The Power of Movement in Plants Part 2

the first day the seedling was placed too far from the vertical gla.s.s; so that the tracing was enormously exaggerated and the movement could not be traced when the cotyledon either rose or sank much; but it was clearly seen that the cotyledons rose thrice and fell twice between 8.15 A.M. and 4.15 P.M. Early on the following morning (June 19th) the apex of a cotyledon was [page 26]

placed only 1 7/8 inch from the vertical gla.s.s. At 6.40 A.M. it stood horizontally; it then fell till 8.35, and then rose. Altogether in the course of 12 h. it rose thrice and fell thrice, as may be seen in Fig. 15.

The great nocturnal rise of the cotyledons usually commences about 4 or 5 P.M., and on the following morning they are expanded or stand horizontally at about 6.30 A.M. In the present instance, however, the great nocturnal rise did not commence till 7 P.M.; but this was due to the hypocotyl having from some unknown cause temporarily bent to the left side, as is shown in the tracing. To ascertain positively that the hypocotyl circ.u.mnutated, a mark was placed at 8.15 P.M. behind the two now closed and vertical cotyledons; and the movement of a gla.s.s filament fixed upright to the top of the hypocotyl was traced until 10.40 P.M. During this time it moved from side to side, as well as backwards and forwards, plainly showing circ.u.mnutation; but the movement was small in extent. Therefore Fig. 15 represents fairly well the movements of the cotyledons alone, with the exception of the one great afternoon curvature to the left.

Oxalis corniculata (var. cuprea).--The cotyledons rise at night to a variable degree above the horizon, generally about 45o: those on some seedlings between 2 and 5 days old were found to be in continued movement all day long; but the movements were more simple than in the last two species. This may have partly resulted from their not being sufficiently illuminated whilst being observed, as was shown by their not beginning to rise until very late in the evening.

Oxalis (Biophytum) sensitiva.--The cotyledons are highly remarkable from the amplitude and rapidity of their movements during the day. The angles at which they stood above or beneath the horizon were measured at short intervals of time; and we regret that their course was not traced during the whole day. We will give only a few of the measurements, which were made whilst the seedlings were exposed to a temperature of 22 1/2o to 24 decrees C. One cotyledon rose 70o in 11 m.; another, on a distinct seedling, fell 80o in 12 m. Immediately before this latter fall the same cotyledon had risen from a vertically downward to a vertically upward position in 1 h. 48 m., and had therefore pa.s.sed through 180o in under 2 h.

We have met with no other instance of a circ.u.mnutating movement of such great amplitude as 180o; nor of such rapidity of movement as the pa.s.sage through 80o in 12 m. The cotyledons of this plant sleep at night by rising [page 27]

vertically and coming into close contact. This upward movement differs from one of the great diurnal oscillations above described only by the position being permanent during the night and by its periodicity, as it always commences late in the evening.

Tropaeolum minus (?) (var. Tom Thumb) (Tropaeoleae).--The cotyledons are hypogean, or never rise above the ground. By removing the soil a buried epicotyl or plumule was found, with its summit arched abruptly downwards, like the arched hypocotyl of the cabbage previously described. A gla.s.s filament with a bead at its end was affixed to the basal half or leg, just above the hypogean cotyledons, which were again almost surrounded by loose earth. The tracing (Fig. 16) shows the course of the bead during 11 h.

After the last dot given in the figure, the bead moved to a great distance, and finally off the gla.s.s, in the direction indicated by the broken line.

This great movement, due to increased growth along the concave surface of the arch, was caused by the basal leg bending backwards from the upper part, that is in a direction opposite to the dependent tip, in the same manner as occurred with the hypocotyl of the cabbage. Another buried and arched epicotyl was observed in the same manner, excepting that the two legs of the arch were tied together with fine silk for the sake of preventing the great movement just mentioned. It moved, however, in the evening in the same direction as before, but the line followed was not so straight. During the morning the tied arch moved in an irregularly circular, strongly zigzag course, and to a greater distance than in the previous case, as was shown in a tracing, magnified 18 times. The movements of a young plant bearing a few leaves and of a mature plant, will hereafter be described.

Fig. 16. Tropaeolum minus (?): circ.u.mnutation of buried and arched epicotyl, traced on a horizontal gla.s.s, from 9.20 A.M. to 8.15 P.M.

Movement of bead of filament magnified 27 times.

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Citrus aurantium (Orange) (Aurantiaceae).--The cotyledons are hypogean. The circ.u.mnutation of an epicotyl, which at the close of our observations was .59 of an inch (15 mm.) in height above the ground, is shown in the annexed figure (Fig. 17), as observed during a period of 44 h. 40 m.

Fig. 17. Citrus aurantium: circ.u.mnutation of epicotyl with a filament fixed transversely near its apex, traced on a horizontal gla.s.s, from 12.13 P.M.

on Feb. 20th to 8.55 A.M. on 22nd. The movement of the bead of the filament was at first magnified 21 times, or 10 1/2, in figure here given, and afterwards 36 times, or 18 as here given; seedling illuminated from above.

Aesculus hippocastanum (Hippocastaneae).--Germinating seeds were placed in a tin box, kept moist internally, with a sloping bank of damp argillaceous sand, on which four smoked gla.s.s-plates rested, inclined at angles of 70o and 65o with the horizon. The tips of the radicles were placed so as just to touch the upper end of the gla.s.s-plates, and, as they grew downwards they pressed lightly, owing to geotropism, on the smoked surfaces, and left tracks of their course. In the middle part of each track the gla.s.s was swept clean, but the margins were much blurred and irregular. Copies of two of these tracks (all four being nearly alike) were made on tracing paper placed over the gla.s.s-plates after they had been varnished; and they are as exact as possible considering the nature of the margins (Fig. 18). They suffice to show that there was some lateral, almost serpentine movement, and that the tips in their downward course pressed with unequal force on the plates, as [page 29]

the tracks varied in breadth. The more perfectly serpentine tracks made by the radicles of Phaseolus multiflorus and Vicia faba (presently to be described), render it almost certain that the radicles of the present plant circ.u.mnutated.

Fig. 18. Aesculus hippocastanum: outlines of tracks left on inclined gla.s.s-plates by tips of radicles. In A the plate was inclined at 70o with the horizon, and the radicle was 1.9 inch in length, and .23 inch in diameter at base. In B the plate was inclined 65o with the horizon, and the radicle was a trifle larger.

Phaseolus multiflorus (Leguminosae).--Four smoked gla.s.s-plates were arranged in the same manner as described under Aesculus, and the tracks left by the tips of four radicles of the present plant, whilst growing downwards, were photographed as transparent objects. Three of them are here exactly copied (Fig. 19). Their serpentine courses show that the tips moved regularly from side to side; they also pressed alternately with greater or less force on the plates, sometimes rising up and leaving them altogether for a very short distance; but this was better seen on the original plates than in the copies. These radicles therefore were continually moving in all directions--that is, they circ.u.mnutated. The distance between the extreme right and left positions of the radicle A, in its lateral movement, was 2 mm., as ascertained by measurement with an eye-piece micrometer.

Fig. 19. Phaseolus multiflorus: tracks left on inclined smoked gla.s.s-plates by tips of radicles in growing downwards. A and C, plates inclined at 60o, B inclined at 68o with the horizon.

Vicia faba (Common Bean) (Leguminosae).--Radicle.--Some beans were allowed to germinate on bare sand, and after one had protruded its radicle to a length of .2 of an inch, it was turned upside down, so that the radicle, which was kept in damp air, now stood upright. A filament, nearly an inch in length, was affixed obliquely near its tip; and the movement of the terminal bead was traced from 8.30 A.M. to 10.30 P.M., as shown in Fig. 18.

The radicle at first changed its course twice [page 30]

abruptly, then made a small loop and then a larger zigzag curve. During the night and till 11 A.M. on the following

Fig. 20. Vicia faba: circ.u.mnutation of a radicle, at first pointing vertically upwards, kept in darkness, traced on a horizontal gla.s.s, during 14 hours. Movement of bead of filament magnified 23 times, here reduced to one-half of original scale.

morning, the bead moved to a great distance in a nearly straight line, in the direction indicated by the broken line in the figure. This resulted from the tip bending quickly downwards, as it had now become much declined, and had thus gained a position highly favourable for the action of geotropism.

Fig. 21. Vicia faba: tracks left on inclined smoked gla.s.s-plates, by tips of radicles in growing downwards. Plate C was inclined at 63o, plates A and D at 71o, plate B at 75o, and plate E at a few degrees beneath the horizon.

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We next experimented on nearly a score of radicles by allowing them to grow downwards over inclined plates of smoked gla.s.s, in exactly the same manner as with Aesculus and Phaseolus. Some of the plates were inclined only a few degrees beneath the horizon, but most of them between 60o and 75o. In the latter cases the radicles in growing downwards were deflected only a little from the direction which they had followed whilst germinating in sawdust, and they pressed lightly on the gla.s.s-plates (Fig. 21). Five of the most distinct tracks are here copied, and they are all slightly sinuous, showing circ.u.mnutation. Moreover, a close examination of almost every one of the tracks clearly showed that the tips in their downward course had alternately pressed with greater or less force on the plates, and had sometimes risen up so as nearly to leave them for short intervals. The distance between the extreme right and left positions of the radicle A was 0.7 mm., ascertained in the same manner as in the case of Phaseolus.

Epicotyl.--At the point where the radicle had protruded from a bean laid on its side, a flattened solid lump projected .1 of an inch, in the same horizontal plane with the bean. This protuberance consisted of the convex summit of the arched epicotyl; and as it became developed the two legs of the arch curved themselves laterally upwards, owing to apogeotropism, at such a rate that the arch stood highly inclined after 14 h., and vertically in 48 h. A filament was fixed to the crown of the protuberance before any arch was visible, but the basal half grew so quickly that on the second morning the end of the filament was bowed greatly downwards. It was therefore removed and fixed lower down. The line traced during these two days extended in the same general direction, and was in parts nearly straight, and in others plainly zigzag, thus giving some evidence of circ.u.mnutation.

As the arched epicotyl, in whatever position it may be placed, bends quickly upwards through apogeotropism, and as the two legs tend at a very early age to separate from one another, as soon as they are relieved from the pressure of the surrounding earth, it was difficult to ascertain positively whether the epicotyl, whilst remaining arched, circ.u.mnutated.

Therefore some rather deeply buried beans were uncovered, and the two legs of the arches were tied together, as had been done with the epicotyl of Tropaeolum and the hypocotyl of the Cabbage. The movements of the tied arches were traced in the usual manner on [page 32]

two occasions during three days. But the tracings made under such unnatural conditions are not worth giving; and it need only be said that the lines were decidedly zigzag, and that small loops were occasionally formed. We may therefore conclude that the epicotyl circ.u.mnutates whilst still arched and before it has grown tall enough to break through the surface of the ground.

In order to observe the movements of the epicotyl at a somewhat more advanced age, a filament was fixed near the base of one which was no longer arched, for its upper half now formed a right angle with the lower half.

This bean had germinated on bare damp sand, and the epicotyl began to straighten itself much sooner than would have occurred if it had been properly planted. The course pursued during 50 h. (from 9 A.M. Dec. 26th, to 11 A.M. 28th) is here shown (Fig. 22); and we see Fig. 22. Vicia faba: circ.u.mnutation of young epicotyl, traced in darkness during 50 hours on a horizontal gla.s.s. Movement of bead of filament magnified 20 times, here reduced to one-half of original scale.

that the epicotyl circ.u.mnutated during the whole time. Its basal part grew so much during the 50 h. that the filament at the end of our observations was attached at the height of .4 inch above the upper surface of the bean, instead of close to it. If the bean had been properly planted, this part of the epicotyl would still have been beneath the soil.

Late in the evening of the 28th, some hours after the above observations were completed, the epicotyl had grown much straighter, for the upper part now formed a widely open angle with the lower part. A filament was fixed to the upright basal part, higher up than before, close beneath the lowest scale-like process or h.o.m.ologue of a leaf; and its movement was traced [page 33]

during 38 h. (Fig. 23). We here again have plain evidence of continued circ.u.mnutation. Had the bean been properly planted, the part of the epicotyl to which the filament was attached, the

Fig. 23. Vicia faba: circ.u.mnutation of the same epicotyl as in Fig. 22, a little more advanced in age, traced under similar conditions as before, from 8.40 A.M. Dec. 28th, to 10.50 A.M. 30th. Movement of bead here magnified 20 times.

movement of which is here shown, would probably have just risen above the surface of the ground.

Lathyrus nissolia (Leguminosae).--This plant was selected for observation from being an abnormal form with gra.s.s-like leaves.

Fig. 24. Lathyrus nissolia: circ.u.mnutation of stem of young seedling, traced in darkness on a horizontal gla.s.s, from 6.45 A.M. Nov. 22nd, to 7 A.M. 23rd. Movement of end of leaf magnified about 12 times, here reduced to one-half of original scale.

The cotyledons are hypogean, and the epicotyl breaks through the ground in an arched form. The movements of a stem, 1.2 inch in height, consisting of three internodes, the lower one almost wholly subterranean, and the upper one bearing a short, [page 34]

narrow leaf, is shown during 24 h., in Fig. 24. No gla.s.s filament was employed, but a mark was placed beneath the apex of the leaf. The actual length of the longer of the two ellipses described by the stem was about .14 of an inch. On the previous day the chief line of movement was nearly at right angles to that shown in the present figure, and it was more simple.

Ca.s.sia tora* (Leguminosae).--A seedling was placed before a

Fig. 25. Ca.s.sia tora: conjoint circ.u.mnutation of cotyledons and hypocotyl, traced on vertical gla.s.s, from 7.10 A.M. Sept. 25th to 7.30 A.M. 26th.

Figure here given reduced to one-half of original scale.

* Seeds of this plant, which grew near the sea-side, were sent to us by Fritz Muller from S. Brazil. The seedlings did not flourish or flower well with us; they were sent to Kew, and were p.r.o.nounced not to be distinguishable from C. tora.

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north-east window; it bent very little towards it, as the hypocotyl which was left free was rather old, and therefore not highly heliotropic. A filament had been fixed to the midrib of one of the cotyledons, and the movement of the whole seedling was traced during two days. The circ.u.mnutation of the hypocotyl is quite insignificant compared with that of the cotyledons. These rise up vertically at night and come into close contact; so that they may be said to sleep. This seedling was so old that a very small true leaf had been developed, which at night was completely hidden by the closed cotyledons. On Sept. 24th, between 8 A.M. and 5 P.M., the cotyledons moved five times up and five times down; they therefore described five irregular ellipses in the course of the 9 h. The great nocturnal rise commenced about 4.30 P.M.

On the following morning (Sept. 25th) the movement of the same cotyledon was again traced in the same manner during 24 h.; and a copy of the tracing is here given (Fig. 25). The morning was cold, and the window had been accidentally left open for a short time, which must have chilled the plant; and this probably prevented it from moving quite as freely as on the previous day; for it rose only four and sank only four times during the day, one of the oscillations being very small. At 7.10 A.M., when the first dot was made, the cotyledons were not fully open or awake; they continued to open till about 9 A.M., by which time they had sunk a little beneath the horizon: by 9.30 A.M. they had risen, and then they oscillated up and down; but the upward and downward lines never quite coincided. At about 4.30 P.M.

the great nocturnal rise commenced. At 7 A.M. on the following morning (Sept. 26th) they occupied nearly the same level as on the previous morning, as shown in the diagram: they then began to open or sink in the usual manner. The diagram leads to the belief that the great periodical daily rise and fall does not differ essentially, excepting in amplitude, from the oscillations during the middle of the day.

Lotus Jacoboeus (Leguminosae).--The cotyledons of this plant, after the few first days of their life, rise so as to stand almost, though rarely quite, vertically at night. They continue to act in this manner for a long time even after the development of some of the true leaves. With seedlings, 3 inches in height, and bearing five or six leaves, they rose at night about 45o. They continued to act thus for about an additional fortnight.

Subsequently they remained horizontal at night, though still green [page 36]

and at last dropped off. Their rising at night so as to stand almost vertically appears to depend largely on temperature; for when the seedlings were kept in a cool house, though they still continued to grow, the cotyledons did not become vertical at night. It is remarkable that the cotyledons do not generally rise at night to any conspicuous extent during the first four or five days after germination; but the period was extremely variable with seedlings kept under the same conditions; and many were observed. Gla.s.s filaments with minute triangles of paper were fixed to the cotyledons (1 mm. in breadth) of two seedlings, only 24 h. old, and the hypocotyl was secured to a stick; their movements greatly magnified were traced, and they certainly circ.u.mnutated the whole time on a small scale, but they did not exhibit any distinct nocturnal and diurnal movement. The hypocotyls, when left free, circ.u.mnutated over a large s.p.a.ce.

Another and much older seedling, bearing a half-developed leaf, had its movements traced in a similar manner during the three first days and nights of June; but seedlings at this age appear to be very sensitive to a deficiency of light; they were observed under a rather dim skylight, at a temperature of between 16o to 17 1/2o C.' and apparently, in consequence of these conditions, the great daily movement of the cotyledons ceased on the third day. During the first two days they began rising in the early afternoon in a nearly straight line, until between 6 and 7 P.M., when they stood vertically. During the latter part of the night, or more probably in the early morning, they began to fall or open, so that by 6.45 A.M. they stood fully expanded and horizontal. They continued to fall slowly for some time, and during the second day described a single small ellipse, between 9 A.M. and 2 P.M., in addition to the great diurnal movement. The course pursued during the whole 24 h. was far less complex than in the foregoing case of Ca.s.sia. On the third morning they fell very much, and then circ.u.mnutated on a small scale round the same spot; by 8.20 P.M. they showed no tendency to rise at night. Nor did the cotyledons of any of the many other seedlings in the same pot rise; and so it was on the following night of June 5th. The pot was then taken back into the hot-house, where it was exposed to the sun, and on the succeeding night all the cotyledons rose again to a high angle, but did not stand quite vertically. On each of the above days the line representing the great nocturnal [page 37]

rise did not coincide with that of the great diurnal fall, so that narrow ellipses were described, as is the usual rule with circ.u.mnutating organs.

The cotyledons are provided with a pulvinus, and its development will hereafter be described.

Mimosa pudica (Leguminosae).--The cotyledons rise up vertically at night, so as to close together. Two seedlings were observed in the greenhouse (temp. 16o to 17o C. or 63o to 65o F.). Their hypocotyls were secured to sticks, and gla.s.s filaments bearing little triangles of paper were affixed to the cotyledons of both. Their movements were traced on a vertical gla.s.s during 24 h. on November 13th. The pot had stood for some time in the same position, and they were chiefly illuminated through the gla.s.s-roof. The cotyledons of one of these seedlings moved downward in the morning till 11.30 A.M., and then rose, moving rapidly in the evening until they stood vertically, so that in this case there was simply a single great daily fall and rise. The other seedling behaved rather differently, for it fell in the morning until 11.30 A.M., and then rose, but after 12.10 P.M. again fell; and the great evening rise did not begin until 1.22 P.M. On the following morning this cotyledon had fallen greatly from its vertical position by 8.15 A.M. Two other seedlings (one seven and the other eight days old) had been previously observed under unfavourable circ.u.mstances, for they had been brought into a room and placed before a north-east window, where the temperature was between only 56o and 57o F. They had, moreover, to be protected from lateral light, and perhaps were not sufficiently illuminated. Under these circ.u.mstances the cotyledons moved simply downwards from 7 A.M. till 2 P.M., after which hour and during a large part of the night they continued to rise. Between 7 and 8 A.M. on the following morning they fell again; but on this second and likewise on the third day the movements became irregular, and between 3 and 10.30 P.M. they circ.u.mnutated to a small extent about the same spot; but they did not rise at night. Nevertheless, on the following night they rose as usual.

Cytisus fragrans (Leguminosae).--Only a few observations were made on this plant. The hypocotyl circ.u.mnutated to a considerable extent, but in a simple manner--namely, for two hours in one direction, and then much more slowly back again in a zigzag course, almost parallel to the first line, and beyond the starting-point. It moved in the same direction all night, but next morning began to return. The cotyledons continually [page 38]

move both up and down and laterally; but they do not rise up at night in a conspicuous manner.

Lupinus luteus (Leguminosae).--Seedlings of this plant were observed because the cotyledons are so thick (about .08 of an inch) that it seemed unlikely that they would move. Our observations were not very successful, as the seedlings are strongly heliotropic, and their circ.u.mnutation could not be accurately observed near a north-east window, although they had been kept during the previous day in the same position. A seedling was then placed in darkness with the hypocotyl secured to a stick; both cotyledons rose a little at first, and then fell during the rest of the day; in the evening between 5 and 6 P.M. they moved very slowly; during the night one continued to fall and the other rose, though only a little. The tracing was not much magnified, and as the lines were plainly zigzag, the cotyledons must have moved a little laterally, that is, they must have circ.u.mnutated.

The hypocotyl is rather thick, about .12 of inch; nevertheless it circ.u.mnutated in a complex course, though to a small extent. The movement of an old seedling with two true leaves partially developed, was observed in the dark. As the movement was magnified about 100 times it is not trustworthy and is not given; but there could be no doubt that the hypocotyl moved in all directions during the day, changing its course 19 times. The extreme actual distance from side to side through which the upper part of the hypocotyl pa.s.sed in the course of 14 hours was only 1/60 of an inch; it sometimes travelled at the rate of 1/50 of an inch in an hour.