The Power of Movement in Plants Part 3

Cucurbita ovifera (Cucurbitaceae).--Radicle: a seed which had

Fig. 26. Cucurbita ovifera: course followed by a radicle in bending geotropically downwards, traced on a horizontal gla.s.s, between 11.25 A.M.

and 10.25 P.M.; the direction during the night is indicated by the broken line. Movement of bead magnified 14 times.

germinated on damp sand was fixed so that the slightly curved radicle, which was only .07 inch in length, stood almost vertically [page 39]

upwards, in which position geotropism would act at first with little power.

A filament was attached near to its base, and projected at about an angle of 45o above the horizon. The general course followed during the 11 hours of observation and during the following night is shown in the accompanying diagram (Fig. 26), and was plainly due to geotropism; but it was also clear that the radicle circ.u.mnutated. By the next morning the tip had curved so much downwards that the filament, instead of projecting at 45o above the horizon, was nearly horizontal. Another germinating seed was turned upside down and covered with damp sand; and a filament was fastened to the radicle so as to project at an angle of about 50o above the horizon; this radicle was .35 of an inch in length and a little curved. The course pursued was mainly governed, as in the last case, by geotropism, but the line traced during 12 hours and magnified as before was more strongly zigzag, again showing circ.u.mnutation.

Four radicles were allowed to grow downwards over plates of smoked gla.s.s, inclined at 70o to the horizon, under the

Fig. 27. Cucurbita ovifera: tracks left by tips of radicles in growing downwards over smoked gla.s.s-plates, inclined at 70o to the horizon.

Fig. 28. Cucurbita ovifera: circ.u.mnutation of arched hypocotyl at a very early age, traced in darkness on a horizontal gla.s.s, from 8 A.M. to 10.20 A.M. on the following day. The movement of the bead magnified 20 times, here reduced to one-half of original scale.

same conditions as in the cases of Aesculus, Phaseolus, and Vicia.

Facsimiles are here given (Fig. 27) of two of these tracks; and a third short one was almost as plainly serpentine as that at A. It was also manifest by a greater or less amount of soot having been swept off the gla.s.ses, that the tips had [page 40]

pressed alternately with greater and less force on them. There must, therefore, have been movement in at least two planes at right angles to one another. These radicles were so delicate that they rarely had the power to sweep the gla.s.ses quite clean. One of them had developed some lateral or secondary rootlets, which projected a few degrees beneath the horizon; and it is an important fact that three of them left distinctly serpentine tracks on the smoked surface, showing beyond doubt that they had circ.u.mnutated like the main or primary radicle. But the tracks were so slight that they could not be traced and copied after the smoked surface had been varnished.

Fig. 29. Cucurbita ovifera: circ.u.mnutation of straight and vertical hypocotyl, with filament fastened transversely across its upper end, traced in darkness on a horizontal gla.s.s, from 8.30 A.M. to 8.30 P.M. The movement of the terminal bead originally magnified about 18 times, here only 4 times.

Hypocotyl.--A seed lying on damp sand was firmly fixed by two crossed wires and by its own growing radicle. The cotyledons were still enclosed within the seed-coats; and the short hypocotyl, between the summit of the radicle and the cotyledons, was as yet only slightly arched. A filament (.85 of inch in length) was attached at an angle of 35o above the horizon to the side of the arch adjoining the cotyledons. This part would ultimately form the upper end of the hypocotyl, after it had grown straight and vertical.

Had the seed been properly planted, the hypocotyl at this stage of growth would have been deeply buried beneath the surface. The course followed by the bead of the filament is shown in Fig. 28. The chief lines of movement from left to right in the figure were parallel to the plane of the two united cotyledons and of the flattened seed; and this movement would aid in dragging them out of the seed-coats, which are held down by a special structure hereafter to be described. The movement at right angles to the above lines was due to the arched hypocotyl becoming more arched as it increased in height. The foregoing observations apply to the leg of the arch next to the cotyledons, but [page 41]

the other leg adjoining the radicle likewise circ.u.mnutated at an equally early age.

The movement of the same hypocotyl after it had become straight and vertical, but with the cotyledons only partially expanded, is shown in Fig.

29. The course pursued during 12 h. apparently represents four and a half ellipses or ovals, with the longer axis of the first at nearly right angles to that of the others. The longer axes of all were oblique to a line joining the opposite cotyledons. The actual extreme distance from side to side over which the summit of the tall hypocotyl pa.s.sed in the course of 12 h. was .28 of an inch. The original figure was traced on a large scale, and from the obliquity of the line of view the outer parts of the diagram are much exaggerated.

Cotyledons.--On two occasions the movements of the cotyledons were traced on a vertical gla.s.s, and as the ascending and descending lines did not quite coincide, very narrow ellipses were formed; they therefore circ.u.mnutated. Whilst young they rise vertically up at night, but their tips always remain reflexed; on the following morning they sink down again.

With a seedling kept in complete darkness they moved in the same manner, for they sank from 8.45 A.M. to 4.30 P.M.; they then began to rise and remained close together until 10 P.M., when they were last observed. At 7 A.M. on the following morning they were as much expanded as at any hour on the previous day. The cotyledons of another young seedling, exposed to the light, were fully open for the first time on a certain day, but were found completely closed at 7 A.M. on the following morning. They soon began to expand again, and continued doing so till about 5 P.M.; they then began to rise, and by 10.30 P.M. stood vertically and were almost closed. At 7 A.M.

on the third morning they were nearly vertical, and again expanded during the day; on the fourth morning they were not closed, yet they opened a little in the course of the day and rose a little on the following night.

By this time a minute true leaf had become developed. Another seedling, still older, bearing a well-developed leaf, had a sharp rigid filament affixed to one of its cotyledons (85 mm. in length), which recorded its own movements on a revolving drum with smoked paper. The observations were made in the hot-house, where the plant had lived, so that there was no change in temperature or light. The record commenced at 11 A.M. on February 18th; and from this hour till 3 P.M. the [page 42]

cotyledon fell; it then rose rapidly till 9 P.M., then very gradually till 3 A.M. February 19th, after which hour it sank gradually till 4.30 P.M.; but the downward movement was interrupted by one slight rise or oscillation about 1.30 P.M. After 4.30 P.M. (19th) the cotyledon rose till 1 A.M. (in the night of February 20th) and then sank very gradually till 9.30 A.M., when our observations ceased. The amount of movement was greater on the 18th than on the 19th or on the morning of the 20th.

Cucurbita aurantia.--An arched hypocotyl was found buried a little beneath the surface of the soil; and in order to prevent it straightening itself quickly, when relieved from the surrounding pressure of the soil, the two legs of the arch were tied together. The seed was then lightly covered with loose damp earth. A filament with a bead at the end was affixed to the basal leg, the movements of which were observed during two days in the usual manner. On the first day the arch moved in a zigzag line towards the side of the basal leg. On the next day, by which time the dependent cotyledons had been dragged above the surface of the soil, the tied arch changed its course greatly nine times in the course of 14 h. It swept a large, extremely irregular, circular figure, returning at night to nearly the same spot whence it had started early in the morning. The line was so strongly zigzag that it apparently represented five ellipses, with their longer axes pointing in various directions. With respect to the periodical movements of the cotyledons, those of several young seedlings formed together at 4 P.M. an angle of about 60o, and at 10 P.M. their lower parts stood vertically and were in contact; their tips, however, as is usual in the genus, were permanently reflexed. These cotyledons, at 7 A.M. on the following morning, were again well expanded.

Lagenaria vulgaris (var. miniature Bottle-gourd) (Cucurbitaceae).--A seedling opened its cotyledons, the movements of which were alone observed, slightly on June 27th and closed them at night: next day, at noon (28th), they included an angle of 53o, and at 10 P.M. they were in close contact, so that each had risen 26 1/2o. At noon, on the 29th, they included an angle of 118o, and at 10 P.M. an angle of 54o, so each had risen 32o. On the following day they were still more open, and the nocturnal rise was greater, but the angles were not measured. Two other seedlings were observed, and behaved during three days in a closely similar manner. The cotyledons, therefore, [page 43]

open more and more on each succeeding day, and rise each night about 30o; consequently during the first two nights of their life they stand vertically and come into contact.

Fig. 30. Lagenaria vulgaris: circ.u.mnutation of a cotyledon, 1 inch in length, apex only 4 3/4 inches from the vertical gla.s.s, on which its movements were traced from 7.35 A.M. July 11th to 9.5 A.M. on the 14th.

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

In order to ascertain more accurately the nature of these movements, the hypocotyl of a seedling, with its cotyledons well expanded, was secured to a little stick, and a filament with triangles of paper was affixed to one of the cotyledons. The observations were made under a rather dim skylight, and the temperature during the whole time was between 17 1/2o to 18o C. (63o to 65o F.). Had the temperature been higher and the light brighter, the movements would probably have been greater. On July 11th (see Fig. 30), the cotyledon fell from 7.35 A.M. till 10 A.M.; it then rose (rapidly after 4 P.M.) till it stood quite vertically at 8.40 P.M. During the early morning of the next day (12th) it fell, and continued to fall till 8 A.M., after which hour it rose, then fell, and again rose, so that by 10.35 P.M. it stood much higher than it did in the morning, but was not vertical as on the preceding night. During the following early morning and whole day (13th) it fell and circ.u.mnutated, but had not risen when observed late in the evening; and this was probably due to the deficiency of heat or light, or of both. We thus see that the cotyledons became more widely open at noon on each succeeding day; and that they rose considerably each night, though not acquiring a vertical position, except during the first two nights.

Cuc.u.mis dudaim (Cucurbitaceae).--Two seedlings had opened [page 44]

their cotyledons for the first time during the day,--one to the extent of 90o and the other rather more; they remained in nearly the same position until 10.40 P.M.; but by 7 A.M. on the following morning the one which had been previously open to the extent of 90o had its cotyledons vertical and completely shut; the other seedling had them nearly shut. Later in the morning they opened in the ordinary manner. It appears therefore that the cotyledons of this plant close and open at somewhat different periods from those of the foregoing species of the allied genera of Cucurbita and Lagenaria.

Fig. 31. Opuntia basilaris: conjoint circ.u.mnutation of hypocotyl and cotyledon; filament fixed longitudinally to cotyledon, and movement traced during 66 h. on horizontal gla.s.s. Movement of the terminal bead magnified about 30 times, here reduced to one-third scale. Seedling kept in hot-house, feebly illuminated from above.

Opuntia basilaris (Cacteae).--A seedling was carefully observed, because, considering its appearance and the nature of the mature plant, it seemed very unlikely that either the hypocotyl or cotyledons would circ.u.mnutate to an appreciable extent. The cotyledons were well developed, being .9 of an inch in length, .22 in breadth, and .15 in thickness. The almost cylindrical hypocotyl, now bearing a minute spinous bud on its summit, was only .45 of an inch in height, and .19 in diameter. The tracing (Fig. 31) shows the combined movement of the hypocotyl and of one of the cotyledons, from 4.45 P.M. on May 28th to 11 A.M. on the 31st. On the 29th a nearly perfect ellipse was completed. On the 30th the hypocotyl moved, from some unknown cause, in the same general direction in a zigzag line; but between 4.30 and 10 P.M. almost completed a second small ellipse. The cotyledons move only a little up and down: thus at 10.15 P.M. they stood only 10o higher than at noon. The chief seat of movement therefore, at least when the cotyledons are rather old as in the present case, lies in the hypocotyl. The ellipse described on the 29th had its longer axis directed at nearly right angles to a line joining the two cotyledons. The actual amount of movement of the bead at the end of the [page 45]

filament was, as far as could be ascertained, about .14 of an inch.

Fig. 32. Helianthus annuus: circ.u.mnutation of hypocotyl, with filament fixed across its summit, traced on a horizontal gla.s.s in darkness, from 8.45 A.M. to 10.45 P.M., and for an hour on following morning. Movement of bead magnified 21 times, here reduced to one-half of original scale.

Helianthus annuus (Compositae).--The upper part of the hypocotyl moved during the day-time in the course shown in the annexed figure (Fig. 32). As the line runs in various directions, crossing itself several times, the movement may be considered as one of circ.u.mnutation. The extreme actual distance travelled was at least .1 of an inch. The movements of the cotyledons of two seedlings were observed; one facing a north-east window, and the other so feebly illuminated from above us as to be almost in darkness. They continued to sink till about noon, when they began to rise; but between 5 and 7 or 8 P.M. they either sank a little, or moved laterally, and then again began to rise. At 7 A.M. on the following morning those on the plant before the north-east window had opened so little that they stood at an angle of 73o above the horizon, and were not observed any longer. Those on the seedling which had been kept in almost complete darkness, sank during the whole day, without rising about mid-day, but rose during the night. On the third and fourth days they continued sinking without any alternate ascending movement; and this, no doubt, was due to the absence of light.

Primula Sinensis (Primulaceae).--A seedling was placed with the two cotyledons parallel to a north-east window on a day when the light was nearly uniform, and a filament was affixed to one of them. From observations subsequently made on another seedling with the stem secured to a stick, the greater part of the movement shown in the annexed figure (Fig.

33), must have been that of the hypocotyl, though the cotyledons certainly move up and down to a certain extent both during the day and night. The movements of the same seedling were traced [page 46]

on the following day with nearly the same result; and there can be no doubt about the circ.u.mnutation of the hypocotyl.

Fig. 33. Primula Sinensis: conjoint circ.u.mnutation of hypocotyl and cotyledon, traced on vertical gla.s.s, from 8.40 A.M. to 10.45 P.M. Movements of bead magnified about 26 times.

Cyclamen Persic.u.m (Primulaceae).--This plant is generally supposed to produce only a single cotyledon, but Dr. H. Gressner* has shown that a second one is developed after a long interval of time. The hypocotyl is converted into a globular corm, even before the first cotyledon has broken through the ground with its blade closely enfolded and with its petiole in the form of an arch, like the arched hypocotyl or epicotyl of any ordinary dicotyledonous plant. A gla.s.s filament was affixed to a cotyledon, .55 of an inch in height, the petiole of which had straightened itself and stood nearly vertical, but with the blade not as yet fully expanded. Its movements were traced during 24 h. on a horizontal gla.s.s, magnified 50 times; and in this interval it described two irregular small circles; it therefore circ.u.mnutates, though on an extremely small scale.

Fig. 34. Stapelia sarpedon: circ.u.mnutation of hypocotyl, illuminated from above, traced on horizontal gla.s.s, from 6.45 A.M. June 26th to 8.45 A.M.

28th. Temp. 23-24o C. Movement of bead magnified 21 times.

Stapelia sarpedon (Asclepiadeae).--This plant, when mature, resembles a cactus. The flattened hypocotyl is fleshy, enlarged in the upper part, and bears two rudimentary cotyledons. It breaks through the ground in an arched form, with the rudimentary cotyledons closed or in contact. A filament was affixed almost

* 'Bot. Zeitung,' 1874, p. 837.

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vertically to the hypocotyl of a seedling half an inch high; and its movements were traced during 50 h. on a horizontal gla.s.s (Fig. 34). From some unknown cause it bowed itself to one side, and as this was effected by a zigzag course, it probably circ.u.mnutated; but with hardly any other seedling observed by us was this movement so obscurely shown.

Ipomoea caerulea vel Pharbitis nil (Convolvulaceae).--Seedlings of this plant were observed because it is a twiner, the upper internodes of which circ.u.mnutate conspicuously; but like other twining plants, the first few internodes which rise above the ground are stiff enough to support themselves, and therefore do not circ.u.mnutate in any plainly recognisable manner.* In this particular instance the fifth internode (including the hypocotyl) was the first which plainly circ.u.mnutated and twined round a stick. We therefore wished to learn whether circ.u.mnutation could be observed in the hypocotyl if carefully observed in our usual manner. Two seedlings were kept in the dark with filaments fixed to the upper part of their hypocotyls; but from circ.u.mstances not worth explaining their movements were traced for only a short time. One moved thrice forwards and twice backwards in nearly opposite directions, in the course of 3 h. 15 m.; and the other twice forwards and twice backwards in 2 h. 22 m. The hypocotyl therefore circ.u.mnutated at a remarkably rapid rate. It may here be added that a filament was affixed transversely to the summit of the second internode above the cotyledons of a little plant 3 inches in height; and its movements were traced on a horizontal gla.s.s. It circ.u.mnutated, and the actual distance travelled from side to side was a quarter of an inch, which was too small an amount to be perceived without the aid of marks.

The movements of the cotyledons are interesting from their complexity and rapidity, and in some other respects. The hypocotyl (2 inches high) of a vigorous seedling was secured to a stick, and a filament with triangles of paper was affixed to one of the cotyledons. The plant was kept all day in the hot-house, and at 4.20 P.M. (June 20th) was placed under a skylight in the house, and observed occasionally during the evening and night. It fell in a slightly zigzag line to a moderate extent from 4.20 P.M. till 10.15 P.M. When looked at shortly after midnight (12.30 P.M.) it had risen a very little, and considerably by

* 'Movements and Habits of Climbing Plants,' p. 33, 1875.

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3.45 A.M. When again looked at, at 6.10 A.M. (21st), it had fallen largely.

A new tracing was now begun (see Fig. 35), and soon afterwards, at 6.42 A.M., the cotyledon had risen a little. During the forenoon it was observed about every hour; but between 12.30 and 6 P.M. every half-hour. If the observations had been made at these short intervals during the whole day, the figure would have been too intricate to have been copied. As it was, the cotyledon moved up and down in the course of 16 h. 20 m. (i.e. between 6.10 A.M. and 10.30 P.M.) thirteen times.

Fig 35. Ipomoea caerulea: circ.u.mnutation of cotyledon, traced on vertical gla.s.s, from 6.10 A.M. June 21st to 6.45 A.M. 22nd. Cotyledon with petiole 1.6 inch in length, apex of blade 4.1 inch from the vertical gla.s.s; so movement not greatly magnified; temp. 20o C.

The cotyledons of this seedling sank downwards during both evenings and the early part of the night, but rose during the latter part. As this is an unusual movement, the cotyledons of twelve other seedlings were observed; they stood almost or quite horizontally at mid-day, and at 10 P.M. were all declined at various angles. The most usual angle was between 30o and 35o; but three stood at about 50o and one at even 70o beneath the horizon. The blades of all these cotyledons had attained almost their full size, viz.

from 1 to 1 inches in length, measured along their midribs. It is a remarkable fact that whilst young--that is, when less than half an inch in length, measured in the same manner--they do not sink [page 49]

downwards in the evening. Therefore their weight, which is considerable when almost fully developed, probably came into play in originally determining the downward movement. The periodicity of this movement is much influenced by the degree of light to which the seedlings have been exposed during the day; for three kept in an obscure place began to sink about noon, instead of late in the evening; and those of another seedling were almost paralysed by having been similarly kept during two whole days. The cotyledons of several other species of Ipomoea likewise sink downwards late in the evening.

Cerinthe major (Boragineae).--The circ.u.mnutation of the hypocotyl of a young seedling with the cotyledons hardly

Fig. 36. Cerinthe major: circ.u.mnutation of hypocotyl, with filament fixed across its summit, illuminated from above, traced on horizontal gla.s.s, from 9.26 A.M. to 9.53 P.M. on Oct. 25th. Movement of the bead magnified 30 times, here reduced to one-third of original scale.

expanded, is shown in the annexed figure (Fig. 36), which apparently represents four or five irregular ellipses, described in the course of a little over 12 hours. Two older seedlings were similarly observed, excepting that one of them was kept in the dark; their hypocotyls also circ.u.mnutated, but in a more simple manner. The cotyledons on a seedling exposed to the light fell from the early morning until a little after noon, and then continued to rise until 10.30 P.M. or later. The cotyledons of this same seedling acted in the same general manner during the two following days. It had previously been tried in the dark, and after being thus kept for only 1 h. 40 m. the cotyledons began at 4.30 P.M. to sink, instead of continuing to rise till late at night.

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Nolana prostrata (Nolaneae).--The movements were not traced, but a pot with seedlings, which had been kept in the dark for an hour, was placed under the microscope, with the micrometer eye-piece so adjusted that each division equalled 1/500th of an inch. The apex of one of the cotyledons crossed rather obliquely four divisions in 13 minutes; it was also sinking, as shown by getting out of focus. The seedlings were again placed in darkness for another hour, and the apex now crossed two divisions in 6 m.

18 s.; that is, at very nearly the same rate as before. After another interval of an hour in darkness, it crossed two divisions in 4 m. 15 s., therefore at a quicker rate. In the afternoon, after a longer interval in the dark, the apex was motionless, but after a time it recommenced moving, though slowly; perhaps the room was too cold. Judging from previous cases, there can hardly be a doubt that this seedling was circ.u.mnutating.