect the leaves. The
inflection excited by the other salts of ammonia is probably due solely
to their nitrogen, on the same principle that nitrogenous organic
fluids act powerfully, whilst non-nitrogenous organic fluids are
powerless. As such minute doses of the salts of ammonia affect the
leaves, we may feel almost sure that Drosera absorbs and profits by the
amount, though small, which is present in rain-water, in the same
manner as other plants absorb these same salts by their roots.
The smallness of the doses of the nitrate, and more especially of the
phosphate of ammonia, which cause the tentacles of immersed leaves to
be inflected, is perhaps the most remarkable fact recorded in this
volume. When we see that much less than the millionth* of a grain of
the phosphate, absorbed by a gland of one of the exterior tentacles,
causes it to bend, it may be thought that the effects of the solution
on the glands of the disc have been overlooked; namely, the
transmission of a motor impulse from them to the exterior tentacles. No
doubt the movements of the latter are thus aided; but the aid thus
rendered must be insignificant; for we know that a drop containing as
much as the 1/3840 of a grain placed on the disc is only just able to
cause the outer tentacles of a highly sensitive leaf to bend. It is
cer-
* It is scarcely possible to realise what a million means. The best
illustration which I have met with is that given by Mr. Croll, who
151