CHAPTER V.

THE EFFECTS OF NON-NITROGENOUS AND NITROGENOUS ORGANIC FLUIDS ON THE
LEAVES.

Non-nitrogenous fluids—Solutions of gum arabic—Sugar—Starch—Diluted
alcohol—Olive oil— Infusion and decoction of tea—Nitrogenous
fluids—Milk—Urine—Liquid albumen—Infusion of raw meat—Impure
mucus—Saliva—Solution of isinglass—Difference in the action of these
two sets of fluids—Decoction of green peas—Decoction and infusion of
cabbage—Decoction of grass leaves.

When, in 1860, I first observed Drosera, and was led to believe that
the leaves absorbed nutritious matter from the insects which they
captured, it seemed to me a good plan to make some preliminary trials
with a few common fluids, containing and not containing nitrogenous
matter; and the results are worth giving.

In all the following cases a drop was allowed to fall from the same
pointed instrument on the centre of the leaf; and by repeated trials
one of these drops was ascertained to be on an average very nearly half
a minim, or 1/960 of a fluid ounce, or .0295 ml. But these measurements
obviously do not pretend to any strict accuracy; moreover, the drops of
the viscid fluids were plainly larger than those of water. Only one
leaf on the same plant was tried, and the plants were collected from
two distant localities. The experiments were made during August and
September. In judging of the effects, one caution is necessary: if a
drop of any adhesive fluid is placed on an old or feeble leaf, the
glands of which have ceased to secrete copiously, the drop sometimes
dries up, especially if the plant [page 77] is kept in a room, and some
of the central and submarginal tentacles are thus drawn together,
giving to them the false appearance of having become inflected. This
sometimes occurs with water, as it is rendered adhesive by mingling
with the viscid secretion. Hence the only safe criterion, and to this
alone I have trusted, is the bending inwards of the exterior tentacles,
which have not been touched by the fluid, or at most only at their
bases. In this case the movement is wholly due to the central glands
having been stimulated by the fluid, and transmitting a motor impulse
to the exterior tentacles. The blade of the leaf likewise often curves
inwards, in the same manner as when an insect or bit of meat is placed
on the disc. This latter movement is never caused, as far as I have
seen, by the mere drying up of an adhesive fluid and the consequent
drawing together of the tentacles.

First for the non-nitrogenous fluids. As a preliminary trial, drops of
distilled water were placed on between thirty and forty leaves, and no
effect whatever was produced; nevertheless, in some other and rare
cases, a few tentacles became for a short time inflected; but this may
have been caused by the glands having been accidentally touched in
getting the leaves into a proper position. That water should produce no
effect might have been anticipated, as otherwise the leaves would have
been excited into movement by every shower of rain.

[Gum arabic.—Solutions of four degrees of strength were made; one of
six grains to the ounce of water (one part to 73); a second rather
stronger, yet very thin; a third moderately thick, and a fourth so
thick that it would only just drop from a pointed instrument. These
were tried on fourteen leaves; the drops being left on the discs from
24 hrs. to 44 hrs.; generally about [page 78] 30 hrs. Inflection was
never thus caused. It is necessary to try pure gum arabic, for a friend
tried a solution bought ready prepared, and this caused the tentacles
to bend; but he afterwards ascertained that it contained much animal
matter, probably glue.

Sugar.—Drops of a solution of white sugar of three strengths (the
weakest containing one part of sugar to 73 of water) were left on
fourteen leaves from 32 hrs. to 48 hrs.; but no effect was produced.

Starch.—A mixture about as thick as cream was dropped on six leaves and
left on them for 30 hrs., no effect being produced. I am surprised at
this fact, as I believe that the starch of commerce generally contains
a trace of gluten, and this nitrogenous substance causes inflection, as
we shall see in the next chapter.

Alcohol, Diluted.—One part of alcohol was added to seven of water, and
the usual drops were placed on the discs of three leaves. No inflection
ensued in the course of 48 hrs. To ascertain whether these leaves had
been at all injured, bits of meat were placed on them, and after 24
hrs. they were closely inflected. I also put drops of sherry-wine on
three other leaves; no inflection was caused, though two of them seemed
somewhat injured. We shall hereafter see that cut off leaves immersed
in diluted alcohol of the above strength do not become inflected.

Olive Oil.—drops were placed on the discs of eleven leaves, and no
effect was produced in from 24 hrs. to 48 hrs. Four of these leaves
were then tested by bits of meat on their discs, and three of them were
found after 24 hrs. with all their tentacles and blades closely
inflected, whilst the fourth had only a few tentacles inflected. It
will, however, be shown in a future place, that cut off leaves immersed
in olive oil are powerfully affected.

Infusion and Decoction of Tea.—Drops of a strong infusion and
decoction, as well as of a rather weak decoction, of tea were placed on
ten leaves, none of which became inflected. I afterwards tested three
of them by adding bits of meat to the drops which still remained on
their discs, and when I examined them after 24 hrs. they were closely
inflected. The chemical principle of tea, namely theine, was
subsequently tried and produced no effect. The albuminous matter which
the leaves must originally have contained, no doubt, had been rendered
insoluble by their having been completely dried.]

We thus see that, excluding the experiments with water, sixty-one
leaves were tried with drops of the [page 79] above-named
non-nitrogenous fluids; and the tentacles were not in a single case
inflected.

[With respect to nitrogenous fluids, the first which came to hand were
tried. The experiments were made at the same time and in exactly the
same manner as the foregoing. As it was immediately evident that these
fluids produced a great effect, I neglected in most cases to record how
soon the tentacles became inflected. But this always occurred in less
than 24 hrs.; whilst the drops of non-nitrogenous fluids which produced
no effect were observed in every case during a considerably longer
period.

Milk.—Drops were placed on sixteen leaves, and the tentacles of all, as
well as the blades of several, soon became greatly inflected. The
periods were recorded in only three cases, namely, with leaves on which
unusually small drops had been placed. Their tentacles were somewhat
inflected in 45 m.; and after 7 hrs. 45 m. the blades of two were so
much curved inwards that they formed little cups enclosing the drops.
These leaves re-expanded on the third day. On another occasion the
blade of a leaf was much inflected in 5 hrs. after a drop of milk had
been placed on it.

Human Urine.—Drops were placed on twelve leaves, and the tentacles of
all, with a single exception, became greatly inflected. Owing, I
presume, to differences in the chemical nature of the urine on
different occasions, the time required for the movements of the
tentacles varied much, but was always effected in under 24 hrs. In two
instances I recorded that all the exterior tentacles were completely
inflected in 17 hrs., but not the blade of the leaf. In another case
the edges of a leaf, after 25 hrs. 30 m., became so strongly inflected
that it was converted into a cup. The power of urine does not lie in
the urea, which, as we shall hereafter see, is inoperative.

Albumen (fresh from a hen’s egg), placed on seven leaves, caused the
tentacles of six of them to be well inflected. In one case the edge of
the leaf itself became much curled in after 20 hrs. The one leaf which
was unaffected remained so for 26 hrs., and was then treated with a
drop of milk, and this caused the tentacles to bend inwards in 12 hrs.

Cold Filtered Infusion of Raw Meat.—This was tried only on a single
leaf, which had most of its outer tentacles and the blade inflected in
19 hrs. During subsequent years, I repeatedly used this infusion to
test leaves which had been experimented [page 80] on with other
substances, and it was found to act most energetically, but as no exact
account of these trials was kept, they are not here introduced.

Mucus.—Thick and thin mucus from the bronchial tubes, placed on three
leaves, caused inflection. A leaf with thin mucus had its marginal
tentacles and blade somewhat curved inward in 5 hrs. 30 m., and greatly
so in 20 hrs. The action of this fluid no doubt is due either to the
saliva or to some albuminous matter* mingled with it, and not, as we
shall see in the next chapter, to mucin or the chemical principle of
mucus.

Saliva.—Human saliva, when evaporated, yields** from 1.14 to 1.19 per
cent. of residue; and this yields 0.25 per cent. of ashes, so that the
proportion of nitrogenous matter which saliva contains must be small.
Nevertheless, drops placed on the discs of eight leaves acted on them
all. In one case all the exterior tentacles, excepting nine, were
inflected in 19 hrs. 30 m.; in another case a few became so in 2 hrs.,
and after 7 hrs. 30 m. all those situated near where the drop lay, as
well as the blade, were acted on. Since making these trials, I have
many scores of times just touched glands with the handle of my scalpel
wetted with saliva, to ascertain whether a leaf was in an active
condition; for this was shown in the course of a few minutes by the
bending inwards of the tentacles. The edible nest of the Chinese
swallow is formed of matter secreted by the salivary glands; two grains
were added to one ounce of distilled water (one part to 218), which was
boiled for several minutes, but did not dissolve the whole. The
usual-sized drops were placed on three leaves, and these in 1 hr. 30 m.
were well, and in 2 hrs. 15 m. closely, inflected.

Isinglass.—Drops of a solution about as thick as milk, and of a still
thicker solution, were placed on eight leaves, and the tentacles of all
became inflected. In one case the exterior tentacles were well curved
in after 6 hrs. 30 m., and the blade of the leaf to a partial extent
after 24 hrs. As saliva acted so efficiently, and yet contains so small
a proportion of nitrogenous matter, I tried how small a quantity of
isinglass would act. One part was dissolved in 218 parts of distilled
water, and drops were placed on four leaves. After 5 hrs. two of these
were considerably and two moderately inflected; after 22 hrs. the
former were greatly and the latter much more inflected. In the course
of 48 hrs.

* Mucus from the air-passages is said in Marshall, ‘Outlines of
Physiology,’ vol. ii. 1867, p. 364, to contain some albumen.

** Müller’s ‘Elements of Physiology,’ Eng. Trans. vol. i., p. 514.
[page 81]

from the time when the drops were placed on the leaves, all four had
almost re-expanded. They were then given little bits of meat, and these
acted more powerfully than the solution. One part of isinglass was next
dissolved in 437 of water; the fluid thus formed was so thin that it
could not be distinguished from pure water. The usual-sized drops were
placed on seven leaves, each of which thus received 1/960 of a grain
(.0295 mg.). Three of them were observed for 41 hrs., but were in no
way affected; the fourth and fifth had two or three of their exterior
tentacles inflected after 18 hrs.; the sixth had a few more; and the
seventh had in addition the edge of the leaf just perceptibly curved
inwards. The tentacles of the four latter leaves began to re-expand
after an additional interval of only 8 hrs. Hence the 1/960 of a grain
of isinglass is sufficient to affect very slightly the more sensitive
or active leaves. On one of the leaves, which had not been acted on by
the weak solution, and on another, which had only two of its tentacles
inflected, drops of the solution as thick as milk were placed; and next
morning, after an interval of 16 hrs., both were found with all their
tentacles strongly inflected.]

Altogether I experimented on sixty-four leaves with the above
nitrogenous fluids, the five leaves tried only with the extremely weak
solution of isinglass not being included, nor the numerous trials
subsequently made, of which no exact account was kept. Of these
sixty-four leaves, sixty-three had their tentacles and often their
blades well inflected. The one which failed was probably too old and
torpid. But to obtain so large a proportion of successful cases, care
must be taken to select young and active leaves. Leaves in this
condition were chosen with equal care for the sixty-one trials with
non-nitrogenous fluids (water not included); and we have seen that not
one of these was in the least affected. We may therefore safely
conclude that in the sixty-four experiments with nitrogenous fluids the
inflection of the exterior tentacles was due to the absorption of [page
82] nitrogenous matter by the glands of the tentacles on the disc.

Some of the leaves which were not affected by the non-nitrogenous
fluids were, as above stated, immediately afterwards tested with bits
of meat, and were thus proved to be in an active condition. But in
addition to these trials, twenty-three of the leaves, with drops of
gum, syrup, or starch, still lying on their discs, which had produced
no effect in the course of between 24 hrs. and 48 hrs., were then
tested with drops of milk, urine, or albumen. Of the twenty-three
leaves thus treated, seventeen had their tentacles, and in some cases
their blades, well inflected; but their powers were somewhat impaired,
for the rate of movement was decidedly slower than when fresh leaves
were treated with these same nitrogenous fluids. This impairment, as
well as the insensibility of six of the leaves, may be attributed to
injury from exosmose, caused by the density of the fluids placed on
their discs.

[The results of a few other experiments with nitrogenous fluids may be
here conveniently given. Decoctions of some vegetables, known to be
rich in nitrogen, were made, and these acted like animal fluids. Thus,
a few green peas were boiled for some time in distilled water, and the
moderately thick decoction thus made was allowed to settle. Drops of
the superincumbent fluid were placed on four leaves, and when these
were looked at after 16 hrs., the tentacles and blades of all were
found strongly inflected. I infer from a remark by Gerhardt* that
legumin is present in peas “in combination with an alkali, forming an
incoagulable solution,” and this would mingle with boiling water. I may
mention, in relation to the above and following experiments, that
according to Schiff** certain forms of albumen

* Watts’ ‘Dictionary of Chemistry,’ vol. iii., p. 568.

** ‘Leçons sur la Phys. de la Digestion,’ tom. i, p. 379; tom. ii. pp.
154, 166, on legumin. [page 83]

exist which are not coagulated by boiling water, but are converted into
soluble peptones.

On three occasions chopped cabbage-leaves* were boiled in distilled
water for 1 hr. or for 1 1/4 hr.; and by decanting the decoction after
it had been allowed to rest, a pale dirty green fluid was obtained. The
usual-sized drops were placed on thirteen leaves. Their tentacles and
blades were inflected after 4 hrs. to a quite extraordinary degree.
Next day the protoplasm within the cells of the tentacles was found
aggregated in the most strongly marked manner. I also touched the
viscid secretion round the glands of several tentacles with minute
drops of the decoction on the head of a small pin, and they became well
inflected in a few minutes. The fluid proving so powerful, one part was
diluted with three of water, and drops were placed on the discs of five
leaves; and these next morning were so much acted on that their blades
were completely doubled over. We thus see that a decoction of
cabbage-leaves is nearly or quite as potent as an infusion of raw meat.

About the same quantity of chopped cabbage-leaves and of distilled
water, as in the last experiment, were kept in a vessel for 20 hrs. in
a hot closet, but not heated to near the boiling-point. Drops of this
infusion were placed on four leaves. One of these, after 23 hrs., was
much inflected; a second slightly; a third had only the submarginal
tentacles inflected; and the fourth was not at all affected. The power
of this infusion is therefore very much less than that of the
decoction; and it is clear that the immersion of cabbage-leaves for an
hour in water at the boiling temperature is much more efficient in
extracting matter which excites Drosera than immersion during many
hours in warm water. Perhaps the contents of the cells are protected
(as Schiff remarks with respect to legumin) by the walls being formed
of cellulose, and that until these are ruptured by boiling-water, but
little of the contained albuminous matter is dissolved. We know from
the strong odour of cooked cabbage-leaves that boiling water produces
some chemical change in them, and that they are thus rendered far more
digestible and nutritious to man. It is therefore an interesting

* The leaves of young plants, before the heart is formed, such as were
used by me, contain 2.1 per cent. of albuminous matter, and the outer
leaves of mature plants 1.6 per cent. Watts’ ‘Dictionary of Chemistry,’
vol. i. p. 653. [page 84]

fact that water at this temperature extracts matter from them which
excites Drosera to an extraordinary degree.

Grasses contain far less nitrogenous matter than do peas or cabbages.
The leaves and stalks of three common kinds were chopped and boiled for
some time in distilled water. Drops of this decoction (after having
stood for 24 hrs.) were placed on six leaves, and acted in a rather
peculiar manner, of which other instances will be given in the seventh
chapter on the salts of ammonia. After 2 hrs. 30 m. four of the leaves
had their blades greatly inflected, but not their exterior tentacles;
and so it was with all six leaves after 24 hrs. Two days afterwards the
blades, as well as the few submarginal tentacles which had been
inflected, all re-expanded; and much of the fluid on their discs was by
this time absorbed. It appears that the decoction strongly excites the
glands on the disc, causing the blade to be quickly and greatly
inflected; but that the stimulus, differently from what occurs in
ordinary cases, does not spread, or only in a feeble degree, to the
exterior tentacles.

I may here add that one part of the extract of belladonna (procured
from a druggist) was dissolved in 437 of water, and drops were placed
on six leaves. Next day all six were somewhat inflected, and after 48
hrs. were completely re-expanded. It was not the included atropine
which produced this effect, for I subsequently ascertained that it is
quite powerless. I also procured some extract of hyoscyamus from three
shops, and made infusions of the same strength as before. Of these
three infusions, only one acted on some of the leaves, which were
tried. Though druggists believe that all the albumen is precipitated in
the preparation of these drugs, I cannot doubt that some is
occasionally retained; and a trace would be sufficient to excite the
more sensitive leaves of Drosera. [page 85]