MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.
Distinction between heliotropism and the effects of light on the
periodicity of the movements of leaves—Heliotropic movements of Beta,
Solanum, Zea, and Avena—Heliotropic movements towards an obscure light
in Apios, Brassica, Phalaris, Tropaeolum, and Cassia—Apheliotropic
movements of tendrils of Bignonia—Of flower-peduncles of
Cyclamen—Burying of the pods—Heliotropism and apheliotropism modified
forms of circumnutation—Steps by which one movement is converted into
the other—Transversal-heliotropismus or diaheliotropism influenced by
epinasty, the weight of the part and apogeotropism—Apogeotropism
overcome during the middle of the day by diaheliotropism—Effects of the
weight of the blades of cotyledons—So called diurnal sleep—Chlorophyll
injured by intense light—Movements to avoid intense light
Sachs first clearly pointed out the important difference between the
action of light in modifying the periodic movements of leaves, and in
causing them to bend towards its source.[1] The latter, or heliotropic
movements are determined by the direction of the light, whilst periodic
movements are affected by changes in its intensity and not by its
direction. The periodicity of the circumnutating movement often
continues for some time in darkness, as we have seen in the last
chapter; whilst heliotropic bending ceases very quickly when the light
fails. Nevertheless, plants which have ceased through long-continued
darkness to move periodically, if re-exposed to the light are still,
according to Sachs, heliotropic.
[1] ‘Physiologie Veg.’ (French Translation), 1868, pp. 42, 517, etc.
Apheliotropism, or, as usually designated, negative
heliotropism, implies that a plant, when unequally illuminated on the
two sides, bends from the light, instead of, as in the last sub-class
of cases, towards it; but apheliotropism is comparatively rare, at
least in a well-marked degree. There is a third and large sub-class of
cases, namely, those of “transversal-Heliotropismus” of Frank, which we
will here call diaheliotropism. Parts of plants, under this influence,
place themselves more or less transversely to the direction whence the
light proceeds, and are thus fully illuminated. There is a fourth
sub-class, as far as the final cause of the movement is concerned; for
the leaves of some plants when exposed to an intense and injurious
amount of light direct themselves, by rising or sinking or twisting, so
as to be less intensely illuminated. Such movements have sometimes been
called diurnal sleep. If thought advisable, they might be called
paraheliotropic, and this term would correspond with our other terms.
It will be shown in the present chapter that all the movements included
in these four sub-classes, consist of modified circumnutation. We do
not pretend to say that if a part of a plant, whilst still growing, did
not circumnutate—though such a supposition is most improbable—it could
not bend towards the light; but, as a matter of fact, heliotropism
seems always to consist of modified circumnutation. Any kind of
movement in relation to light will obviously be much facilitated by
each part circumnutating or bending successively in all directions, so
that an already existing movement has only to be increased in some one
direction, and to be lessened or stopped in the other directions, in
order that it should become heliotropic, apheliotropic, etc., as the
case may be. In the next chapter some observations on the sensitiveness
of plants to light, their
rate of bending towards it, and the accuracy with which they point
towards its source, etc., will be given. Afterwards it will be
shown—and this seems to us a point of much interest—that sensitiveness
to light is sometimes confined to a small part of the plant; and that
this part when stimulated by light, transmits an influence to distant
parts, exciting them to bend.
Heliotropism.—When a plant which is strongly heliotropic (and species
differ much in this respect) is exposed to a bright lateral light, it
bends quickly towards it, and the course pursued by the stem is quite
or nearly straight. But if the light is much dimmed, or occasionally
interrupted, or admitted in only a slightly oblique direction, the
course pursued is more or less zigzag; and as we have seen and shall
again see, such zigzag movement results from the elongation or drawing
out of the ellipses, loops, etc., which the plant would have described,
if it had been illuminated from above. On several occasions we were
much struck with this fact, whilst observing the circumnutation of
highly sensitive seedlings, which were unintentionally illuminated
rather obliquely, or only at successive intervals of time.
Fig. 168. Beta vulgaris: circumnutation of hypocotyl, deflected by the
light being slightly lateral, traced on a horizontal glass from 8.30
A.M. to 5.30 P.M. Direction of the lighted taper by which it was
illuminated shown by a line joining the first and penultimate dots.
Figure reduced to one-third of the original scale.
For instance two young seedlings of Beta vulgaris were placed in the
middle of a room with north-east windows, and were kept covered up,
except during each observation which lasted for only a minute or two;
but the result was that their hypocotyls bowed themselves to the side,
whence some light occasionally entered, in lines which were
only slightly zigzag. Although not a single ellipse was even
approximately formed, we inferred from the zigzag lines—and, as it
proved, correctly—that their hypocotyls were circumnutating, for on the
following day these same seedlings were placed in a completely darkened
room, and were observed each time by the aid of a small wax taper held
almost directly above them, and their movements were traced on a
horizontal glass above; and now their hypocotyls clearly circumnutated
(Fig. 168, and Fig. 39, formerly given, p. 52); yet they moved a short
distance towards the side where the taper was held up. If we look at
these diagrams, and suppose that the taper had been held more on one
side, and that the hypocotyls, still circumnutating, had bent
themselves within the same time much more towards the light, long
zigzag lines would obviously have been the result.
Fig. 169. Avena sativa: heliotropic movement and circumnutation of
sheath-like cotyledon (1½ inch in height) traced on horizontal glass
from 8 A.M. to 10.25 P.M. Oct. 16th.
Again, two seedlings of Solanum lycopersicum were illuminated from
above, but accidentally a little more light entered on one than on any
other side, and their hypocotyls became slightly bowed towards the
brighter side; they moved in a zigzag line and described in their
course two little triangles, as seen in Fig. 37 (p. 50), and in another
tracing not given. The sheath-like cotyledons of Zea mays behaved,
under nearly similar circumstances, in a nearly similar manner as
described in our first chapter (p. 64), for they bowed themselves
during the whole day towards one side, making, however, in their course
some conspicuous flexures. Before we knew how greatly ordinary
circumnutation was modified by a lateral light, some seedling oats,
with rather old and therefore not highly sensitive cotyledons, were
placed in front of a north-east window, towards which they bent all day
in a strongly zigzag course. On the following day they continued to
bend in the same direction (Fig. 169), but zigzagged much less. The
sky, however, became between 12.40 and 2.35 P.M.
overcast with extraordinarily dark thunder-clouds, and it was
interesting to note how plainly the cotyledons circumnutated during
this interval.
The foregoing observations are of some value, from having been made
when we were not attending to heliotropism; and they led us to
experiment on several kinds of seedlings, by exposing them to a dim
lateral light, so as to observe the gradations between ordinary
circumnutation and heliotropism. Seedlings in pots were placed in front
of, and about a yard from, a north-east window; on each side and over
the pots black boards were placed; in the rear the pots were open to
the diffused light of the room, which had a second north-east and a
north-west window. By hanging up one or more blinds before the window
where the seedlings stood, it was easy to dim the light, so that very
little more entered on this side than on the opposite one, which
received the diffused light of the room. Late in the evening the blinds
were successively removed, and as the plants had been subjected during
the day to a very obscure light, they continued to bend towards the
window later in the evening than would otherwise have occurred. Most of
the seedlings were selected because they were known to be highly
sensitive to light, and some because they were but little sensitive, or
had become so from having grown old. The movements were traced in the
usual manner on a horizontal glass cover; a fine glass filament with
little triangles of paper having been cemented in an upright position
to the hypocotyls. Whenever the stem or hypocotyl became much bowed
towards the light, the latter part of its course had to be traced on a
vertical glass, parallel to the window, and at right angles to the
horizontal glass cover.
Fig. 170. Apios graveolens: heliotropic movement of hypocotyl (.45 of
inch in height) towards a moderately bright lateral light, traced on a
horizontal glass from 8.30 A.M. to 11.30 A.M. Sept. 18th. Figure
reduced to one-third of original scale.
Apios graveolens.—The hypocotyl bends in a few hours
rectangularly towards a bright lateral light. In order to ascertain how
straight a course it would pursue when fairly well illuminated on one
side, seedlings were first placed before a south-west window on a
cloudy and rainy morning; and the movement of two hypocotyls were
traced for 3 h., during which time they became greatly bowed towards
the light. One of these tracings is given on p. 422 (Fig. 170), and the
course may be seen to be almost straight. But the amount of light on
this occasion was superfluous, for two seedlings were placed before a
north-east window, protected by an ordinary linen and two muslin
blinds, yet their hypocotyls moved towards this rather dim light in
only slightly zigzag lines; but after 4 P.M., as the light waned, the
lines became distinctly zigzag. One of these seedlings, moreover,
described in the afternoon an ellipse of considerable size, with its
longer axis directed towards the window.
We now determined that the light should be made dim enough, so we began
by exposing several seedlings before a north-east window, protected by
one linen blind, three muslin blinds, and a towel. But so little light
entered that a pencil cast no perceptible shadow on a white card, and
the hypocotyls did not bend at all towards the window. During this
time, from 8.15 to 10.50 A.M., the hypocotyls zigzagged or
circumnutated near the same spot, as may be seen at A, in Fig. 171. The
towel, therefore, was removed at 10.50 A.M., and replaced by two muslin
blinds, and now the light passed through one ordinary linen and four
muslin blinds. When a pencil was held upright on a card close to the
seedlings, it cast a shadow (pointing from the window) which could only
just be distinguished. Yet this very slight excess of light on one side
sufficed to cause the hypocotyls of all the seedlings immediately to
begin bending in zigzag lines towards the window. The course of one is
shown at A (Fig. 171): after moving towards the window from 10.50 A.M.
to 12.48 P.M. it bent from the window, and then returned in a nearly
parallel line; that is, it almost completed between 12.48 and 2 P.M. a
narrow ellipse. Late in the evening, as the light waned, the hypocotyl
ceased to bend towards the window, and circumnutated on a small scale
round the same spot; during the night it moved considerably backwards,
that is, became more upright, through the action of apogeotropism. At
B, we have a tracing of the movements of another seedling from the hour
(10.50 A.M.) when the towel was removed; and it is in all essential
respects
similar to the previous one. In these two cases there could be no doubt
that the ordinary circumnutating movement of the hypocotyl was modified
and rendered heliotropic.
Fig. 171. Apios graveolens: heliotropic movement and circumnutation of
the hypocotyls of two seedlings towards a dim lateral light, traced on
a horizontal glass during the day. The broken lines show their return
nocturnal courses. Height of hypocotyl of A .5, and of B .55 inch.
Figure reduced to one-half of original scale.
Brassica oleracea.—The hypocotyl of the cabbage, when not disturbed by
a lateral light, circumnutates in a complicated
manner over nearly the same space, and a figure formerly given is here
reproduced (Fig. 172). If the hypocotyl is exposed to a moderately
strong lateral light it moves quickly towards this side, travelling in
a straight, or nearly straight, line. But when the lateral light is
very dim its course is extremely tortuous, and evidently consists of
modified circumnutation. Seedlings were placed before a north-east
window, protected by a linen and muslin blind and by a towel. The sky
was cloudy, and whenever the clouds grew a little lighter an additional
muslin blind was temporarily suspended. The light from the window was
thus so much obscured that, judging by the unassisted eye, the
seedlings appeared to receive more light from the interior of the room
than from the window; but this was not really the case, as was shown by
a very faint shadow cast by a pencil on a card. Nevertheless, this
extremely small excess of light on one side caused the hypocotyls,
which in the morning had stood upright, to bend at right angles towards
the window, so that in the evening (after 4.23 P.M.) their course had
to be traced on a vertical glass parallel to the window. It should be
stated that at 3.30 P.M., by which time the sky had become darker, the
towel was removed and replaced by an additional muslin blind, which
itself was removed at 4 P.M., the other two
blinds being left suspended. In Fig. 173 the course pursued, between
8.9 A.M. and 7.10 P.M., by one of the hypocotyls thus exposed is shown.
It may be observed that during the first 16 m. the hypocotyl moved
obliquely from the light, and this,
no doubt, was due to its then circumnutating in this direction. Similar
cases were repeatedly observed, and a dim light rarely or never
produced any effect until from a quarter to three-quarters of an hour
had elapsed. After 5.15 P.M., by which time the light had become
obscure, the hypocotyl began to circumnutate about the same spot. The
contrast between the two figures (172 and 173) would have been more
striking, if they had been originally drawn on the same scale, and had
been equally reduced. But the movements shown in Fig. 172 were at first
more magnified, and have been reduced to only one-half of the original
scale; whereas those in Fig. 173 were at first less magnified, and have
been reduced to a one-third scale. A tracing made at the same time with
the last of the movements of a second hypocotyl, presented a closely
analogous appearance; but it did not bend quite so much towards the
light, and it circumnutated rather more plainly.
Fig. 172. Brassica oleracea: ordinary circumnutating movement of the
hypocotyl of a seedling plant.
Fig. 173. Brassica oleracea: heliotropic movement and circumnutation of
a hypocotyl towards a very dim lateral light, traced during 11 hours,
on a horizontal glass in the morning, and on a vertical glass in the
evening. Figure reduced to one-third of the original scale.
Phalaris Canariensis.—The sheath-like cotyledons of this
monocotyledonous plant were selected for trial, because they are very
sensitive to light and circumnutate well, as formerly shown (see Fig.
49, p. 63). Although we felt no doubt about the result, some seedlings
were first placed before a south-west window on a moderately bright
morning, and the movements of one were traced. As is so common, it
moved
for the first 45 m. in a zigzag line; it then felt the full influence
of the light, and travelled towards it for the next 2 h. 30 m. in an
almost straight line. The tracing has not been given, as it was almost
identical with that of Apios under similar circumstances (Fig. 170). By
noon it had bowed itself to its full extent; it then circumnutated
about the same spot and described two ellipses; by 5 P.M. it had
retreated considerably from the light, through the action of
apogeotropism. After some preliminary trials for ascertaining the right
degree of obscurity, some seedlings were placed (Sept. 16th) before a
north-east window, and light was admitted through an ordinary linen and
three muslin blinds. A pencil held close by the pot now cast a very
faint shadow on a white card, pointing from the window. In the evening,
at 4.30 and again at 6 P.M., some of the blinds were removed. In Fig.
174 we see the course pursued under these circumstances by a rather old
and not very sensitive cotyledon, 1.9 inch in height, which became much
bowed, but was never rectangularly bent towards the light. From 11
A.M., when the sky became rather duller, until 6.30 P.M., the
zigzagging was conspicuous, and evidently consisted of drawn-out
ellipses. After 6.30 P.M. and during the night, it retreated in a
crooked line from the window. Another and younger seedling moved during
the same time much more quickly and to a much greater distance, in an
only slightly zigzag line towards the light; by 11 A.M. it was bent
almost rectangularly in this direction, and now circumnutated about the
same place.
Fig. 174. Phalaris Canariensis: heliotropic movement and circumnutation
of a rather old cotyledon, towards a dull lateral light, traced on a
horizontal glass from 8.15 A.M. Sept. 16th to 7.45 A.M. 17th. Figure
reduced to one-third of original scale.
Tropaeolum majus.—Some very young seedlings, bearing only two leaves,
and therefore not as yet arrived at the climbing stage of growth, were
first tried before a north-east window without any blind. The epicotyls
bowed themselves towards the light so rapidly that in little more than
3 h. their tips pointed rectangularly towards it. The lines traced were
either nearly straight or slightly zigzag; and in this latter case we
see that a trace of circumnutation was retained even under the
influence of a moderately bright light. Twice whilst these epicotyls
were bending towards the window, dots were made every 5 or 6 minutes,
in order to detect any trace of lateral movement, but there was hardly
any; and the lines formed by their junction were nearly straight, or
only very slightly zigzag, as in the other parts of the figures. After
the epicotyls had bowed themselves to the full extent towards the
light, ellipses of considerable size were described in the usual
manner.
After having seen how the epicotyls moved towards a moderately bright
light, seedlings were placed at 7.48 A.M. (Sept. 7th) before a
north-east window, covered by a towel, and shortly afterwards by an
ordinary linen blind, but the epicotyls still moved towards the window.
At 9.13 A.M. two additional muslin blinds were suspended, so that the
seedlings received very little more light from the window than from the
interior of the room. The sky varied in brightness, and the seedlings
occasionally received for a short time less light from the window than
from the opposite side (as ascertained by the shadow cast), and then
one of the blinds was temporarily removed. In the evening the blinds
were taken away, one by one. the course pursued by an epicotyl under
these circumstances is shown in Fig. 175. During the whole day, until
6.45 P.M., it plainly bowed itself towards the light; and the tip moved
over a considerable space. After 6.45 P.M. it moved backwards, or from
the window, till
10.40 P.M., when the last dot was made. Here, then, we have a distinct
heliotropic movement, effected by means of six elongated figures (which
if dots had been made every few minutes would have been more or less
elliptic) directed towards the light, with the apex of each successive
ellipse nearer to the window than the previous one. Now, if the light
had been only a little brighter, the epicotyl would have bowed itself
more to the light, as we may safely conclude from the previous trials;
there would also have been less lateral movement, and the ellipses or
other figures would have been drawn out into a strongly marked zigzag
line, with probably one or two small loops still formed. If the light
had been much brighter, we should have had a slightly zigzag line, or
one quite straight, for there would have been more movement in the
direction of the light, and much less from side to side.
Fig. 175. Tropaeolum majus: heliotropic movement and circumnutation of
the epicotyl of a young seedling towards a dull lateral light, traced
on a horizontal glass from 7.48 A.M. to 10.40 P.M. Figure reduced to
one-half of the original scale.
Fig. 176. Tropaeolum majus: heliotropic movement and circumnutation of
an old internode towards a lateral light, traced on a horizontal glass
from 8 A.M. Nov. 2nd to 10.20 A.M. Nov. 4th. Broken lines show the
nocturnal course.
Sachs states that the older internodes of this Tropaeolum are
apheliotropic; we therefore placed a plant, 11 3/4 inches high, in a
box, blackened within, but open on one side in front of a north-east
window without any blind. A filament was fixed to the third internode
from the summit on one plant, and to the fourth internode of another.
These internodes were either not old enough, or the light was not
sufficiently bright, to induce apheliotropism, for both plants bent
slowly towards, instead of from the window during four days. The
course, during two days of the first-mentioned internode, is given in
Fig. 176; and we see that it either circumnutated on a small scale, or
travelled in a zigzag line towards the light. We have thought this case
of feeble heliotropism in one of the older internodes of a plant,
which, whilst young, is so extremely sensitive to light, worth giving.
Fig. 177. Cassia tora: heliotropic movement and circumnutation of a
hypocotyl (1½ inch in height) traced on a horizontal glass from 8 A.M.
to 10.10 P.M. Oct. 7th. Also its circumnutation in darkness from 7 A.M.
Oct. 8th to 7.45 A.M. Oct. 9th.
Cassia tora.—The cotyledons of this plant are extremely sensitive to
light, whilst the hypocotyls are much less sensitive than those of most
other seedlings, as we had often observed with surprise. It seemed
therefore worth while to trace their movements. They were exposed to a
lateral light before a north-east window, which was at first covered
merely by a muslin blind, but as the sky grew brighter about 11 A.M.,
an additional linen blind was suspended. After 4 P.M. one blind and
then the other was removed. The seedlings were protected on each side
and above, but were open to the diffused light of the room in the rear.
Upright filaments were fixed to the hypocotyls of two seedlings, which
stood vertically in the morning. The accompanying figure (Fig. 177)
shows the course pursued by one of them during two days; but it should
be particularly noticed that during the second day the seedlings were
kept in darkness, and they then circumnutated round nearly the same
small space. On the first day (Oct. 7th) the hypocotyl moved from 8
A.M. to 12.23 P.M., toward the light in a zigzag line, then turned
abruptly to the left and afterwards described a small ellipse. Another
irregular
ellipse was completed between 3 P.M. and about 5.30 P.M., the hypocotyl
still bending towards the light. The hypocotyl was straight and upright
in the morning, but by 6 P.M. its upper half was bowed towards the
light, so that the chord of the arc thus formed stood at an angle of
20° with the perpendicular. After 6 P.M. its course was reversed
through the action of apogeotropism, and it continued to bend from the
window during the night, as shown by the broken line. On the next day
it was kept in the dark (excepting when each observation was made by
the aid of a taper), and the course followed from 7 A.M. on the 8th to
7.45 A.M. on the 9th is here likewise shown. The difference between the
two parts of the figure (177), namely that described during the daytime
on the 7th, when exposed to a rather dim lateral light, and that on the
8th in darkness, is striking. The difference consists in the lines
during the first day having been drawn out in the direction of the
light. The movements of the other seedling, traced under the same
circumstances, were closely similar.
Apheliotropism.—We succeeded in observing only two cases of
apheliotropism, for these are somewhat rare; and the movements are
generally so slow that they would have been very troublesome to trace.
Fig. 178. Bignonia capreolata: apheliotropic movement of a tendril,
traced on a horizontal glass from 6.45 A.M. July 19th to 10 A.M. 20th.
Movements as originally traced, little magnified, here reduced to
two-thirds of the original scale.
Bignonia capreolata.—No organ of any plant, as far as we have seen,
bends away so quickly from the light as do the tendrils of this
Bignonia. They are also remarkable from circumnutating much less
regularly than most other tendrils, often remaining stationary; they
depend on apheliotropism for coming into
contact with the trunks of trees.[2] The stem of a young plant was tied
to a stick at the base of a pair of fine tendrils, which projected
almost vertically upwards; and it was placed in front of a north-east
window, being protected on all other sides from the light. The first
dot was made at 6.45 A.M., and by 7.35 A.M. both tendrils felt the full
influence of the light, for they moved straight away from it until 9.20
A.M., when they circumnutated for a time, still moving, but only a
little, from the light (see Fig. 178 of the left-hand tendril). After 3
P.M. they again moved rapidly away from the light in zigzag lines. By a
late hour in the evening both had moved so far, that they pointed in a
direct line from the light. During the night they returned a little in
a nearly opposite direction. On the following morning they again moved
from the light and converged, so that by the evening they had become
interlocked, still pointing from the light. The right-hand tendril,
whilst converging, zigzagged much more than the one figured. Both
tracings showed that the apheliotropic movement was a modified form of
circumnutation.
[2] ‘The Movements and Habits of Climbing Plants,’ 1875, p. 97.
Cyclamen Persicum.—Whilst this plant is in flower the peduncles stand
upright, but their uppermost part is hooked so that the flower itself
hangs downwards. As soon as the pods begin to swell, the peduncles
increase much in length and slowly curve downwards, but the short,
upper, hooked part straightens itself. Ultimately the pods reach the
ground, and if this is covered with moss or dead leaves, they bury
themselves. We have often seen saucer-like depressions formed by the
pods in damp sand or sawdust; and one pod (.3 of inch in diameter)
buried itself in sawdust for three-quarters of its length.[3] We shall
have occasion hereafter to consider the object gained by this burying
process. The peduncles can change the direction of their curvature, for
if a pot, with plants having their peduncles already bowed downwards,
be placed horizontally, they slowly bend at right angles to their
former direction towards the centre of the earth. We therefore at first
attributed the movement to geotropism; but a pot which had lain
horizontally with the pods
all pointing to the ground, was reversed, being still kept horizontal,
so that the pods now pointed directly upwards; it was then placed in a
dark cupboard, but the pods still pointed upwards after four days and
nights. The pot, in the same position, was next brought back into the
light, and after two days there was some bending downwards of the
peduncles, and on the fourth day two of them pointed to the centre of
the earth, as did the others after an additional day or two. Another
plant, in a pot which had always stood upright, was left in the dark
cupboard for six days; it bore 3 peduncles, and only one became within
this time at all bowed downwards, and that doubtfully. The weight,
therefore, of the pods is not the cause of the bending down. This pot
was then brought back into the light, and after three days the
peduncles were considerably bowed downwards. We are thus led to infer
that the downward curvature is due to apheliotropism; though more
trials ought to have been made.
[3] The peduncles of several other species of Cyclamen twist
themselves into a spire, and according to Erasmus Darwin (‘Botanic
Garden,’ Canto., iii. p. 126), the pods forcibly penetrate the earth.
See also Grenier and Godron, ‘Flore de France,’ tom. ii. p. 459.
Fig. 179. Cyclamen Persicum: downward apheliotropic movement of a
flower-peduncle, greatly magnified (about 47 times?), traced on a
horizontal glass from 1 P.M. Feb. 18th to 8 A.M. 21st.
In order to observe the nature of this movement, a peduncle bearing a
large pod which had reached and rested on the ground, was lifted a
little up and secured to a stick. A filament was fixed across the pod
with a mark beneath, and its
movement, greatly magnified, was traced on a horizontal glass during 67
h. The plant was illuminated during the day from above. A copy of the
tracing is given on p. 434 (Fig. 179); and there can be no doubt that
the descending movement is one of modified circumnutation, but on an
extremely small scale. The observation was repeated on another pod,
which had partially buried itself in sawdust, and which was lifted up a
quarter of an inch above the surface; it described three very small
circles in 24 h. Considering the great length and thinness of the
peduncles and the lightness of the pods, we may conclude that they
would not be able to excavate saucer-like depressions in sand or
sawdust, or bury themselves in moss, etc., unless they were aided by
their continued rocking or circumnutating movement.
Relation between Circumnutation and Heliotropism.—Any one who will look
at the foregoing diagrams, showing the movements of the stems of
various plants towards a lateral and more or less dimmed light, will be
forced to admit that ordinary circumnutation and heliotropism graduate
into one another. When a plant is exposed to a dim lateral light and
continues during the whole day bending towards it, receding late in the
evening, the movement unquestionably is one of heliotropism. Now, in
the case of Tropaeolum (Fig. 175) the stem or epicotyl obviously
circumnutated during the whole day, and yet it continued at the same
time to move heliotropically; this latter movement being effected by
the apex of each successive elongated figure or ellipse standing nearer
to the light than the previous one. In the case of Cassia (Fig. 177)
the comparison of the movement of the hypocotyl, when exposed to a dim
lateral light and to darkness, is very instructive; as is that between
the ordinary circumnutating movement of a seedling Brassica (Figs. 172,
173), or that of Phalaris (Figs. 49, 174), and their heliotropic
movement towards a window protected by blinds. In both these cases,
and in many others, it was interesting to notice how gradually the
stems began to circumnutate as the light waned in the evening. We have
therefore many kinds of gradations from a movement towards the light,
which must be considered as one of circumnutation very slightly
modified and still consisting of ellipses or circles,—though a movement
more or less strongly zigzag, with loops or ellipses occasionally
formed,—to a nearly straight, or even quite straight, heliotropic
course.
A plant, when exposed to a lateral light, though this may be bright,
commonly moves at first in a zigzag line, or even directly from the
light; and this no doubt is due to its circumnutating at the time in a
direction either opposite to the source of the light, or more or less
transversely to it. As soon, however, as the direction of the
circumnutating movement nearly coincides with that of the entering
light, the plant bends in a straight course towards the light, if this
is bright. The course appears to be rendered more and more rapid and
rectilinear, in accordance with the degree of brightness of the
light—firstly, by the longer axes of the elliptical figures, which the
plant continues to describe as long as the light remains very dim,
being directed more or less accurately towards its source, and by each
successive ellipse being described nearer to the light. Secondly, if
the light is only somewhat dimmed, by the acceleration and increase of
the movement towards it, and by the retardation or arrestment of that
from the light, some lateral movement being still retained, for the
light will interfere less with a movement at right angles to its
direction, than with one in its own direction.[4]
The result is that the course is rendered more or less zigzag and
unequal in rate. Lastly, when the light is very bright all lateral
movement is lost; and the whole energy of the plant is expended in
rendering the circumnutating movement rectilinear and rapid in one
direction alone, namely, towards the light.
[4] In his paper, ‘Ueber orthotrope und plagiotrope Pflanzentheile’
(‘Arbeiten des Bot. Inst. in Würzburg,’ Band ii. Heft ii. 1879), Sachs
has discussed the manner in which geotropism and heliotropism are
affected by differences in the angles at which the organs of plants
stand with respect to the direction of the incident force.
The common view seems to be that heliotropism is a quite distinct kind
of movement from circumnutation; and it may be urged that in the
foregoing diagrams we see heliotropism merely combined with, or
superimposed on, circumnutation. But if so, it must be assumed that a
bright lateral light completely stops circumnutation, for a plant thus
exposed moves in a straight line towards it, without describing any
ellipses or circles. If the light be somewhat obscured, though amply
sufficient to cause the plant to bend towards it, we have more or less
plain evidence of still-continued circumnutation. It must further be
assumed that it is only a lateral light which has this extraordinary
power of stopping circumnutation, for we know that the several plants
above experimented on, and all the others which were observed by us
whilst growing, continue to circumnutate, however bright the light may
be, if it comes from above. Nor should it be forgotten that in the life
of each plant, circumnutation precedes heliotropism, for hypocotyls,
epicotyls, and petioles circumnutate before they have broken through
the ground and have ever felt the influence of light.
We are therefore fully justified, as it seems to us, in believing that
whenever light enters laterally, it is the
movement of circumnutation which gives rise to, or is converted into,
heliotropism and apheliotropism. On this view we need not assume
against all analogy that a lateral light entirely stops circumnutation;
it merely excites the plant to modify its movement for a time in a
beneficial manner. The existence of every possible gradation, between a
straight course towards a lateral light and a course consisting of a
series of loops or ellipses, becomes perfectly intelligible. Finally,
the conversion of circumnutation into heliotropism or apheliotropism,
is closely analogous to what takes place with sleeping plants, which
during the daytime describe one or more ellipses, often moving in
zigzag lines and making little loops; for when they begin in the
evening to go to sleep, they likewise expend all their energy in
rendering their course rectilinear and rapid. In the case of
sleep-movements, the exciting or regulating cause is a difference in
the intensity of the light, coming from above, at different periods of
the twenty-four hours; whilst with heliotropic and apheliotropic
movements, it is a difference in the intensity of the light on the two
sides of the plant.
_Transversal-heliotropismus_ (_of Frank_[5]) _or Diaheliotropism_.—The
cause of leaves placing themselves more or less transversely to the
light, with their upper surfaces directed towards it, has been of late
the subject of much controversy. We do not here refer to the object of
the movement, which no doubt is that their upper surfaces may be fully
illuminated, but the means by which this position is gained. Hardly a
better or more simple instance can be given
of diaheliotropism than that offered by many seedlings, the cotyledons
of which are extended horizontally. When they first burst from their
seed-coats they are in contact and stand in various positions, often
vertically upwards; they soon diverge, and this is effected by
epinasty, which, as we have seen, is a modified form of circumnutation.
After they have diverged to their full extent, they retain nearly the
same position, though brightly illuminated all day long from above,
with their lower surfaces close to the ground and thus much shaded.
There is therefore a great contrast in the degree of illumination of
their upper and lower surfaces, and if they were heliotropic they would
bend quickly upwards. It must not, however, be supposed that such
cotyledons are immovably fixed in a horizontal position. When seedlings
are exposed before a window, their hypocotyls, which are highly
heliotropic, bend quickly towards it, and the upper surfaces of their
cotyledons still remain exposed at right angles to the light; but if
the hypocotyl is secured so that it cannot bend, the cotyledons
themselves change their position. If the two are placed in the line of
the entering light, the one furthest from it rises up and that nearest
to it often sinks down; if placed transversely to the light, they twist
a little laterally; so that in every case they endeavour to place their
upper surfaces at right angles to the light. So it notoriously is with
the leaves on plants nailed against a wall, or grown in front of a
window. A moderate amount of light suffices to induce such movements;
all that is necessary is that the light should steadily strike the
plants in an oblique direction. With respect to the above twisting
movement of cotyledons, Frank has given many and much more striking
instances in the case of the leaves on
branches which had been fastened in various positions or turned upside
down.
[5] ‘Die natürliche Wagerechte Richtung von Pflanzentheilen,’ 1870.
See also some interesting articles by the same author, “Zur Frage über
Transversal-Geo-und Heliotropismus,” ‘Bot. Zeitung,’ 1873, p. 17 et
seq.
In our observations on the cotyledons of seedling plants, we often felt
surprise at their persistent horizontal position during the day, and
were convinced before we had read Frank’s essay, that some special
explanation was necessary. De Vries has shown[6] that the more or less
horizontal position of leaves is in most cases influenced by epinasty,
by their own weight, and by apogeotropism. A young cotyledon or leaf
after bursting free is brought down into its proper position, as
already remarked, by epinasty, which, according to De Vries, long
continues to act on the midribs and petioles. Weight can hardly be
influential in the case of cotyledons, except in a few cases presently
to be mentioned, but must be so with large and thick leaves. With
respect to apogeotropism, De Vries maintains that it generally comes
into play, and of this fact we shall presently advance some indirect
evidence. But over these and other constant forces we believe that
there is in many cases, but we do not say in all, a preponderant
tendency in leaves and cotyledons to place themselves more or less
transversely with respect to the light.
[6] ‘Arbeiten des Bot. Instituts in Würzburg,’ Heft. ii. 1872, pp.
223–277.
In the cases above alluded to of seedlings exposed to a lateral light
with their hypocotyls secured, it is impossible that epinasty, weight
and apogeotropism, either in opposition or combined, can be the cause
of the rising of one cotyledon, and of the sinking of the other, since
the forces in question act equally on both; and since epinasty, weight
and apogeotropism all act in a vertical plane, they cannot cause the
twisting of the petioles, which occurs in seedlings under the
above conditions of illumination. All these movements evidently depend
in some manner on the obliquity of the light, but cannot be called
heliotropic, as this implies bending towards the light; whereas the
cotyledon nearest to the light bends in an opposed direction or
downwards, and both place themselves as nearly as possible at right
angles to the light. The movement, therefore, deserves a distinct name.
As cotyledons and leaves are continually oscillating up and down, and
yet retain all day long their proper position with their upper surfaces
directed transversely to the light, and if displaced reassume this
position, diaheliotropism must be considered as a modified form of
circumnutation. This was often evident when the movements of cotyledons
standing in front of a window were traced. We see something analogous
in the case of sleeping leaves or cotyledons, which after oscillating
up and down during the whole day, rise into a vertical position late in
the evening, and on the following morning sink down again into their
horizontal or diaheliotropic position, in direct opposition to
heliotropism. This return into their diurnal position, which often
requires an angular movement of 90°, is analogous to the movement of
leaves on displaced branches, which recover their former positions. It
deserves notice that any force such as apogeotropism, will act with
different degrees of power[7] in the different positions of those
leaves or cotyledons which oscillate largely up and down during the
day; and yet they recover their horizontal or diaheliotropic position.
[7] See former note, in reference to Sachs’ remarks on this subject.
We may therefore conclude that diaheliotropic movements cannot be fully
explained by the direct action of light, gravitation, weight, etc., any
more
than can the nyctitropic movements of cotyledons and leaves. In the
latter case they place themselves so that their upper surfaces may
radiate at night as little as possible into open space, with the upper
surfaces of the opposite leaflets often in contact. These movements,
which are sometimes extremely complex, are regulated, though not
directly caused, by the alternations of light and darkness. In the case
of diaheliotropism, cotyledons and leaves place themselves so that
their upper surfaces may be exposed to the light, and this movement is
regulated, though not directly caused, by the direction whence the
light proceeds. In both cases the movement consists of circumnutation
modified by innate or constitutional causes, in the same manner as with
climbing plants, the circumnutation of which is increased in amplitude
and rendered more circular, or again with very young cotyledons and
leaves which are thus brought down into a horizontal position by
epinasty.
We have hitherto referred only to those leaves and cotyledons which
occupy a permanently horizontal position; but many stand more or less
obliquely, and some few upright. the cause of these differences of
position is not known; but in accordance with Wiesner’s views,
hereafter to be given, it is probable that some leaves and cotyledons
would suffer, if they were fully illuminated by standing at right
angles to the light.
We have seen in the second and fourth chapters that those cotyledons
and leaves which do not alter their positions at night sufficiently to
be said to sleep, commonly rise a little in the evening and fall again
on the next morning, so that they stand during the night at a rather
higher inclination than during the middle of the day. It is incredible
that a rising movement of 2° or 3°, or even of 10° or 20°, can be of
any service to the plant, so as to have been specially acquired. It
must be the result of some periodical change in the conditions to which
they are subjected, and there can hardly be a doubt that this is the
daily alternations of light and darkness. De Vries states in the paper
before referred to, that most petioles and midribs are apogeotropic;[8]
and apogeotropism would account for the above rising movement, which is
common to so many widely distinct species, if we suppose it to be
conquered by diaheliotropism during the middle of the day, as long as
it is of importance to the plant that its cotyledons and leaves should
be fully exposed to the light. The exact hour in the afternoon at which
they begin to bend slightly upwards, and the extent of the movement,
will depend on their degree of sensitiveness to gravitation and on
their power of resisting its action during the middle of the day, as
well as on the amplitude of their ordinary circumnutating movements;
and as these qualities differ much in different species, we might
expect that the hour in the afternoon at which they begin to rise would
differ much in different species, as is the case. Some other agency,
however, besides apogeotropism, must come into play, either directly or
indirectly, in this upward movement. Thus a young bean (Vicia faba),
growing in a small pot, was placed in front of a window in a klinostat;
and at night the leaves rose a little, although
the action of apogeotropism was quite eliminated. Nevertheless, they
did not rise nearly so much at night, as when subjected to
apogeotropism. Is it not possible, or even probable, that leaves and
cotyledons, which have moved upwards in the evening through the action
of apogeotropism during countless generations, may inherit a tendency
to this movement? We have seen that the hypocotyls of several
Leguminous plants have from a remote period inherited a tendency to
arch themselves; and we know that the sleep-movements of leaves are to
a certain extent inherited, independently of the alternations of light
and darkness.
[8] According to Frank (‘Die nat. Wagerechte Richtung von
Pflanzentheilen,’ 1870, p. 46) the root-leaves of many plants, kept in
darkness, rise up and even become vertical; and so it is in some cases
with shoots. (See Rauwenhoff, ‘Archives Néerlandaises,’ tom. xii. p.
32.) These movements indicate apogeotropism; but when organs have been
long kept in the dark, the amount of water and of mineral matter which
they contain is so much altered, and their regular growth is so much
disturbed, that it is perhaps rash to infer from their movements what
would occur under normal conditions. (See Godlewski, ‘Bot. Zeitung,’
Feb. 14th, 1879.)
In our observations on the circumnutation of those cotyledons and
leaves which do not sleep at night, we met with hardly any distinct
cases of their sinking a little in the evening, and rising again in the
morning,—that is, of movements the reverse of those just discussed. We
have no doubt that such cases occur, inasmuch as the leaves of many
plants sleep by sinking vertically downwards. How to account for the
few cases which were observed must be left doubtful. The young leaves
of Cannabis sativa sink at night between 30° and 40° beneath the
horizon; and Kraus attributes this to epinasty in conjunction with the
absorption of water. Whenever epinastic growth is vigorous, it might
conquer diaheliotropism in the evening, at which time it would be of no
importance to the plant to keep its leaves horizontal. The cotyledons
of Anoda Wrightii, of one variety of Gossypium, and of several species
of Ipomœa, remain horizontal in the evening whilst they are very young;
as they grow a little older they curve a little downwards, and when
large and heavy sink so much that they come under our definition of
sleep. In the case of
the Anoda and of some species of Ipomœa, it was proved that the
downward movement did not depend on the weight of the cotyledons; but
from the fact of the movement being so much more strongly pronounced
after the cotyledons have grown large and heavy, we may suspect that
their weight aboriginally played some part in determining that the
modification of the circumnutating movement should be in a downward
direction.
The so-called Diurnal Sleep of Leaves, Or Paraheliotropism.—This is
another class of movements, dependent on the action of light, which
supports to some extent the belief that the movements above described
are only indirectly due to its action. We refer to the movements of
leaves and cotyledons which when moderately illuminated are
diaheliotropic; but which change their positions and present their
edges to the light, when the sun shines brightly on them. These
movements have sometimes been called diurnal sleep, but they differ
wholly with respect to the object gained from those properly called
nyctitropic; and in some cases the position occupied during the day is
the reverse of that during the night.
It has long been known[9] that when the sun shines brightly on the
leaflets of Robinia, they rise up and present their edges to the light;
whilst their position at night is vertically downwards. We have
observed the same movement, when the sun shone brightly on the leaflets
of an Australian Acacia. Those of Amphicarpæa monoica turned their
edges to the sun; and an analogous movement of the little almost
rudimentary basal leaflets of Mimosa albida was on one occasion so
rapid that it could be distinctly seen through a lens. the elongated,
unifoliate, first leaves of Phaseolus Roxburghii stood at 7 A.M. at 20°
above the horizon, and no doubt they afterwards sank a little lower. At
noon, after having been exposed for about 2 h. to
a bright sun, they stood at 56° above the horizon; they were then
protected from the rays of the sun, but were left well illuminated from
above, and after 30 m. they had fallen 40°, for they now stood at only
16° above the horizon. Some young plants of Phaseolus Hernandesii had
been exposed to the same bright sunlight, and their broad, unifoliate,
first leaves now stood up almost or quite vertically, as did many of
the leaflets on the trifoliate secondary leaves; but some of the
leaflets had twisted round on their own axes by as much as 90° without
rising, so as to present their edges to the sun. The leaflets on the
same leaf sometimes behaved in these two different manners, but always
with the result of being less intensely illuminated. These plants were
then protected from the sun, and were looked at after 1½ h.; and now
all the leaves and leaflets had reassumed their ordinary sub-horizontal
positions. The copper-coloured cotyledons of some seedlings of Cassia
mimosoides were horizontal in the morning, but after the sun had shone
on them, each had risen 45½° above the horizon. the movement in these
several cases must not be confounded with the sudden closing of the
leaflets of Mimosa pudica, which may sometimes be noticed when a plant
which has been kept in an obscure place is suddenly exposed to the sun;
for in this case the light seems to act, as if it were a touch.
[9] Pfeffer gives the names and dates of several ancient writers in
his ‘Die Periodischen Bewegungen,’ 1875, p. 62.
From Prof. Wiesner’s interesting observations, it is probable that the
above movements have been acquired for a special purpose. the
chlorophyll in leaves is often injured by too intense a light, and
Prof. Wiesner[10] believes that it is protected by the most diversified
means, such as the presence of hairs, colouring matter, etc., and
amongst other means by the leaves presenting their edges to the sun, so
that the blades then receive much less light. He experimented on the
young leaflets of Robinia, by fixing them in such a position that they
could not escape being intensely illuminated, whilst others were
allowed to place themselves obliquely; and the former began to suffer
from the light in the course of two days.
[10] ‘Die Näturlichen Einrichtungen zum Schutze des Chlorophylls,’
etc., 1876. Pringsheim has recently observed under the microscope the
destruction of chlorophyll in a few minutes by the action of
concentrated light from the sun, in the presence of oxygen. See, also,
Stahl on the protection of chlorophyll from intense light, in ‘Bot.
Zeitung,’ 1880.
In the cases above given, the leaflets move either upwards
or twist laterally, so as to place their edges in the direction of the
sun’s light; but Cohn long ago observed that the leaflets of Oxalis
bend downwards when fully exposed to the sun. We witnessed a striking
instance of this movement in the very large leaflets of O. Ortegesii. A
similar movement may frequently be observed with the leaflets of
Averrhoa bilimbi (a member of the Oxalidæ); and a leaf is here
represented (Fig. 180) on which the sun had shone. A diagram (Fig. 134)
was given in the last chapter, representing the oscillations by which a
leaflet rapidly descended under these circumstances; and the movement
may be seen closely to resemble that (Fig. 133) by which it assumed its
nocturnal position. It is an interesting fact in relation to our
present subject that, as Prof. Batalin informs us in a letter, dated
February, 1879, the leaflets of Oxalis acetosella may be daily exposed
to the sun during many weeks, and they do not suffer if they are
allowed to depress themselves; but if this be prevented, they lose
their colour and wither in two or three days. Yet the duration of a
leaf is about two months, when subjected only to diffused light; and in
this case the leaflets never sink downwards during the day.
Fig. 180. Averrhoa bilimbi: leaf with leaflets depressed after exposure
to sunshine; but the leaflets are sometimes more depressed than is here
shown. Figure much reduced.
As the upward movements of the leaflets of Robinia, and the downward
movements of those of Oxalis, have been proved to be highly beneficial
to these plants when subjected to bright sunshine, it seems probable
that they have been acquired for the special purpose of avoiding too
intense an illumination. As it would have been very troublesome in all
the above cases to
have watched for a fitting opportunity and to have traced the movement
of the leaves whilst they were fully exposed to the sunshine, we did
not ascertain whether paraheliotropism always consisted of modified
circumnutation; but this certainly was the case with the Averrhoa, and
probably with the other species, as their leaves were continually
circumnutating.