MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION.
Means of observation—Apogeotropism—Cytisus—Verbena—Beta—Gradual
conversion of the movement of circumnutation into apogeotropism in
Rubus, Lilium, Phalaris, Avena, and Brassica—Apogeotropism retarded by
heliotropism—Effected by the aid of joints or pulvini—Movements of
flower-peduncles of Oxalis—General remarks on
apogeotropism—Geotropism—Movements of radicles—Burying of
seed-capsules—Use of process—Trifolium
subterraneum—Arachis—Amphicarpæa—Diageotropism—Conclusion
Our object in the present chapter is to show that geotropism,
apogeotropism, and diageotropism are modified forms of circumnutation.
Extremely fine filaments of glass, bearing two minute triangles of
paper, were fixed to the summits of young stems, frequently to the
hypocotyls of seedlings, to flower-peduncles, radicles, etc., and the
movements of the parts were then traced in the manner already described
on vertical and horizontal glass-plates. It should be remembered that
as the stems or other parts become more and more oblique with respect
to the glasses, the figures traced on them necessarily become more and
more magnified. The plants were protected from light, excepting whilst
each observation was being made, and then the light, which was always a
dim one, was allowed to enter so as to interfere as little as possible
with the movement in progress; and we did not detect any evidence of
such interference.
When observing the gradations between
circumnutation and heliotropism, we had the great advantage of being
able to lessen the light; but with geotropism analogous experiments
were of course impossible. We could, however, observe the movements of
stems placed at first only a little from the perpendicular, in which
case geotropism did not act with nearly so much power, as when the
stems were horizontal and at right angles to the force. Plants, also,
were selected which were but feebly geotropic or apogeotropic, or had
become so from having grown rather old. Another plan was to place the
stems at first so that they pointed 30 or 40° beneath the horizon, and
then apogeotropism had a great amount of work to do before the stem was
rendered upright; and in this case ordinary circumnutation was often
not wholly obliterated. Another plan was to observe in the evening
plants which during the day had become greatly curved heliotropically;
for their stems under the gradually waning light very slowly became
upright through the action of apogeotropism; and in this case modified
circumnutation was sometimes well displayed.
Apogeotropism.—Plants were selected for observation almost by chance,
excepting that they were taken from widely different families. If the
stem of a plant which is even moderately sensitive to apogeotropism be
placed horizontally, the upper growing part bends quickly upwards, so
as to become perpendicular; and the line traced by joining the dots
successively made on a glass-plate, is generally almost straight. For
instance, a young Cytisus fragrans, 12 inches in height, was placed so
that the stem projected 10° beneath the horizon, and its course was
traced during 72 h. At first it bent a very little downwards (Fig.
182), owing no doubt to the weight of the stem, as this occurred with
most of the other plants observed, though, as they were of course
circumnutating, the short downward lines were often oblique. After
three-quarters of an hour the stem began to curve upwards, quickly
during the first two hours, but much more slowly during the afternoon
and night,
and on the following day. During the second night it fell a little, and
circumnutated during the following day; but it also moved a short
distance to the right, which was caused by a little light having been
accidentally admitted on this side. The stem was now inclined 60° above
the horizon, and had therefore risen 70°. With time allowed it would
probably have become upright, and no doubt would have continued
circumnutating. The sole remarkable feature in the figure here given is
the straightness of the course pursued. The stem, however, did not move
upwards at an equable rate, and it sometimes stood almost or quite
still. Such periods probably represent attempts to circumnutate in a
direction opposite to apogeotropism.
Fig. 182. Cytisus fragrans: apogeotropic movement of stem from 10°
beneath to 60° above horizon, traced on vertical glass, from 8.30 A.M.
March 12th to 10.30 P.M. 13th. The subsequent circumnutating movement
is likewise shown up to 6.45 A.M. on the 15th. Nocturnal course
represented, as usual, by a broken line. Movement not greatly
magnified, and tracing reduced to two-thirds of original scale.
The herbaceous stem of a Verbena melindres (?) laid horizontally, rose
in 7 h. so much that it could no longer be observed on the vertical
glass which stood in front of the plant. The long line which was traced
was almost absolutely straight. After the 7 h. it still continued to
rise, but now circumnutated slightly. On the following day it stood
upright, and circumnutated regularly, as shown in Fig. 82, given in the
fourth chapter. The stems of several other plants which were highly
sensitive to apogeotropism rose up in almost straight lines, and
then suddenly began to circumnutate. A partially etiolated and somewhat
old hypocotyl of a seedling cabbage (2 3/4 inches in height) was so
sensitive that when placed at an angle of only 23° from the
perpendicular, it became vertical in 33 minutes. As it could not have
been strongly acted upon by apogeotropism in the above slightly
inclined position, we expected that it would have circumnutated, or at
least have moved in a zigzag course. Accordingly, dots were made every
3 minutes; but, when these were joined, the line was nearly straight.
After this hypocotyl had become upright it still moved onwards for half
an hour in the same general direction, but in a zigzag manner. During
the succeeding 9 h. it circumnutated regularly, and described 3 large
ellipses. In this case apogeotropism, although acting at a very
unfavourable angle, quite overcame the ordinary circumnutating
movement.
Fig. 183. Beta vulgaris: apogeotropic movement of hypocotyl from 19°
beneath horizon to a vertical position, with subsequent circumnutation,
traced on a vertical and on a horizontal glass-plate, from 8.28 A.M.
Sept. 28th to 8.40 A.M. 29th. Figure reduced to one-third of original
scale.
The hypocotyls of Beta vulgaris are highly sensitive to apogeotropism.
One was placed so as to project 19° beneath the horizon; it fell at
first a very little (see Fig. 183), no doubt owing to its weight; but
as it was circumnutating the line was
oblique. During the next 3 h. 8 m. it rose in a nearly straight line,
passing through an angle of 109°, and then (at 12.3 P.M.) stood
upright. It continued for 55 m. to move in the same general direction
beyond the perpendicular, but in a zigzag course. It returned also in a
zigzag line, and then circumnutated regularly, describing three large
ellipses during the remainder of the day. It should be observed that
the ellipses in this figure are exaggerated in size, relatively to the
length of the upward straight line, owing to the position of the
vertical and horizontal glass-plates. Another and somewhat old
hypocotyl was placed so as to stand at only 31° from the perpendicular,
in which position apogeotropism acted on it with little force, and its
course accordingly was slightly zigzag.
The sheath-like cotyledons of Phalaris Canariensis are extremely
sensitive to apogeotropism. One was placed so as to project 40° beneath
the horizon. Although it was rather old and 1.3 inch in height, it
became vertical in 4 h. 30 m., having passed through an angle of 130°
in a nearly straight line. It then suddenly began to circumnutate in
the ordinary manner. The cotyledons of this plant, after the first leaf
has begun to protrude, are but slightly apogeotropic, though they still
continue to circumnutate. One at this stage of development was placed
horizontally, and did not become upright even after 13 h., and its
course was slightly zigzag. So, again, a rather old hypocotyl of Cassia
tora (1 1/4 inch in height) required 28 h. to become upright, and its
course was distinctly zigzag; whilst younger hypocotyls moved much more
quickly and in a nearly straight line.
When a horizontally placed stem or other organ rises in a zigzag line,
we may infer from the many cases given in our previous chapters, that
we have a modified form of circumnutation; but when the course is
straight, there is no evidence of circumnutation, and any one might
maintain that this latter movement had been replaced by one of a wholly
distinct kind. This view seems the more probable when (as sometimes
occurred with the hypocotyls of Brassica and Beta, the stems of
Cucurbita, and the cotyledons of Phalaris) the part in question, after
bending up in a straight course, suddenly begins to circumnutate to the
full extent and in the usual manner. A fairly good instance of a sudden
change of this kind—that is, from a nearly straight upward movement to
one of circumnutation—is shown in Fig. 183; but more striking instances
were occasionally observed with Beta, Brassica, and Phalaris.
We will now describe a few cases in which it may be
seen how gradually circumnutation becomes changed into apogeotropism,
under circumstances to be specified in each instance.
Rubus idæus (hybrid).—A young plant, 11 inches in height, growing in a
pot, was placed horizontally; and the upward movement was traced during
nearly 70 h.; but the plant, though growing vigorously, was not highly
sensitive to apogeotropism, or it was not capable of quick movement,
for during the above time it rose only 67°. We may see in the diagram
(Fig. 184) that during the first day of 12 h. it rose in a nearly
straight line. When placed horizontally, it was evidently
circumnutating, for it rose at first a little, notwithstanding the
weight of the stem, and then sank down; so that it did not start on its
permanently upward course until 1 h. 25 m. had elapsed. On the second
day, by which time it had risen considerably, and when apogeotropism
acted on it with somewhat less power, its course during 15½ h. was
clearly zigzag, and the rate of the upward movement was not equable.
During the third day, also of 15½ h., when apogeotropism acted on it
with still less power, the stem plainly circumnutated, for it moved
during this day 3 times up and 3 times down, 4 times to the left and 4
to the right. But the course was so complex that it could hardly be
traced on the glass. We can, however, see that the successively formed
irregular ellipses rose higher and higher. Apogeotropism continued to
act on the fourth morning, as the stem was still rising, though it now
stood only 23° from the perpendicular. In this diagram the several
stages may be followed by which an almost rectilinear, upward,
apogeotropic course first becomes zigzag, and then changes into a
circumnutating movement, with most of the successively formed,
irregular ellipses directed upwards.
Fig 184: Rubus idæus (hybrid): apogeotropic movement of stem, traced on
a vertical glass during 3 days and 3 nights, from 10.40 A.M. March 18th
to 8 A.M. 21st. Figure reduced to one-half of the original scale.
Lilium auratum.—A plant 23 inches in height was placed
horizontally, and the upper part of the stem rose 58° in 46 h., in the
manner shown in the accompanying diagram (Fig. 185). We here see that
during the whole of the second day of 15½ h., the stem plainly
circumnutated whilst bending upwards through apogeotropism. It had
still to rise considerably, for when the last dot in the figure was
made, it stood 32° from an upright position.
Fig. 185. Lilium auratum: apogeotropic movement of stem, traced on a
vertical glass during 2 days and 2 nights, from 10.40 A.M. March 18th
to 8 A.M. 20th. Figure reduced to one-half of the original scale.
Phalaris Canariensis.—A cotyledon of this plant (1.3 inch in height)
has already been described as rising in 4 h. 30 m. from 40° beneath the
horizon into a vertical position, passing through an angle of 130° in a
nearly straight line, and then abruptly beginning to circumnutate.
Another somewhat old cotyledon of the same height (but from which a
true leaf had not yet protruded), was similarly placed at 40° beneath
the horizon. For the first 4 h. it rose in a nearly straight course
(Fig. 186), so that by 1.10 P.M. it was highly inclined, and now
apogeotropism acted on it with much less power than before, and it
began to zigzag. At 4.15 P.M. (i.e. in 7 h. from the commencement) it
stood vertically, and afterwards continued to circumnutate in the usual
manner about the same spot. Here then we have a graduated change from a
straight upward apogeotropic course into circumnutation, instead of an
abrupt change, as in the former case.
Avena sativa.—The sheath-like cotyledons, whilst young, are strongly
apogeotropic; and some which were placed at 45° beneath the horizon
rose 90° in 7 or 8 h. in lines almost absolutely straight. An oldish
cotyledon, from which the first leaf began to
protrude whilst the following observations were being made, was placed
at 10° beneath the horizon, and it rose only 59° in 24h. It behaved
rather differently from any other plant, observed by us, for during the
first 4½ h. it rose in a line not far from straight; during the next 6½
h. it circumnutated, that is, it descended and again ascended in a
strongly marked zigzag course; it then resumed its upward movement in a
moderately straight line, and, with time allowed, no doubt would have
become upright. In this case, after the first 4½ h., ordinary
circumnutation almost completely conquered for a time apogeotropism.
Fig 186. Phalaris Canariensis: apogeotropic movement of cotyledon,
traced on a vertical and horizontal glass, from 9.10 A.M. Sept. 19th to
9 A.M. 20th. Figure here reduced to one-fifth of original scale.
Brassica oleracea.—The hypocotyls of several young seedlings placed
horizontally, rose up vertically in the course of 6 or 7 h. in nearly
straight lines. A seedling which had grown in darkness to a height of 2
1/4 inches, and was therefore rather old and not highly sensitive, was
placed so that the hypocotyl projected at between 30° and 40° beneath
the horizon. The upper part alone became curved
upwards, and rose during the first 3 h. 10 m. in a nearly straight line
(Fig. 187); but it was not possible to trace the upward movement on the
vertical glass for the first 1 h. 10 m., so that the nearly straight
line in the diagram ought to have been much longer. During the next 11
h. the hypocotyl circumnutated, describing irregular figures, each of
which rose a little above the one previously formed. During the night
and following early morning it continued to rise in a zigzag course, so
that apogeotropism was still acting. At the close of our observations,
after 23 h. (represented by the highest dot in the diagram) the
hypocotyl was still 32° from the perpendicular. There can be little
doubt that it would ultimately have become upright by describing an
additional number of irregular ellipses, one above the other.
Fig 187. Brassica oleracea: apogeotropic movement of hypocotyl, traced
on vertical glass, from 9.20 A.M., Sept. 12th to 8.30 A.M. 13th. The
upper part of the figure is more magnified than the lower part. If the
whole course had been traced, the straight upright line would have been
much longer. Figure here reduced to one-third of the original scale.
Apogeotropism retarded by Heliotropism.—When the stem of any plant
bends during the day towards a lateral light, the movement is opposed
by apogeotropism; but as the light gradually wanes in the evening the
latter power slowly gains the upper hand, and draws the stem back into
a vertical position. Here then we have a good opportunity for observing
how apogeotropism acts when very nearly balanced by an opposing force.
For instance, the plumule of Tropaeolum majus (see former Fig. 175)
moved towards the dim evening light in a slightly zigzag line until
6.45 P.M., it then returned on its course until
10.40 P.M., during which time it zigzagged and described an ellipse of
considerable size. The hypocotyl of Brassica oleracea (see former Fig.
173) moved in a straight line to the light until 5.15 P.M., and then
from the light, making in its backward course a great rectangular bend,
and then returned for a short distance towards the former source of the
light; no observations were made after 7.10 P.M., but during the night
it recovered its vertical position. A hypocotyl of Cassia tora moved in
the evening in a somewhat zigzag line towards the failing light until 6
P.M., and was now bowed 20° from the perpendicular; it then returned on
its course, making before 10.30 P.M. four great, nearly rectangular
bends and almost completing an ellipse. Several other analogous cases
were casually observed, and in all of them the apogeotropic movement
could be seen to consist of modified circumnutation.
Apogeotropic Movements effected by the aid of joints or
pulvini.—Movements of this kind are well known to occur in the
Gramineæ, and are effected by means of the thickened bases of their
sheathing leaves; the stem within being in this part thinner than
elsewhere.[1] According to the analogy of all other pulvini, such
joints ought to continue circumnutating for a long period, after the
adjoining parts have ceased to grow. We therefore wished to ascertain
whether this was the case with the Gramineæ; for if so, the upward
curvature of their stems, when extended horizontally or laid prostrate,
would be explained in accordance with our view—namely, that
apogeotropism results from modified circumnutation. After these joints
have curved upwards, they are fixed in their new position by increased
growth along their lower sides.
[1] This structure has been recently described by De Vries in an
interesting article, ‘Ueber die Aufrichtung des gelagerten Getreides,’
in ‘Landwirthschaftliche Jahrbücher,’ 1880, p. 473.
Lolium perenne.—A young stem, 7 inches in height, consisting of 3
internodes, with the flower-head not yet protruded, was selected for
observation. A long and very thin glass filament was cemented
horizontally to the stem close above the second joint, 3 inches above
the ground. This joint was subsequently proved to be in an active
condition, as its lower side swelled much through the action of
apogeotropism (in the manner described by De Vries) after the haulm had
been fastened down for 24 h. in a horizontal position. The pot was
so placed that the end of the filament stood beneath the 2-inch object
glass of a microscope with an eye-piece micrometer, each division of
which equalled 1/500 of an inch. The end of the filament was repeatedly
observed during 6 h., and was seen to be in constant movement; and it
crossed 5 divisions of the micrometer (1/100 inch) in 2 h. Occasionally
it moved forwards by jerks, some of which were 1/1000 inch in length,
and then slowly retreated a little, afterwards again jerking forwards.
These oscillations were exactly like those described under Brassica and
Dionaea, but they occurred only occasionally. We may therefore conclude
that this moderately old joint was continually circumnutating on a
small scale.
Alopecurus pratensis.—A young plant, 11 inches in height, with the
flower-head protruded, but with the florets not yet expanded, had a
glass filament fixed close above the second joint, at a height of only
2 inches above the ground. The basal internode, 2 inches in length, was
cemented to a stick to prevent any possibility of its circumnutating.
The extremity of the filament, which projected about 50° above the
horizon, was often observed during 24 h. in the same manner as in the
last case. Whenever looked at, it was always in movement, and it
crossed 30 divisions of the micrometer (3/50 inch) in 3½ h.; but it
sometimes moved at a quicker rate, for at one time it crossed 5
divisions in 1½ h. The pot had to be moved occasionally, as the end of
the filament travelled beyond the field of vision; but as far as we
could judge it followed during the daytime a semicircular course; and
it certainly travelled in two different directions at right angles to
one another. It sometimes oscillated in the same manner as in the last
species, some of the jerks forwards being as much as 1/1000 of an inch.
We may therefore conclude that the joints in this and the last species
of grass long continue to circumnutate; so that this movement would be
ready to be converted into an apogeotropic movement, whenever the stem
was placed in an inclined or horizontal position.
Movements of the Flower-peduncles of Oxalis carnosa, due to
apogeotropism and other forces.—The movements of the main peduncle, and
of the three or four sub-peduncles which each main peduncle of this
plant bears, are extremely complex, and are determined by several
distinct causes. Whilst the flowers are expanded, both kinds of
peduncles circumnutate about the same spot, as we have seen (Fig. 91)
in the fourth chapter. But soon after the flowers have begun to wither
the
sub-peduncles bend downwards, and this is due to epinasty; for on two
occasions when pots were laid horizontally, the sub-peduncles assumed
the same position relatively to the main peduncle, as would have been
the case if they had remained upright; that is, each of them formed
with it an angle of about 40°. If they had been acted on by geotropism
or apheliotropism (for the plant was illuminated from above), they
would have directed themselves to the centre of the earth. A main
peduncle was secured to a stick in an upright position, and one of the
upright sub-peduncles which had been observed circumnutating whilst the
flower was expanded, continued to do so for at least 24 h. after it had
withered. It then began to bend downwards, and after 36 h. pointed a
little beneath the horizon. A new figure was now begun (A, Fig. 188),
and the sub-peduncle was traced descending in a zigzag line from 7.20
P.M. on the 19th to 9 A.M. on the 22nd. It now pointed almost
perpendicularly downwards, and the glass filament had to be removed and
fastened transversely across the base of the young capsule. We expected
that the sub-peduncle would have been motionless in its new position;
but it continued slowly to swing, like a pendulum, from side to side,
that is, in a plane at right angles to that in which it had descended.
This circumnutating movement was observed from 9 A.M. on 22nd to 9 A.M.
24th, as shown at B in the diagram. We were not able to observe this
particular sub-peduncle any longer; but it would certainly have gone on
circumnutating until the capsule was nearly ripe (which requires only a
short time), and it would then have moved upwards.
The upward movement (C, Fig. 188) is effected in part by the whole
sub-peduncle rising in the same manner as it had previously descended
through epinasty—namely, at the joint where united to the main
peduncle. As this upward movement occurred with plants kept in the dark
and in whatever position the main peduncle was fastened, it could not
have been caused by heliotropism or apogeotropism, but by hyponasty.
Besides this movement at the joint, there is another of a very
different kind, for the sub-peduncle becomes upwardly bent in the
middle part. If the sub-peduncle happens at the time to be inclined
much downwards, the upward curvature is so great that the whole forms a
hook. The upper end bearing the capsule, thus always places itself
upright, and as this occurs in darkness, and in whatever position the
main peduncle may have been secured,
the upward curvature cannot be due to heliotropism or hyponasty, but to
apogeotropism.
Fig. 188. Oxalis carnosa: movements of flower-peduncle, traced on a
vertical glass: A, epinastic downward movement; B, circumnutation
whilst depending vertically; C, subsequent upward movement, due to
apogeotropism and hyponasty combined.
In order to trace this upward movement, a filament was fixed to a
sub-peduncle bearing a capsule nearly ripe, which was beginning to bend
upwards by the two means just described. Its course was traced (see C,
Fig 188) during 53 h., by which time it had become nearly upright. The
course is seen to be strongly zigzag, together with some little loops.
We may therefore conclude that the movement consists of modified
circumnutation.
The several species of Oxalis probably profit in the following manner
by their sub-peduncles first bending downwards and then upwards. They
are known to scatter their seeds by the bursting of the capsule; the
walls of which are so extremely thin, like silver paper, that they
would easily be permeated by rain. But as soon as the petals wither,
the sepals rise up and enclose the young capsule, forming a perfect
roof over it as soon as the sub-peduncle has bent itself downwards. By
its subsequent upward movement, the capsule stands when ripe at a
greater height above the ground by twice the length of the
sub-peduncle, than it did when dependent, and is thus able to scatter
its seeds to a greater distance. The sepals, which enclose the ovarium
whilst it is young, present an additional adaptation by expanding
widely when the seeds are ripe, so as not to interfere with their
dispersal. In the case of Oxalis acetosella, the capsules are said
sometimes to bury themselves under loose leaves or moss on the ground,
but this cannot occur with those of O. carnosa, as the woody stem is
too high.
Oxalis acetosella.—The peduncles are furnished with a joint in the
middle, so that the lower part answers to the main peduncle,
and the upper part to one of the sub-peduncles of O. carnosa. The upper
part bends downwards, after the flower has begun to wither, and the
whole peduncle then forms a hook; that this bending is due to epinasty
we may infer from the case of O. carnosa. When the pod is nearly ripe,
the upper part straightens itself and becomes erect; and this is due to
hyponasty or apogeotropism, or both combined, and not to heliotropism,
for it occurred in darkness. The short, hooked part of the peduncle of
a cleistogamic flower, bearing a pod nearly ripe, was observed in the
dark during three days. The apex of the pod at first pointed
perpendicularly down, but in the course of three days rose 90°, so that
it now projected horizontally. The course during the two latter days is
shown in Fig. 189; and it may be seen how greatly the peduncle, whilst
rising, circumnutated. The lines of chief movement were at right angles
to the plane of the originally hooked part. The tracing was not
continued any longer; but after two additional days, the peduncle with
its capsule had become straight and stood upright.
Fig. 189. Oxalis acetosella: course pursued by the upper part of a
peduncle, whilst rising, traced from 11 A.M. June 1st to 9 A.M. 3rd.
Figure here reduced to one-half of the original scale.
Concluding Remarks on Apogeotropism.—When apogeotropism is rendered by
any means feeble, it acts, as shown in the several foregoing cases, by
increasing the always present circumnutating movement in a direction
opposed to gravity, and by diminishing that in the direction of
gravity, as well as that to either side. The upward movement thus
becomes unequal in rate, and is sometimes interrupted by stationary
periods. Whenever irregular ellipses or loops are still formed, their
longer axes are almost always directed in the line of gravity, in an
analogous manner as occurred with heliotropic movements in reference to
the light. As apogeotropism acts more and more energetically, ellipses
or loops cease to be formed, and the course becomes at first strongly,
and then less and less zigzag, and finally rectilinear. From this
gradation in the nature of the movement, and more especially from all
growing parts, which alone (except when pulvini are present) are acted
on by apogeotropism,
continually circumnutating, we may conclude that even a rectilinear
course is merely an extremely modified form of circumnutation. It is
remarkable that a stem or other organ which is highly sensitive to
apogeotropism, and which has bowed itself rapidly upwards in a straight
line, is often carried beyond the vertical, as if by momentum. It then
bends a little backwards to a point round which it finally
circumnutates. Two instances of this were observed with the hypocotyls
of Beta vulgaris, one of which is shown in Fig. 183, and two other
instances with the hypocotyls of Brassica. This momentum-like movement
probably results from the accumulated effects of apogeotropism. For the
sake of observing how long such after-effects lasted, a pot with
seedlings of Beta was laid on its side in the dark, and the hypocotyls
in 3 h. 15 m. became highly inclined. The pot, still in the dark, was
then placed upright, and the movements of the two hypocotyls were
traced; one continued to bend in its former direction, now in
opposition to apogeotropism, for about 37 m., perhaps for 48 m.; but
after 61 m. it moved in an opposite direction. The other hypocotyl
continued to move in its former course, after being placed upright, for
at least 37 m.
Different species and different parts of the same species are acted on
by apogeotropism in very different degrees. Young seedlings, most of
which circumnutate quickly and largely, bend upwards and become
vertical in much less time than do any older plants observed by us; but
whether this is due to their greater sensitiveness to apogeotropism, or
merely to their greater flexibility we do not know. A hypocotyl of Beta
traversed an angle of 109° in 3 h. 8 m., and a cotyledon of Phalaris an
angle of 130° in 4 h. 30 m. On the other hand, the stem of a herbaceous
Verbena rose 90° in about 24 h.; that of Rubus 67°, in 70 h.; that of
Cytisus 70°, in 72 h.; that of a young American Oak only 37°, in 72 h.
The stem of a young Cyperus alternifolius rose only 11° in 96 h.; the
bending being confined to near its base. Though the sheath-like
cotyledons of Phalaris are so extremely sensitive to apogeotropism, the
first true leaves which protrude from them exhibited only a trace of
this action. Two fronds of a fern, Nephrodium molle, both of them young
and one with the tip still inwardly curled, were kept in a horizontal
position for 46 h., and during this time they rose so little that it
was doubtful whether there was any true apogeotropic movement.
The most curious case known to us of a difference in sensitiveness to
gravitation, and consequently of movement, in different parts of the
same organ, is that offered by the petioles of the cotyledons of Ipomœa
leptophylla. The basal part for a short length where united to the
undeveloped hypocotyl and radicle is strongly geotropic, whilst the
whole upper part is strongly apogeotropic. But a portion near the
blades of the cotyledons is after a time acted on by epinasty and
curves downwards, for the sake of emerging in the form of an arch from
the ground; it subsequently straightens itself, and is then again acted
on by apogeotropism.
A branch of Cucurbita ovifera, placed horizontally, moved upwards
during 7 h. in a straight line, until it stood at 40° above the
horizon; it then began to circumnutate, as if owing to its trailing
nature it had no tendency to rise any higher. Another upright branch
was secured to a stick, close to the base of a tendril, and the pot was
then laid horizontally in the dark. In this position the tendril
circumnutated and made
several large ellipses during 14 h., as it likewise did on the
following day; but during this whole time it was not in the least
affected by apogeotropism. On the other hand, when branches of another
Cucurbitaceous plant, Echinocytis lobata, were fixed in the dark so
that the tendrils depended beneath the horizon, these began immediately
to bend upwards, and whilst thus moving they ceased to circumnutate in
any plain manner; but as soon as they had become horizontal they
recommenced to revolve conspicuously.[2] The tendrils of Passiflora
gracilis are likewise apogeotropic. Two branches were tied down so that
their tendrils pointed many degrees beneath the horizon. One was
observed for 8 h., during which time it rose, describing two circles,
one above the other. The other tendril rose in a moderately straight
line during the first 4 h., making however one small loop in its
course; it then stood at about 45° above the horizon, where it
circumnutated during the remaining 8 h. of observation.
[2] For details see ‘The Movements and Habits of Climbing Plants,’
1875, p. 131.
A part or organ which whilst young is extremely sensitive to
apogeotropism ceases to be so as it grows old; and it is remarkable, as
showing the independence of this sensitiveness and of the
circumnutating movement, that the latter sometimes continues for a time
after all power of bending from the centre of the earth has been lost.
Thus a seedling Orange bearing only 3 young leaves, with a rather stiff
stem, did not curve in the least upwards during 24 h. whilst extended
horizontally; yet it circumnutated all the time over a small space. The
hypocotyl of a young seedling of Cassia tora, similarly placed, became
vertical in 12 h.; that of an older seedling, 1 1/4 inch in height,
became so in 28 h.; and that of another still older one, 1½ inch in
height, remained horizontal during two days, but distinctly
circumnutated during this whole time.
When the cotyledons of Phalaris or Avena are laid horizontally, the
uppermost part first bends upwards, and then the lower part;
consequently, after the lower part has become much curved upwards, the
upper part is compelled to curve backwards in an opposite direction, in
order to straighten itself and to stand vertically; and this subsequent
straightening process is likewise due to apogeotropism. The upper part
of 8 young cotyledons of Phalaris were made rigid by being cemented to
thin glass rods, so that this part could not bend in the least;
nevertheless, the basal part was not prevented from curving upward. A
stem or other organ which bends upwards through apogeotropism exerts
considerable force; its own weight, which has of course to be lifted,
was sufficient in almost every instance to cause the part at first to
bend a little downwards; but the downward course was often rendered
oblique by the simultaneous circumnutating movement. The cotyledons of
Avena placed horizontally, besides lifting their own weight, were able
to furrow the soft sand above them, so as to leave little crescentic
open spaces on the lower sides of their bases; and this is a remarkable
proof of the force exerted.
As the tips of the cotyledons of Phalaris and Avena bend upwards
through the action of apogeotropism before the basal part, and as these
same tips when excited by a lateral light transmit some influence to
the lower part, causing it to bend, we thought that the same rule might
hold good with apogeotropism. Consequently, the tips of 7 cotyledons of
Phalaris were
cut off for a length in three cases of .2 inch and in the four other
cases of .14, .12, .1, and .07 inch. But these cotyledons, after being
extended horizontally, bowed themselves upwards as effectually as the
unmutilated specimens in the same pots, showing that sensitiveness to
gravitation is not confined to their tips.
GEOTROPISM.
This movement is directly the reverse of apogeotropism. Many organs
bend downwards through epinasty or apheliotropism or from their own
weight; but we have met with very few cases of a downward movement in
sub-aërial organs due to geotropism. We shall however, give one good
instance in the following section, in the case of Trifolium
subterraneum, and probably in that of Arachis hypogaea.
On the other hand, all roots which penetrate the ground (including the
modified root-like petioles of Megarrhiza and Ipomœa leptophylla) are
guided in their downward course by geotropism; and so are many aërial
roots, whilst others, as those of the Ivy, appear to be indifferent to
its action. In our first chapter the movements of the radicles of
several seedlings were described. We may there see (Fig. 1) how a
radicle of the cabbage, when pointing vertically upwards so as to be
very little acted on by geotropism, circumnutated; and how another
(Fig. 2) which was at first placed in an inclined position bowed itself
downwards in a zigzag line, sometimes remaining stationary for a time.
Two other radicles of the cabbage travelled downwards in almost
rectilinear courses. A radicle of the bean placed upright (Fig. 20)
made a great sweep and zigzagged; but as it sank downwards and was more
strongly acted on by geotropism, it moved in an
almost straight course. A radicle of Cucurbita, directed upwards (Fig.
26), also zigzagged at first, and described small loops; it then moved
in a straight line. Nearly the same result was observed with the
radicles of Zea mays. But the best evidence of the intimate connection
between circumnutation and geotropism was afforded by the radicles of
Phaseolus, Vicia, and Quercus, and in a less degree by those of Zea and
Æsculus (see Figs. 18, 19, 21, 41, and 52); for when these were
compelled to grow and slide down highly inclined surfaces of smoked
glass, they left distinctly serpentine tracks.
The Burying of Seed-capsules: Trifolium subterraneum.—The flower-heads
of this plant are remarkable from producing only 3 or 4 perfect
flowers, which are situated exteriorly. All the other many flowers
abort, and are modified into rigid points, with a bundle of vessels
running up their centres. After a time 5 long, elastic, claw-like
projections, which represent the divisions of the calyx, are developed
on their summits. As soon as the perfect flowers wither they bend
downwards, supposing the peduncle to stand upright, and they then
closely surround its upper part. This movement is due to epinasty, as
is likewise the case with the flowers of T. repens. The imperfect
central flowers ultimately follow, one after the other, the same
course. Whilst the perfect flowers are thus bending down, the whole
peduncle curves downwards and increases much in length, until the
flower-head reaches the ground. Vaucher[3] says that when the plant is
so placed that the heads cannot soon reach the ground, the peduncles
grow to the extraordinary length of from 6 to 9 inches. In whatever
position the branches may be placed, the upper part of the peduncle at
first bends vertically upwards through heliotropism; but as soon as the
flowers begin to wither the downward curvature of the whole peduncle
commences. As this latter movement occurred in complete darkness, and
with peduncles arising from upright and from dependent branches, it
cannot be due to apheliotropism or to epinasty, but must be attributed
to geotropism. Nineteen
upright flower-heads, arising from branches in all sorts of positions,
on plants growing in a warm greenhouse, were marked with thread, and
after 24 h. six of them were vertically dependent; these therefore had
travelled through 180° in this time. Ten were extended
sub-horizontally, and these had moved through about 90°. Three very
young peduncles had as yet moved only a little downwards, but after an
additional 24 h. were greatly inclined.
[3] ‘Hist. Phys. des Plantes d’Europe,’ tom. ii. 1841, p. 106.
At the time when the flower-heads reach the ground, the younger
imperfect flowers in the centre are still pressed closely together, and
form a conical projection; whereas the perfect and imperfect flowers on
the outside are upturned and closely surround the peduncle. They are
thus adapted to offer as little resistance, as the case admits of, in
penetrating the ground, though the diameter of the flower-head is still
considerable. The means by which this penetration is effected will
presently be described. The flower-heads are able to bury themselves in
common garden mould, and easily in sand or in fine sifted cinders
packed rather closely. The depth to which they penetrated, measured
from the surface to the base of the head, was between 1/4 and ½ inch,
but in one case rather above 0.6 inch. With a plant kept in the house,
a head partly buried itself in sand in 6 h.: after 3 days only the tips
of the reflexed calyces were visible, and after 6 days the whole had
disappeared. But with plants growing out of doors we believe, from
casual observations, that they bury themselves in a much shorter time.
After the heads have buried themselves, the central aborted flowers
increase considerably in length and rigidity, and become bleached. They
gradually curve, one after the other, upwards or towards the peduncle,
in the same manner as did the perfect flowers at first. In thus moving,
the long claws on their summits carry with them some earth. Hence a
flower-head which has been buried for a sufficient time, forms a rather
large ball, consisting of the aborted flowers, separated from one
another by earth, and surrounding the little pods (the product of the
perfect flowers) which lie close round the upper part of the peduncle.
The calyces of the perfect and imperfect flowers are clothed with
simple and multicellular hairs, which have the power of absorption; for
when placed in a weak solution of carbonate of ammonia (2 gr. to 1 oz.
of water) their protoplasmic contents immediately became aggregated and
afterwards displayed the usual slow movements. This clover generally
grows in dry soil, but whether the power of absorption by the hairs on
the buried flower-heads is of any importance to them we do not know.
Only a few of the flower-heads, which from their position are not able
to reach the ground and bury themselves, yield seeds; whereas the
buried ones never failed, as far as we observed, to produce as many
seeds as there had been perfect flowers.
We will now consider the movements of the peduncle whilst curving down
to the ground. We have seen in Chap. IV., Fig. 92, p. 225, that an
upright young flower-head circumnutated conspicuously; and that this
movement continued after the peduncle had begun to bend downwards. The
same peduncle was observed when inclined at an angle of 19° above the
horizon, and it circumnutated during two days. Another
which was already curved 36° beneath the horizon, was observed from 11
A.M. July 22nd to the 27th, by which latter date it had become
vertically dependent. Its course during the first 12 h. is shown in
Fig. 190, and its position on the three succeeding mornings until the
25th, when it was nearly vertical. During the first day the peduncle
clearly circumnutated, for it moved 4 times down and 3 times up; and on
each succeeding day, as it sank downwards, the same movement continued,
but was only occasionally observed and was less strongly marked. It
should be stated that these peduncles were observed under a double
skylight in the house, and that they generally moved downwards very
much more slowly than those on plants growing out of doors or in the
greenhouse.
Fig. 190. Trifolium subterraneum: downward movement of peduncle from
19° beneath the horizon to a nearly vertically dependent position,
traced from 11 A.M. July 22nd to the morning of 25th. Glass filament
fixed transversely across peduncle, at base of flower-head.
Fig. 191. Trifolium subterraneum: circumnutating movement of peduncle,
whilst the flower-head was burying itself in sand, with the reflexed
tips of the calyx still visible; traced from 8 A.M. July 26th to 9 A.M.
on 27th. Glass filament fixed transversely across peduncle, near
flower-head.
Fig. 192. Trifolium subterraneum: movement of same peduncle, with
flower-head completely buried beneath the sand; traced from 8 A.M. to
7.15 P.M. on July 29th.
The movement of another vertically dependent peduncle with the
flower-head standing half an inch above the ground, was traced, and
again when it first touched the ground; in both cases irregular
ellipses were described every 4 or 5 h. A peduncle on a plant which had
been brought into the house, moved from an upright into a vertically
dependent position in a single day; and here the course during the
first 12 h. was nearly straight, but with a few well-marked zigzags
which betrayed the essential nature of the movement. Lastly the
circumnutation of a peduncle was traced during 51 h. whilst in the act
of burying itself obliquely in a little heap of sand. After it had
buried itself to such a depth that the tips of the sepals were alone
visible, the above figure (Fig 191) was traced during 25 h. When the
flower-head had completely disappeared beneath the sand, another
tracing was made during 11 h. 45 m. (Fig. 192); and here again we see
that the peduncle was circumnutating.
Any one who will observe a flower-head burying itself, will be
convinced that the rocking movement, due to the continued
circumnutation of the peduncle, plays an important part in the act.
Considering that the flower-heads are very light, that the peduncles
are long, thin, and flexible, and that they arise from flexible
branches, it is incredible that an object as blunt as one of these
flower-heads could penetrate the ground by means of the growing force
of the peduncle, unless it were aided by the rocking movement. After a
flower-head has penetrated the ground to a small depth, another and
efficient agency comes into play; the central rigid aborted flowers,
each terminating in five long claws, curve up towards the peduncle; and
in doing so can hardly fail to drag the head down to a greater depth,
aided as this action is by the circumnutating movement, which continues
after the flower-head has completely buried itself. The aborted flowers
thus act something like the hands of the mole, which force the earth
backwards and the body forwards.
It is well known that the seed-capsules of various widely distinct
plants either bury themselves in the ground, or are produced from
imperfect flowers developed beneath the surface. Besides the present
case, two other well-marked instances will be immediately given. It is
probable that one chief good thus gained is the protection of the seeds
from animals which prey on them. In the case of T. subterraneum, the
seeds are not only concealed by being buried, but are likewise
protected by being closely surrounded by the rigid, aborted flowers. We
may the more confidently infer that protection is here aimed at,
because the seeds of several species in this same genus are protected
in other ways;[4] namely, by the swelling and closure of the calyx, or
by the persistence and bending down of the standard-petal, etc. But the
most curious instance is that of T. globosum, in which the upper
flowers are sterile, as in T. subterraneum, but are here developed into
large brushes of hairs which envelop and protect the seed-bearing
flowers. Nevertheless, in all these cases the capsules, with their
seeds, may profit, as Mr. T. Thiselton Dyer has remarked,[5] by their
being kept somewhat damp; and the advantage of such dampness perhaps
throws light on the presence of the absorbent hairs on the buried
flower-heads of T. subterraneum. According to Mr. Bentham, as quoted by
Mr. Dyer,
the prostrate habit of Helianthemum prostratum “brings the capsules in
contact with the surface of the ground, postpones their maturity, and
so favours the seeds attaining a larger size.” The capsules of Cyclamen
and of Oxalis acetosella are only occasionally buried, and this only
beneath dead leaves or moss. If it be an advantage to a plant that its
capsules should be kept damp and cool by being laid on the ground, we
have in these latter cases the first step, from which the power of
penetrating the ground, with the aid of the always present movement of
circumnutation, might afterwards have been gained.
[4] Vaucher, ‘Hist. Phys. des Plantes d’Europe,’ tom. ii. p. 110.
[5] See his interesting article in ‘Nature,’ April 4th, 1878, p. 446.
Arachis hypogoea.—The flowers which bury themselves, rise from stiff
branches a few inches above the ground, and stand upright. After they
have fallen off, the gynophore, that is the part which supports the
ovarium, grows to a great length, even to 3 or 4 inches, and bends
perpendicularly downwards. It resembles closely a peduncle, but has a
smooth and pointed apex, which contains the ovules, and is at first not
in the least enlarged. The apex after reaching the ground penetrates
it, in one case observed by us to a depth of 1 inch, and in another to
0.7 inch. It there becomes developed into a large pod. Flowers which
are seated too high on the plant for the gynophore to reach the ground
are said[6] never to produce pods.
[6] ‘Gard. Chronicle,’ 1857, p. 566.
The movement of a young gynophore, rather under an inch in length and
vertically dependent, was traced during 46 H. by means of a glass
filament (with sights) fixed transversely a little above the apex. It
plainly circumnutated (Fig. 193) whilst increasing in length and
growing downwards. It was then raised up, so as to be extended almost
horizontally, and the terminal part curved itself downwards, following
a nearly straight course during 12 h., but with one attempt to
circumnutate, as shown in Fig. 194. After 24 h. it had become nearly
vertical. Whether the exciting cause of the downward movement is
geotropism or apheliotropism was not ascertained; but probably it is
not apheliotropism, as all the gynophores grew straight down towards
the ground, whilst the light in the hot-house entered from one side as
well as from above. Another and older gynophore, the apex of which had
nearly reached the ground, was observed during 3 days in the same
manner as the first-mentioned short one; and it was found to be always
circumnutating. During the first 34 h. it described a figure which
represented four ellipses. Lastly, a long gynophore, the apex of which
had buried itself to the depth of about half an inch, was pulled up and
extended horizontally: it quickly began to curve downwards in a zigzag
line; but on the following day the
terminal bleached portion was a little shrivelled. As the gynophores
are rigid and arise from stiff branches, and as they terminate in sharp
smooth points, it is probable that they could penetrate the ground by
the mere force of growth. But this action must be aided by the
circumnutating movement, for fine sand, kept moist, was pressed close
round the apex of a gynophore which had reached the ground, and after a
few hours it was surrounded by a narrow open crack. After three weeks
this gynophore was uncovered, and the apex was found at a depth of
rather above half an inch developed into a small, white, oval pod.
Fig. 193 Arachis hypogoea: circumnutation of vertically dependent young
gynophore, traced on a vertical glass from 10 A.M. July 31st to 8 A.M.
Aug. 2nd.
Fig. 194. Arachis hypogoea: downward movement of same young gynophore,
after being extended horizontally; traced on a vertical glass from 8.30
A.M. to 8.30 P.M. Aug. 2nd.
Amphicarpoea monoica.—This plant produces long thin shoots, which twine
round a support and of course circumnutate. Early in the summer shorter
shoots are produced from the lower parts of the plant, which grow
perpendicularly downwards and penetrate the ground. One of these,
terminating in a minute bud, was observed to bury itself in sand to a
depth of 0.2 inch in 24 h. It was lifted up and fixed in an inclined
position about 25° beneath the horizon, being feebly illuminated from
above. In this position it described two vertical ellipses in 24 h.;
but on the following day, when brought into the house, it circumnutated
only a very little round the same spot. Other branches were seen to
penetrate the ground, and were afterwards found running like roots
beneath the surface for a length of nearly two inches, and they had
grown thick. One of these, after thus running, had emerged into the
air. How far circumnutation aids these delicate branches in entering
the ground we do not know; but the reflexed hairs with which they are
clothed will assist in the work. This plant produces pods in the air,
and others beneath the ground; which differ greatly in appearance. Asa
Gray says[7] that it is the imperfect flowers on the creeping branches
near the base of the plant which produce the subterranean pods; these
flowers, therefore, must bury themselves like those of Arachis. But it
may be suspected that the branches which were seen by us to penetrate
the ground also produce subterranean flowers and pods.
[7] ‘Manual of the Botany of the Northern United States,’ 1856, p.
106.
DIAGEOTROPISM.
Besides geotropism and apogeotropism, there is, according to Frank, an
allied form of movement,
namely, “transverse-geotropism,” or diageotropism, as we may call it
for the sake of matching our other terms. Under the influence of
gravitation certain parts are excited to place themselves more or less
transversely to the line of its action.[8] We made no observations on
this subject, and will here only remark that the position of the
secondary radicles of various plants, which extend horizontally or are
a little inclined downwards, would probably be considered by Frank as
due to transverse-geotropism. As it has been shown in Chap. I. that the
secondary radicles of Cucurbita made serpentine tracks on a smoked
glass-plate, they clearly circumnutated, and there can hardly be a
doubt that this holds good with other secondary radicles. It seems
therefore highly probable that they place themselves in their
diageotropic position by means of modified circumnutation.
[8] Elfving has lately described (‘Arbeiten des Bot. Instituts in
Würzburg,’ B. ii. 1880, p. 489) an excellent instance of such
movements in the rhizomes of certain plants.
Finally, we may conclude that the three kinds of movement which have
now been described and which are excited by gravitation, consist of
modified circumnutation. Different parts or organs on the same plant,
and the same part in different species, are thus excited to act in a
widely different manner. We can see no reason why the attraction of
gravity should directly modify the state of turgescence and subsequent
growth of one part on the upper side and of another part on the lower
side. We are therefore led to infer that both geotropic, apogeotropic,
and diageotropic movements, the purpose of which we can generally
understand,
have been acquired for the advantage of the plant by the modification
of the ever-present movement of circumnutation. This, however, implies
that gravitation produces some effect on the young tissues sufficient
to serve as a guide to the plant.