The Power of Movement in Plants. Charles Darwin
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growth; but after a time one grew to a height of five inches, measured to the tips of the small partially unfolded leaves on the summit, and now looked vigorous. It consisted of six very thin internodes of unequal lengths. Considering these circumstances and the nature of the plant, we hardly expected that it would circumnutate; but the annexed figure (Fig. 40) shows that it did so in a conspicuous manner, changing its course many times and travelling in all directions during the 48 h. of observation. The figure seems to represent 5 or 6 irregular ovals or ellipses. The actual amount of movement from side to side (excluding one great bend to the left) was about .2 of an inch; but this was difficult to estimate, as owing to the rapid growth of the stem, the attached filament was much further from the mark beneath at the close than at the commencement of the observations. It deserves notice that the pot was placed in a north-east room within a deep box, the top of which was not at first covered up, so that the inside facing [page 54] the windows was a little more illuminated than the opposite side; and during the first morning the stem travelled to a greater distance in this direction (to the left in the figure) than it did afterwards when the box was completely protected from light.
Fig. 40. Quercus (American sp.): circumnutation of young stem, traced on horizontal glass, from 12.50 P.m. Feb. 22nd to 12.50 P.m. 24th. Movement of bead greatly magnified at first, but slightly towards the close of the observations—about 10 times on an average.
Quercus robur.—Observations were made only on the movements of the radicles from germinating acorns, which were allowed to grow downwards in the manner previously described, over plates of smoked glass, inclined at angles between 65o and 69o to the horizon. In four cases the tracks left were almost straight, but the tips had pressed sometimes with more and sometimes with less force on the glass, as shown by the varying thickness of the tracks and by little bridges of soot left across them. In the fifth case the track was slightly serpentine, that is, the tip had moved a little from side to side. In the sixth case (Fig. 41, A) it was plainly serpentine, and the tip had pressed almost equably on the glass in its whole course. In the seventh case (B) the tip had moved both laterally and had pressed [page 55] alternately with unequal force on the glass; so that it had moved a little in two planes at right angles to one another. In the eighth and last case (C) it had moved very little laterally, but had alternately left the glass and come into contact with it again. There can be no doubt that in the last four cases the radicle of the oak circumnutated whilst growing downwards.
Fig. 41. Quercus robur: tracks left on inclined smoked glass-plates by tips of radicles in growing downwards. Plates A and C inclined at 65o and plate B at 68o to the horizon.
Corylus avellana (Corylaceae).—The epicotyl breaks through the ground in an arched form; but in the specimen which was first examined, the apex had become decayed, and the epicotyl grew to some distance through the soil, in a tortuous, almost horizontal direction, like a root. In consequence of this injury it had emitted near the hypogean cotyledons two secondary shoots, and it was remarkable that both of these were arched, like the normal epicotyl in ordinary cases. The soil was removed from around one of these arched secondary shoots, and a glass filament was affixed to the basal leg. The whole was kept damp beneath a metal-box with a glass lid, and was thus illuminated only from above. Owing apparently to the lateral pressure of the earth being removed, the terminal and bowed-down part of the shoot began at once to move upwards, so that after 24 h. it formed a right angle with the lower part. This lower part, to which the filament was attached, also straightened itself, and moved a little backwards from the upper part. Consequently a long line was traced on the horizontal glass; and [page 56] this was in parts straight and in parts decidedly zigzag, indicating circumnutation.
On the following day the other secondary shoot was observed; it was a little more advanced in age, for the upper part, instead of depending vertically downwards, stood at an angle of 45o above the horizon. The tip of the shoot projected obliquely .4 of an inch above the ground, but by the close of our observations, which lasted 47 h., it had grown, chiefly towards its base, to a height of .85 of an inch. The filament was fixed transversely to the basal and almost upright half of the shoot, close beneath the lowest scale-like appendage. The circumnutating course pursued is shown in the accompanying figure (Fig. 42). The actual distance traversed from side to side was about .04 of an inch.
Fig. 42. Corylus avellana: circumnutation of a young shoot emitted from the epicotyl, the apex of which had been injured, traced on a horizontal glass, from 9 A.m. Feb. 2nd to 8 A.m. 4th. Movement of bead magnified about 27 times.
Pinus pinaster (Coniferae).—A young hypocotyl, with the tips of the cotyledons still enclosed within the seed-coats, was at first only .35 of an inch in height; but the upper part grew so rapidly that at the end of our observations it was .6 in height,
Fig. 43. Pinus pinaster: circumnutation of hypocotyl, with filament fixed across its summit, traced on horizontal glass, from 10 A.m. March 21st to 9 A.m. 23rd. Seedling kept in darkness. Movement of bead magnified about 35 times. [page 57]
and by this time the filament was attached some way down the little stem. From some unknown cause, the hypocotyl moved far towards the left, but there could be no doubt (Fig. 43) that it circumnutated. Another hypocotyl was similarly observed, and it likewise moved in a strongly zigzag line to the same side. This lateral movement was not caused by the attachment of the glass filaments, nor by the action of light; for no light was allowed to enter when each observation was made, except from vertically above.
The hypocotyl of a seedling was secured to a little stick; it bore nine in appearance distinct cotyledons, arranged in a circle. The movements of two nearly opposite ones were observed. The tip of one was painted white, with a mark placed below, and the figure described (Fig. 44, A) shows that it made an irregular
Fig. 44. Pinus pinaster: circumnutation of two opposite cotyledons, traced on horizontal glass in darkness, from 8.45 A.m. to 8.35 P.m. Nov. 25th. Movement of tip in A magnified about 22 times, here reduced to one-half of original scale.
circle in the course of about 8 h. during the night it travelled to a considerable distance in the direction indicated by the broken line. A glass filament was attached longitudinally to the other cotyledon, and this nearly completed (Fig, 44, B) an irregular circular figure in about 12 hours. During the night it also moved to a considerable distance, in the direction indicated by the broken line. The cotyledons therefore circumnutate independently of the movement of the hypocotyl. Although they moved much during the night, they did not approach each other so as to stand more vertically than during the day. [page 58]
Cycas pectinata (Cycadeae).—The large seeds of this plant in germinating first protrude a single leaf, which breaks through the ground with the petiole bowed into an arch and with the leaflets involuted. A leaf in this condition, which at the close of our observations was 2½ inches in height, had its movements traced in a warm greenhouse by means of a glass filament bearing paper triangles attached across its tip. The tracing (Fig. 45) shows how large, complex, and rapid were the circum-
Fig. 45. Cycas pectinata: circumnutation of young leaf whilst emerging from the ground, feebly illuminated from above, traced on vertical glass, from 5 P.m. May 28th to 11 A.m. 31st. Movement magnified 7 times, here reduced to two-thirds of original scale.
nutating movements. The extreme distance from side to side which it passed over amounted to between .6 and .7 of an inch.
Canna Warscewiczii (Cannaceae).—A seedling with the plumule projecting one inch above the ground was observed, but not under fair conditions, as it was brought out of the hot-house and kept in a room not sufficiently warm. Nevertheless the tracing (Fig. 46) shows that it made two or three incomplete irregular circles or ellipses in the course of 48 hours. The plumule is straight; and this was the first instance observed [page 59] by us of the part that first breaks through the ground not being arched.
Fig. 46. Canna Warscewiczii: circumnutation of plumule with filament affixed obliquely to outer sheath-like leaf, traced in darkness on horizontal glass from 8.45 A.m. Nov. 9th to 8.10 A.m. 11th. Movement of bead magnified 6 times.
Allium cepa (Liliaceae).—The narrow green leaf, which protrudes from the seed of the common onion as a cotyledon,* breaks through the ground in the form of an arch, in the same manner as the hypocotyl or epicotyl of a dicotyledonous plant. Long after the arch has risen