The Power of Movement in Plants Part 4

Fig. 37. Solanum lycopersic.u.m: circ.u.mnutation of hypocotyl, with filament fixed across its summit, traced on horizontal gla.s.s, from 10 A.M. to 5 P.M.

Oct. 24th. Illuminated obliquely from above. Movement of bead magnified about 35 times, here reduced to one-third of original scale.

Solanum lycopersic.u.m (Solaneae).--The movements of the hypocotyls of two seedling tomatoes were observed during seven hours, and there could be no doubt that both circ.u.mnutated. They were illuminated from above, but by an accident a little light entered on one side, and in the accompanying figure (Fig. 37) it may be seen that the hypocotyl moved to this side (the upper one in the figure), making small loops and zigzagging in its course. The movements of the cotyledons were also traced both on vertical and horizontal gla.s.ses; their angles with the horizon were likewise measured at various hours. They fell from 8.30 A.M. (October 17th) to about noon; then moved laterally in a zigzag line, and at about 4 P.M. began to rise; they continued to do so until 10.30 P.M., by which hour they stood vertically and were asleep. At what hour of the night or early morning they began to fall was not ascertained. Owing to the lateral movement shortly after mid-day, the descending and ascending lines did not coincide, and irregular ellipses were described during each 24 h. The regular periodicity of these movements is destroyed, as we shall hereafter see, if the seedlings are kept in the dark.

[page 51]

Solanum palinacanthum.--Several arched hypocotyls rising nearly .2 of an inch above the ground, but with the cotyledons still buried beneath the surface, were observed, and the tracings showed that they circ.u.mnutated.

Moreover, in several cases little open circular s.p.a.ces or cracks in the argillaceous sand which surrounded the arched hypocotyls were visible, and these appeared to have been made by the hypocotyls having bent first to one and then to another side whilst growing upwards. In two instances the vertical arches were observed to move to a considerable distance backwards from the point where the cotyledons lay buried; this movement, which has been noticed in some other cases, and which seems to aid in extracting the cotyledons from the buried seed-coats, is due to the commencement of the straightening of the hypocotyl. In order to prevent this latter movement, the two legs of an arch, the

Fig. 38. Solanum palinacanthum: circ.u.mnutation of an arched hypocotyl, just emerging from the ground, with the two legs tied together, traced in darkness on a horizontal gla.s.s, from 9.20 A.M. Dec. 17th to 8.30 A.M. 19th.

Movement of bead magnified 13 times; but the filament, which was affixed obliquely to the crown of the arch, was of unusual length.

summit of which was on a level with the surface of the soil, were tied together; the earth having been previously removed to a little depth all round. The movement of the arch during 47 hours under these unnatural circ.u.mstances is exhibited in the annexed figure.

The cotyledons of some seedlings in the hot-house were horizontal about noon on December 13th; and at 10 P.M. had risen to an angle of 27o above the horizon; at 7 A.M. on the following [page 52]

morning, before it was light, they had risen to 59o above the horizon; in the afternoon of the same day they were found again horizontal.

Beta vulgaris (Chenopodeae).--The seedlings are excessively sensitive to light, so that although on the first day they were uncovered only during two or three minutes at each observation, they all moved steadily towards the side of the room whence the light proceeded, and the tracings consisted only of slightly zigzag lines directed towards the light. On the next day the plants were placed in a completely darkened room, and at each observation were illuminated as much as possible from vertically above by a small wax taper. The annexed figure (Fig. 39) shows the movement of the hypocotyl during 9 h. under these circ.u.mstances. A second seedling was similarly observed at the same time, and the tracing had the same peculiar character, due to the hypocotyl often moving and returning in nearly parallel lines. The movement of a third hypocotyl differed greatly.

Fig. 39. Beta vulgaris: circ.u.mnutation of hypocotyl, with filament fixed obliquely across its summit, traced in darkness on horizontal gla.s.s, from 8.25 A.M. to 5.30 P.M. Nov. 4th. Movement of bead magnified 23 times, here reduced to one-third of original scale.

We endeavoured to trace the movements of the cotyledons, and for this purpose some seedlings were kept in the dark, but they moved in an abnormal manner; they continued rising from 8.45 A.M. to 2 P.M., then moved laterally, and from 3 to 6 P.M. descended; whereas cotyledons which have been exposed all the day to the light rise in the evening so as to stand vertically at night; but this statement applies only to young seedlings.

For instance, six seedlings in the greenhouse had their cotyledons partially open for the first time on the morning of November 15th, and at 8.45 P.M. all were completely closed, so that they might properly be said to be asleep. Again, on the morning of November 27th, the cotyledons of four other seedlings, which were surrounded by a collar of brown paper so that they received light only from above, were open to the extent of 39o; at 10 P.M. they were completely closed; next morning (November 28th) at 6.45 A.M. whilst it was still dark, two of them [page 53]

were partially open and all opened in the course of the morning; but at 10.20 P.M. all four (not to mention nine others which had been open in the morning and six others on another occasion) were again completely closed.

On the morning of the 29th they were open, but at night only one of the four was closed, and this only partially; the three others had their cotyledons much more raised than during the day. On the night of the 30th the cotyledons of the four were only slightly raised.

Ricinus Borboniensis (Euphorbiaceae).--Seeds were purchased under the above name--probably a variety of the common castor-oil plant. As soon as an arched hypocotyl had risen clear above the ground, a filament was attached to the upper leg bearing the cotyledons which were still buried beneath the surface, and the movement of the bead was traced on a horizontal gla.s.s during a period of 34 h. The lines traced were strongly zigzag, and as the bead twice returned nearly parallel to its former course in two different directions, there could be no doubt that the arched hypocotyl circ.u.mnutated. At the close of the 34 h. the upper part began to rise and straighten itself, dragging the cotyledons out of the ground, so that the movements of the bead could no longer be traced on the gla.s.s.

Quercus (American sp.) (Cupuliferae).--Acorns of an American oak which had germinated at Kew were planted in a pot in the greenhouse. This transplantation checked their 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 circ.u.mstances and the nature of the plant, we hardly expected that it would circ.u.mnutate; 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.): circ.u.mnutation of young stem, traced on horizontal gla.s.s, 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 gla.s.s, 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 gla.s.s, 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 gla.s.s 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 gla.s.s; 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 gla.s.s and come into contact with it again. There can be no doubt that in the last four cases the radicle of the oak circ.u.mnutated whilst growing downwards.

Fig. 41. Quercus robur: tracks left on inclined smoked gla.s.s-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 gla.s.s filament was affixed to the basal leg. The whole was kept damp beneath a metal-box with a gla.s.s 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 gla.s.s; and [page 56]

this was in parts straight and in parts decidedly zigzag, indicating circ.u.mnutation.

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 circ.u.mnutating 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: circ.u.mnutation of a young shoot emitted from the epicotyl, the apex of which had been injured, traced on a horizontal gla.s.s, 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: circ.u.mnutation of hypocotyl, with filament fixed across its summit, traced on horizontal gla.s.s, 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 circ.u.mnutated. 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 gla.s.s 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: circ.u.mnutation of two opposite cotyledons, traced on horizontal gla.s.s 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 gla.s.s 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 circ.u.mnutate 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 2 inches in height, had its movements traced in a warm greenhouse by means of a gla.s.s filament bearing paper triangles attached across its tip. The tracing (Fig. 45) shows how large, complex, and rapid were the circ.u.m-

Fig. 45. Cycas pectinata: circ.u.mnutation of young leaf whilst emerging from the ground, feebly illuminated from above, traced on vertical gla.s.s, 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 pa.s.sed 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: circ.u.mnutation of plumule with filament affixed obliquely to outer sheath-like leaf, traced in darkness on horizontal gla.s.s 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 above the surface the apex remains within the seed-coats, evidently absorbing the still abundant contents. The summit or crown of the arch, when it first protrudes from the seed and is still buried beneath the ground, is simply rounded; but before it reaches the surface it is developed into a conical protuberance of a white colour (owing to the absence of chlorophyll), whilst the adjoining parts are green, with the epidermis apparently rather thicker and tougher than elsewhere. We may therefore conclude that this conical protuberance is a special adaptation for breaking through the ground,** and answers the same end as the knife-like white crest on the summit of the straight cotyledon of the Gramineae.

* This is the expression used by Sachs in his 'Text-book of Botany.'

** Haberlandt has briefly described ('Die Schutzeinrichtungen...Keimpflanze,' 1877, p. 77) this curious structure and the purpose which it subserves. He states that good figures of the cotyledon of the onion have been given by t.i.ttmann and by Sachs in his 'Experimental Physiologie,' p. 93.

[page 60]

After a time the apex is drawn out of the empty seed-coats, and rises up, forming a right angle, or more commonly a still larger angle with the lower part, and occasionally the whole becomes nearly straight. The conical protuberance, which originally formed the crown of the arch, is now seated on one side, and appears like a joint or knee, which from acquiring chlorophyll becomes green, and increases in size. In rarely or never becoming perfectly straight, these cotyledons differ remarkably from the ultimate condition of the arched hypocotyls or epicotyls of dicotyledons.

It is, also, a singular circ.u.mstance that the attenuated extremity of the upper bent portion invariably withers and dies.

A filament, 1.7 inch in length, was affixed nearly upright beneath the knee to the basal and vertical portion of a cotyledon; and its movements were traced during 14 h. in the usual manner. The tracing here given (Fig. 47) indicates circ.u.mnutation. The movement of the upper part above the knee of the same cotyledon, which projected at about an angle of 45o above the horizon, was observed at the same time. A filament was not affixed to it, but a mark was placed beneath the apex, which was almost white from beginning to wither, and its movements were thus traced. The figure described resembled pretty closely that above given; and this shows that the chief seat of movement is in the lower or basal part of the cotyledon.

Fig. 47. Allium cepa: circ.u.mnutation of basal half of arched cotyledon, traced in darkness on horizontal gla.s.s, from 8.15 A.M. to 10 P.M. Oct.

31st. Movement of bead magnified about 17 times.

Asparagus officinalis (Asparageae).--The tip of a straight plumule or cotyledon (for we do not know which it should be called) was found at a depth of .1 inch beneath the surface, and the earth was then removed all round to the dept of .3 inch. a gla.s.s filament was affixed obliquely to it, and the movement of the bead, magnified 17 times, was traced in darkness.

During the first 1 h. 15 m. the plumule moved to the right, and during the next two hours it returned in a roughly parallel but strongly zigzag course. From some unknown cause it had grown up through the soil in an inclined direction, and now through apogeotropism it moved during nearly 24 h. in [page 61]

the same general direction, but in a slightly zigzag manner, until it became upright. On the following morning it changed its course completely.

There can therefore hardly be a doubt that the plumule circ.u.mnutates, whilst buried beneath the ground, as much as the pressure of the surrounding earth will permit. The surface of the soil in the pot was now covered with a thin layer of very fine argillaceous sand, which was kept damp; and after the tapering seedlings had grown a few tenths of an inch in height, each was found surrounded by a little open s.p.a.ce or circular crack; and this could be accounted for only by their having circ.u.mnutated and thus pushed away the sand on all sides; for there was no vestige of a crack in any other part.

In order to prove that there was circ.u.mnutation, the move-

Fig. 48. Asparagus officinalis: circ.u.mnutation of plumules with tips whitened and marks placed beneath, traced on a horizontal gla.s.s. A, young plumule; movement traced from 8.30 A.M. Nov. 30th to 7.15 A.M. next morning; magnified about 35 times. B, older plumule; movement traced from 10.15 A.M. to 8.10 P.M. Nov. 29th; magnified 9 times, but here reduced to one-half of original scale.

ments of five seedlings, varying in height from .3 inch to 2 inches, were traced. They were placed within a box and illuminated from above; but in all five cases the longer axes of the figures described were directed to nearly the same point; so that more light seemed to have come through the gla.s.s roof of the greenhouse on one side than on any other. All five tracings resembled each other to a certain extent, and it will suffice to give two of them. In A (Fig. 48) the seedling was only .45 of an [page 62]

inch in height, and consisted of a single internode bearing a bud on its summit. The apex described between 8.30 A.M. and 10.20 P.M. (i.e. during nearly 14 hours) a figure which would probably have consisted of 3 ellipses, had not the stem been drawn to one side until 1 P.M., after which hour it moved backwards. On the following morning it was not far distant from the point whence it had first started. The actual amount of movement of the apex from side to side was very small, viz. about 1/18th of an inch.

The seedling of which the movements are shown in Fig. 48, B, was 1 3/4 inch in height, and consisted of three internodes besides the bud on the summit.