THE EFFECTS OF HEAT ON THE LEAVES.
Nature of the experiments—Effects of boiling water—Warm water causes
rapid inflection—Water at a higher temperature does not cause immediate
inflection, but does not kill the leaves, as shown by their subsequent
re-expansion and by the aggregation of the protoplasm—A still higher
temperature kills the leaves and coagulates the albuminous contents of
the glands.
In my observations on Drosera rotundifolia, the leaves seemed to be
more quickly inflected over animal substances, and to remain inflected
for a longer period during very warm than during cold weather. I
wished, therefore, to ascertain whether heat alone would induce
inflection, and what temperature was the most efficient. Another
interesting point presented itself, namely, at what degree life was
extinguished; for Drosera offers unusual facilities in this respect,
not in the loss of the power of inflection, but in that of subsequent
re-expansion, and more especially in the failure of the protoplasm to
become aggregated, when the leaves after being heated are immersed in a
solution of carbonate of ammonia.*
* When my experiments on the effects of heat were made, I was not aware
that the subject had been carefully investigated by several observers.
For instance, Sachs is convinced (‘Traité de Botanique,’ 1874, pp. 772,
854) that the most different kinds of plants all perish if kept for 10
m. in water at 45° to 46° Cent., or 113° to 115° Fahr.; and he
concludes that the protoplasm within their cells always coagulates, if
in a damp condition, at a temperature of between 50° and 60° Cent., or
122° to 140° Fahr. Max Schultze and Kühne (as quoted by Dr. Bastian in
‘Contemp. Review,’ 1874, p. 528) “found that the protoplasm of
plant-cells, with which they experimented, was always killed and [[page
67]] altered by a very brief exposure to a temperature of 118 1/2°
Fahr. as a maximum.” As my results are deduced from special phenomena,
namely, the subsequent aggregation of the protoplasm and the
re-expansion of the tentacles, they seem to me worth giving. We shall
find that Drosera resists heat somewhat better than most other plants.
That there should be considerable differences in this respect is not
surprising, considering that some low vegetable organisms grow in hot
springs—cases of which have been collected by Prof. Wyman (‘American
Journal of Science,’ vol. xliv. 1867). Thus, Dr. Hooker found Confervae
in water at 168° Fahr.; Humboldt, at 185° Fahr.; and Descloizeaux, at
208° Fahr.) [page 67]
[My experiments were tried in the following manner. Leaves were cut
off, and this does not in the least interfere with their powers; for
instance, three cut off leaves, with bits of meat placed on them, were
kept in a damp atmosphere, and after 23 hrs. closely embraced the meat
both with their tentacles and blades; and the protoplasm within their
cells was well aggregated. Three ounces of doubly distilled water was
heated in a porcelain vessel, with a delicate thermometer having a long
bulb obliquely suspended in it. The water was gradually raised to the
required temperature by a spirit-lamp moved about under the vessel; and
in all cases the leaves were continually waved for some minutes close
to the bulb. They were then placed in cold water, or in a solution of
carbonate of ammonia. In other cases they were left in the water, which
had been raised to a certain temperature, until it cooled. Again in
other cases the leaves were suddenly plunged into water of a certain
temperature, and kept there for a specified time. Considering that the
tentacles are extremely delicate, and that their coats are very thin,
it seems scarcely possible that the fluid contents of their cells
should not have been heated to within a degree or two of the
temperature of the surrounding water. Any further precautions would, I
think, have been superfluous, as the leaves from age or constitutional
causes differ slightly in their sensitiveness to heat.
It will be convenient first briefly to describe the effects of
immersion for thirty seconds in boiling water. The leaves are rendered
flaccid, with their tentacles bowed backwards, which, as we shall see
in a future chapter, is probably due to their outer surfaces retaining
their elasticity for a longer period than their inner surfaces retain
the power of contraction. The purple fluid within the cells of the
pedicels is rendered finely granular, but there is no true aggregation;
nor does this follow [page 68] when the leaves are subsequently placed
in a solution of carbonate of ammonia. But the most remarkable change
is that the glands become opaque and uniformly white; and this may be
attributed to the coagulation of their albuminous contents.
My first and preliminary experiment consisted in putting seven leaves
in the same vessel of water, and warming it slowly up to the
temperature of 110° Fahr. (43°.3 Cent.); a leaf being taken out as soon
as the temperature rose to 80° (26°.6 Cent.), another at 85°, another
at 90°, and so on. Each leaf, when taken out, was placed in water at
the temperature of my room, and the tentacles of all soon became
slightly, though irregularly, inflected. They were now removed from the
cold water and kept in damp air, with bits of meat placed on their
discs. The leaf which had been exposed to the temperature of 110o
became in 15 m. greatly inflected; and in 2 hrs. every single tentacle
closely embraced the meat. So it was, but after rather longer
intervals, with the six other leaves. It appears, therefore, that the
warm bath had increased their sensitiveness when excited by meat.
I next observed the degree of inflection which leaves underwent within
stated periods, whilst still immersed in warm water, kept as nearly as
possible at the same temperature; but I will here and elsewhere give
only a few of the many trials made. A leaf was left for 10 m. in water
at 100° (37°.7 Cent.), but no inflection occurred. A second leaf,
however, treated in the same manner, had a few of its exterior
tentacles very slightly inflected in 6 m., and several irregularly but
not closely inflected in 10 m. A third leaf, kept in water at 105° to
106° (40°.5 to 41°.1 Cent.), was very moderately inflected in 6 m. A
fourth leaf, in water at 110° (43°.3 Cent.), was somewhat inflected in
4 m., and considerably so in from 6 to 7 m.
Three leaves were placed in water which was heated rather quickly, and
by the time the temperature rose to 115°-116° (46°.1 to 46°.06 Cent.),
all three were inflected. I then removed the lamp, and in a few minutes
every single tentacle was closely inflected. The protoplasm within the
cells was not killed, for it was seen to be in distinct movement; and
the leaves, having been left in cold water for 20 hrs., re-expanded.
Another leaf was immersed in water at 100o (37.7° Cent.), which was
raised to 120° (48°.8 Cent.); and all the tentacles, except the extreme
marginal ones, soon became closely inflected. The leaf was now placed
in cold water, and in 7 hrs. 30 m. it had partly, and in 10 hrs. fully,
re-expanded. On the following morning it was immersed in a weak
solution of carbonate of [page 69] ammonia, and the glands quickly
became black, with strongly marked aggregation in the tentacles,
showing that the protoplasm was alive, and that the glands had not lost
their power of absorption. Another leaf was placed in water at 110o
(43°.3 Cent.) which was raised to 120° (48°.8 Cent.); and every
tentacle, excepting one, was quickly and closely inflected. This leaf
was now immersed in a few drops of a strong solution of carbonate of
ammonia (one part to 109 of water); in 10 m. all the glands became
intensely black, and in 2 hrs. the protoplasm in the cells of the
pedicels was well aggregated. Another leaf was suddenly plunged, and as
usual waved about, in water at 120°, and the tentacles became inflected
in from 2 m. to 3 m., but only so as to stand at right angles to the
disc. The leaf was now placed in the same solution (viz. one part of
carbonate of ammonia to 109 of water, or 4 grs. to 1 oz., which I will
for the future designate as the strong solution), and when I looked at
it again after the interval of an hour, the glands were blackened, and
there was well-marked aggregation. After an additional interval of 4
hrs. the tentacles had become much more inflected. It deserves notice
that a solution as strong as this never causes inflection in ordinary
cases. Lastly a leaf was suddenly placed in water at 125° (51°.6
Cent.), and was left in it until the water cooled; the tentacles were
rendered of a bright red and soon became inflected. The contents of the
cells underwent some degree of aggregation, which in the course of
three hours increased; but the masses of protoplasm did not become
spherical, as almost always occurs with leaves immersed in a solution
of carbonate of ammonia.]
We learn from these cases that a temperature of from 120° to 125°
(48°.8 to 51°.6 Cent.) excites the tentacles into quick movement, but
does not kill the leaves, as shown either by their subsequent
re-expansion or by the aggregation of the protoplasm. We shall now see
that a temperature of 130° (54°.4 Cent.) is too high to cause immediate
inflection, yet does not kill the leaves.
[Experiment 1.—A leaf was plunged, and as in all cases waved about for
a few minutes, in water at 130° (54°.4 Cent.), but there was no trace
of inflection; it was then placed in cold water, and after an interval
of 15 m. very slow movement was [page 70] distinctly seen in a small
mass of protoplasm in one of the cells of a tentacle.* After a few
hours all the tentacles and the blade became inflected.
Experiment 2.—Another leaf was plunged into water at 130o to 131o, and
as before there was no inflection. After being kept in cold water for
an hour, it was placed in the strong solution of ammonia, and in the
course of 55 m. the tentacles were considerably inflected. The glands,
which before had been rendered of a brighter red, were now blackened.
The protoplasm in the cells of the tentacles was distinctly aggregated;
but the spheres were much smaller than those generated in unheated
leaves when subjected to carbonate of ammonia. After an additional 2
hrs. all the tentacles, excepting six or seven, were closely inflected.
Experiment 3.—A similar experiment to the last, with exactly the same
results.
Experiment 4.—A fine leaf was placed in water at 100° (37°.7 Cent.),
which was then raised to 145° (62°.7 Cent.). Soon after immersion,
there was, as might have been expected, strong inflection. The leaf was
now removed and left in cold water; but from having been exposed to so
high a temperature, it never re-expanded.
Experiment 5.—Leaf immersed at 130° (54°.4 Cent.), and the water raised
to 145° (62°.7 Cent.), there was no immediate inflection; it was then
placed in cold water, and after 1 hr. 20 m. some of the tentacles on
one side became inflected. This leaf was now placed in the strong
solution, and in 40 m. all the submarginal tentacles were well
inflected, and the glands blackened. After an additional interval of 2
hrs. 45 m. all the tentacles, except eight or ten, were closely
inflected, with their cells exhibiting a slight degree of aggregation;
but the spheres of protoplasm were very small, and the cells of the
exterior tentacles contained some pulpy or disintegrated brownish
matter.
Experiments 6 and 7.—Two leaves were plunged in water at 135° (57°.2
Cent.) which was raised to 145° (62°.7 Cent.); neither became
inflected. One of these, however, after having been left for 31 m. in
cold water, exhibited some slight inflection, which increased after an
additional interval of 1 hr. 45 m., until
* Sachs states (‘Traité de Botanique,’ 1874, p. 855) that the movements
of the protoplasm in the hairs of a Cucurbita ceased after they were
exposed for 1 m. in water to a temperature of 47° to 48° Cent., or 117°
to 119° Fahr. [page 71]
all the tentacles, except sixteen or seventeen, were more or less
inflected; but the leaf was so much injured that it never re-expanded.
The other leaf, after having been left for half an hour in cold water,
was put into the strong solution, but no inflection ensued; the glands,
however, were blackened, and in some cells there was a little
aggregation, the spheres of protoplasm being extremely small; in other
cells, especially in the exterior tentacles, there was much
greenish-brown pulpy matter.
Experiment 8.—A leaf was plunged and waved about for a few minutes in
water at 140° (60° Cent.), and was then left for half an hour in cold
water, but there was no inflection. It was now placed in the strong
solution, and after 2 hrs. 30 m. the inner submarginal tentacles were
well inflected, with their glands blackened, and some imperfect
aggregation in the cells of the pedicels. Three or four of the glands
were spotted with the white porcelain-like structure, like that
produced by boiling water. I have seen this result in no other instance
after an immersion of only a few minutes in water at so low a
temperature as 140°, and in only one leaf out of four, after a similar
immersion at a temperature of 145° Fahr. On the other hand, with two
leaves, one placed in water at 145° (62°.7 Cent.), and the other in
water at 140° (60° Cent.), both being left therein until the water
cooled, the glands of both became white and porcelain-like. So that the
duration of the immersion is an important element in the result.
Experiment 9.—A leaf was placed in water at 140° (60° Cent.), which was
raised to 150° (65°.5 Cent.); there was no inflection; on the contrary,
the outer tentacles were somewhat bowed backwards. The glands became
like porcelain, but some of them were a little mottled with purple. The
bases of the glands were often more affected than their summits. This
leaf having been left in the strong solution did not undergo any
inflection or aggregation.
Experiment 10.—A leaf was plunged in water at 150° to 150 1/2° (65°.5
Cent.); it became somewhat flaccid, with the outer tentacles slightly
reflexed, and the inner ones a little bent inwards, but only towards
their tips; and this latter fact shows that the movement was not one of
true inflection, as the basal part alone normally bends. The tentacles
were as usual rendered of a very bright red, with the glands almost
white like porcelain, yet tinged with pink. The leaf having been placed
in the strong solution, the cell-contents of the tentacles became of a
muddy-brown, with no trace of aggregation. [page 72]
Experiment 11.—A leaf was immersed in water at 145° (62°.7 Cent.),
which was raised to 156° (68°.8 Cent.). The tentacles became bright red
and somewhat reflexed, with almost all the glands like porcelain; those
on the disc being still pinkish, those near the margin quite white. The
leaf being placed as usual first in cold water and then in the strong
solution, the cells in the tentacles became of a muddy greenish brown,
with the protoplasm not aggregated. Nevertheless, four of the glands
escaped being rendered like porcelain, and the pedicels of these glands
were spirally curled, like a French horn, towards their upper ends; but
this can by no means be considered as a case of true inflection. The
protoplasm within the cells of the twisted portions was aggregated into
distinct though excessively minute purple spheres. This case shows
clearly that the protoplasm, after having been exposed to a high
temperature for a few minutes, is capable of aggregation when
afterwards subjected to the action of carbonate of ammonia, unless the
heat has been sufficient to cause coagulation.]
Concluding Remarks.—As the hair-like tentacles are extremely thin and
have delicate walls, and as the leaves were waved about for some
minutes close to the bulb of the thermometer, it seems scarcely
possible that they should not have been raised very nearly to the
temperature which the instrument indicated. From the eleven last
observations we see that a temperature of 130° (54°.4 Cent.) never
causes the immediate inflection of the tentacles, though a temperature
from 120° to 125° (48°.8 to 51°.6 Cent.) quickly produces this effect.
But the leaves are paralysed only for a time by a temperature of 130°,
as afterwards, whether left in simple water or in a solution of
carbonate of ammonia, they become inflected and their protoplasm
undergoes aggregation. This great difference in the effects of a higher
and lower temperature may be compared with that from immersion in
strong and weak solutions of the salts of ammonia; for the former do
not excite movement, whereas the latter act energetically. A temporary
suspension of the [page 73] power of movement due to heat is called by
Sachs* heat-rigidity; and this in the case of the sensitive-plant
(Mimosa) is induced by its exposure for a few minutes to humid air,
raised to 120°-122° Fahr., or 49° to 50° Cent. It deserves notice that
the leaves of Drosera, after being immersed in water at 130° Fahr., are
excited into movement by a solution of the carbonate so strong that it
would paralyse ordinary leaves and cause no inflection.
The exposure of the leaves for a few minutes even to a temperature of
145° Fahr. (62°.7 Cent.) does not always kill them; as when afterwards
left in cold water, or in a strong solution of carbonate of ammonia,
they generally, though not always, become inflected; and the protoplasm
within their cells undergoes aggregation, though the spheres thus
formed are extremely small, with many of the cells partly filled with
brownish muddy matter. In two instances, when leaves were immersed in
water, at a lower temperature than 130° (54°.4 Cent.), which was then
raised to 145° (62°.7 Cent.), they became during the earlier period of
immersion inflected, but on being afterwards left in cold water were
incapable of re-expansion. Exposure for a few minutes to a temperature
of 145o sometimes causes some few of the more sensitive glands to be
speckled with the porcelain-like appearance; and on one occasion this
occurred at a temperature of 140° (60° Cent.). On another occasion,
when a leaf was placed in water at this temperature of only 140o, and
left therein till the water cooled, every gland became like porcelain.
Exposure for a few minutes to a temperature of 150° (65°.5 Cent.)
generally produces this effect, yet many glands retain a
* ‘Traité de Bot.’ 1874, p. 1034. [page 74]
pinkish colour, and many present a speckled appearance. This high
temperature never causes true inflection; on the contrary, the
tentacles commonly become reflexed, though to a less degree than when
immersed in boiling water; and this apparently is due to their passive
power of elasticity. After exposure to a temperature of 150° Fahr., the
protoplasm, if subsequently subjected to carbonate of ammonia, instead
of undergoing aggregation, is converted into disintegrated or pulpy
discoloured matter. In short, the leaves are generally killed by this
degree of heat; but owing to differences of age or constitution, they
vary somewhat in this respect. In one anomalous case, four out of the
many glands on a leaf, which had been immersed in water raised to 156°
(68°.8 Cent.), escaped being rendered porcellanous;* and the protoplasm
in the cells close beneath these glands underwent some slight, though
imperfect, degree of aggregation.
Finally, it is a remarkable fact that the leaves of Drosera
rotundifolia, which flourishes on bleak upland moors throughout Great
Britain, and exists (Hooker) within the Arctic Circle, should be able
to withstand for even a short time immersion in water heated to a
temperature of 145°.**
It may be worth adding that immersion in cold
* As the opacity and porcelain-like appearance of the glands is
probably due to the coagulation of the albumen, I may add, on the
authority of Dr. Burdon Sanderson, that albumen coagulates at about
155o, but, in presence of acids, the temperature of coagulation is
lower. The leaves of Drosera contain an acid, and perhaps a difference
in the amount contained may account for the slight differences in the
results above recorded.
** It appears that cold-blooded animals are, as might have been
expected, far more sensitive to an increase of temperature than is
Drosera. Thus, as I hear from Dr. Burdon Sanderson, a frog begins to be
distressed in water at a temperature of only 85° Fahr. At 95° the
muscles become rigid, and the animal dies in a stiffened condition.
[page 75]
water does not cause any inflection: I suddenly placed four leaves,
taken from plants which had been kept for several days at a high
temperature, generally about 75° Fahr. (23°.8 Cent.), in water at 45°
(7°.2 Cent.), but they were hardly at all affected; not so much as some
other leaves from the same plants, which were at the same time immersed
in water at 75°; for these became in a slight degree inflected. [page
76]