The Power of Movement in Plants Part 17

We see from these three cases that stolons or runners circ.u.mnutate in a very complex manner. The lines generally extend in a vertical plane, and this may probably be attributed to the effect of the weight of the unsupported end of the stolon; but there is always some, and occasionally a considerable, amount of lateral movement. The circ.u.mnutation is so great in amplitude that it may almost be compared with that of climbing plants. That the stolons are thus aided in pa.s.sing over obstacles and in winding between the stems of the surrounding plants, the observations above given render almost certain. If they had not circ.u.mnutated, their tips would have been liable to have been doubled up, as often as they met with obstacles in their path; but as it is, they easily avoid them. This must be a considerable advantage to the plant in spreading from its parent-stock; but we are far from supposing that the power has been gained by the stolons for this purpose, for circ.u.mnutation seems to be of universal occurrence with all growing parts; but it is not improbable that the amplitude of the movement may have been specially increased for this purpose.

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CIRc.u.mNUTATION OF FLOWER-STEMS.

We did not think it necessary to make any special observations on the circ.u.mnutation of flower-stems, these being axial in their nature, like stems or stolons; but some were incidentally made whilst attending to other subjects, and these we will here briefly give. A few observations have also been made by other botanists. These taken together suffice to render it probable that all peduncles and sub-peduncles circ.u.mnutate whilst growing.

[Oxalis carnosa.--The peduncle which springs from the thick and woody stem of this plant bears three or four sub-peduncles.

Fig. 91. Oxalis carnosa: flower-stem, feebly illuminated from above, its circ.u.mnutation traced from 9 A.M. April 13th to 9 A.M. 15th. Summit of flower 8 inches beneath the horizontal gla.s.s. Movement probably magnified about 6 times.

A filament with little triangles of paper was fixed within the calyx of a flower which stood upright. Its movements were observed for 48 h.; during the first half of this time the flower was fully expanded, and during the second half withered. The figure here given (Fig. 91) represents 8 or 9 ellipses. Although the main peduncle circ.u.mnutated, and described one large and [page 224]

two smaller ellipses in the course of 24 h., yet the chief seat of movement lies in the sub-peduncles, which ultimately bend vertically downwards, as will be described in a future chapter. The peduncles of Oxalis acetosella likewise bend downwards, and afterwards, when the pods are nearly mature, upwards; and this is effected by a circ.u.mnutating movement.

It may be seen in the above figure that the flower-stem of O. carnosa circ.u.mnutated during two days about the same spot. On the other hand, the flower-stem of O. sensitiva undergoes a strongly marked, daily, periodical change of position, when kept at a proper temperature. In the middle of the day it stands vertically up, or at a high angle; in the afternoon it sinks, and in the evening projects horizontally, or almost horizontally, rising again during the night. This movement continues from the period when the flowers are in bud to when, as we believe, the pods are mature: and it ought perhaps to have been included amongst the so-called sleep-movements of plants. A tracing was not made, but the angles were measured at successive periods during one whole day; and these showed that the movement was not continuous, but that the peduncle oscillated up and down. We may therefore conclude that it circ.u.mnutated. At the base of the peduncle there is a ma.s.s of small cells, forming a well-developed pulvinus, which is exteriorly coloured purple and hairy. In no other genus, as far as we know, is the peduncle furnished with a pulvinus. The peduncle of O. Ortegesii behaved differently from that of O. sensitiva, for it stood at a less angle above the horizon in the middle of the day, then in the morning or evening.

By 10.20 P.M. it had risen greatly. During the middle of the day it oscillated much up and down.

Trifolium subterraneum.--A filament was fixed vertically to the uppermost part of the peduncle of a young and upright flower-head (the stem of the plant having been secured to a stick); and its movements were traced during 36 h. Within this time it described (see Fig. 92) a figure which represents four ellipses; but during the latter part of the time the peduncle began to bend downwards, and after 10.30 P.M. on the 24th it curved so rapidly down, that by 6.45 A.M. on the 25th it stood only 19o above the horizon. It went on circ.u.mnutating in nearly the same position for two days. Even after the flower-heads have buried themselves in the ground they continue, as will hereafter be shown, to circ.u.mnutate. It will also be seen in the next chapter that the sub-peduncles of the separate flowers of [page 225]

Trifolium repens circ.u.mnutate in a complicated course during several days.

I may add that the gynoph.o.r.e of Arachis hypogoea,

Fig. 92. Trifolium subterraneum: main flower-peduncle, illuminated from above, circ.u.mnutation traced on horizontal gla.s.s, from 8.40 A.M. July 23rd to 10.30 P.M. 24th.

which looks exactly like a peduncle, circ.u.mnutates whilst growing vertically downwards, in order to bury the young pod in the ground.

The movements of the flowers of Cyclamen Persic.u.m were not observed; but the peduncle, whilst the pod is forming, increases much in length, and bows itself down by a circ.u.mnutating movement. A young peduncle of Maurandia semperflorens, 1 inch in length, was carefully observed during a whole day, and it made 4 narrow, vertical, irregular and short ellipses, each at an average rate of about 2 h. 25 m. An adjoining peduncle described during the same time similar, though fewer, ellipses.* According to Sachs**

the flower-stems, whilst growing,

* 'The Movements and Habits of Climbing Plants,' 2nd edit., 1875, p. 68.

** 'Text-Book of Botany,' 1875, [[page 226]]

p. 766. Linnaeus and Trevira.n.u.s (according to Pfeffer, 'Die Periodischen Bewegungen,' etc., p. 162) state that the flower-stalks of many plants occupy different positions by night and day, and we shall see in the chapter on the Sleep of Plants that this implies circ.u.mnutation.

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of many plants, for instance, those of Bra.s.sica napus, revolve or circ.u.mnutate; those of Allium porrum bend from side to side, and, if this movement had been traced on a horizontal gla.s.s, no doubt ellipses would have been formed. Fritz Muller has described* the spontaneous revolving movements of the flower-stems of an Alisma, which he compares with those of a climbing plant.

We made no observations on the movements of the different parts of flowers.

Morren, however, has observed** in the stamens of Sparmannia and Cereus a "fremiss.e.m.e.nt spontane," which, it may be suspected, is a circ.u.mnutating movement. The circ.u.mnutation of the gynostemium of Stylidium, as described by Gad,*** is highly remarkable, and apparently aids in the fertilisation of the flowers. The gynostemium, whilst spontaneously moving, comes into contact with the viscid labellum, to which it adheres, until freed by the increasing tension of the parts or by being touched.]

We have now seen that the flower-stems of plants belonging to such widely different families as the Cruciferae, Oxalidae, Leguminosae, Primulaceae, Scrophularineae, Alismaceae, and Liliaceae, circ.u.mnutate; and that there are indications of this movement in many other families. With these facts before us, bearing also in mind that the tendrils of not a few plants consist of modified peduncles, we may admit without much doubt that all growing flower-stems circ.u.mnutate.

CIRc.u.mNUTATION OF LEAVES: DICOTYLEDONS.

Several distinguished botanists, Hofmeister, Sachs, Pfeffer, De Vries, Batalin, Millardet, etc., have ob-

* 'Jenaische Zeitsch.,' B. v. p. 133.

** 'N. Mem. de l'Acad. R. de Bruxelles,' tom. xiv. 1841, p. 3.

*** 'Sitzungbericht des bot. Vereins der P. Brandenburg,' xxi. p. 84.

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served, and some of them with the greatest care, the periodical movements of leaves; but their attention has been chiefly, though not exclusively, directed to those which move largely and are commonly said to sleep at night. From considerations hereafter to be given, plants of this nature are here excluded, and will be treated of separately. As we wished to ascertain whether all young and growing leaves circ.u.mnutated, we thought that it would be sufficient if we observed between 30 and 40 genera, widely distributed throughout the vegetable series, selecting some unusual forms and others on woody plants. All the plants were healthy and grew in pots.

They were illuminated from above, but the light perhaps was not always sufficiently bright, as many of them were observed under a skylight of ground-gla.s.s. Except in a few specified cases, a fine gla.s.s filament with two minute triangles of paper was fixed to the leaves, and their movements were traced on a vertical gla.s.s (when not stated to the contrary) in the manner already described. I may repeat that the broken lines represent the nocturnal course. The stem was always secured to a stick, close to the base of the leaf under observation. The arrangement of the species, with the number of the Family appended, is the same as in the case of stems.

Fig. 93. Sarracenia purpurea: circ.u.mnutation of young pitcher, traced from 8 A.M. July 3rd to 10.15 A.M. 4th. Temp. 17o - 18o C. Apex of pitcher 20 inches from gla.s.s, so movement greatly magnified.

(1.) Sarracenia purpurea (Sarraceneae, Fam. 11).--A young leaf, or pitcher, 8 inches in height, with the bladder swollen but with the hood not as yet open, had a filament fixed transversely [page 228]

across its apex; it was observed for 48 h., and during the whole of this time it circ.u.mnutated in a nearly similar manner, but to a very small extent. The tracing given (Fig. 93) relates only to the movement during the first 26 h.

(2) Glaucium luteum (Papaveraceae, Fam. 12).--A young plant, bearing only 8 leaves, had a filament attached to the youngest leaf but one, which was 3 inches in length, including the petiole. The circ.u.mnutating movement was traced during 47 h. On both days the leaf descended from before 7 A.M.

until about 11 A.M., and then ascended slightly during the rest of the day and the early part of the night. During the latter part of the night it fell greatly. It did not ascend so much during the second as during the first day, and it descended considerably lower on the second night than on the first. This difference was probably due to the illumination from above having been insufficient during the two days of observation. Its course during the two days is shown in Fig. 94.

Fig. 94. Glaucium luteum: circ.u.mnutation of young leaf, traced from 9.30 A.M. June 14th to 8.30 A.M. 16th. Tracing not much magnified, as apex of leaf stood only 5 inches from the gla.s.s.

(3.) Crambe maritima (Cruciferae, Fam. 14).--A leaf 9 inches in length on a plant not growing vigorously was first observed. Its apex was in constant movement, but this could hardly be traced, from being so small in extent.

The apex, however, certainly changed its course at least 6 times in the course of 14 h. A more vigorous young plant, bearing only 4 leaves, was then selected, and a filament was affixed to the midrib of the third leaf from the base, which, with the petiole, was 5 inches in length. The leaf stood up almost vertically, but the tip [page 229]

was deflected, so that the filament projected almost horizontally, and its movements were traced during 48 h. on a vertical gla.s.s as shown in the accompanying figure (Fig. 95). We here plainly see that the leaf was continually circ.u.mnutating; but the proper periodicity of its movements was disturbed by its being only dimly illuminated from above through a double skylight. We infer that this was the case, because two leaves on plants growing out of doors, had their angles above the horizon measured in the middle of the day and at 9 to about 10 P.M. on successive nights, and they were found at this latter hour to have risen by an average angle of 9o above their mid-day position: on the following morning they fell to their former position. Now it may be observed in the diagram that the leaf rose during the second night, so that it stood at 6.40 A.M. higher than at 10.20 P.M. on the preceding night; and this may be attributed to the leaf adjusting itself to the dim light, coming exclusively from above.

Fig. 95. Crambe maritima: circ.u.mnutation of leaf, disturbed by being insufficiently illuminated from above, traced from 7.50 A.M. June 23rd to 8 A.M. 25th. Apex of leaf 15 1/4 inches from the vertical gla.s.s, so that the tracing was much magnified, but is here reduced to one-fourth of original scale.

(4.) Bra.s.sica oleracea (Cruciferae).--Hofmeister and Batalin* state that the leaves of the cabbage rise at night, and fall by day. We covered a young plant, bearing 8 leaves, under a large bell-gla.s.s, placing it in the same position with respect to the

* 'Flora,' 1873, p. 437.

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light in which it had long remained, and a filament was fixed at the distance of .4 of an inch from the apex of a young leaf nearly 4 inches in length. Its movements were then traced during three days, but the tracing is not worth giving. The leaf fell during the whole morning, and rose in the evening and during the early part of the night. The ascending and descending lines did not coincide, so that an irregular ellipse was formed each 24 h. The basal part of the midrib did not move, as was ascertained by measuring at successive periods the angle which it formed with the horizon, so that the movement was confined to the terminal portion of the leaf, which moved through an angle of 11o in the course of 24 h., and the distance travelled by the apex, up and down, was between .8 and .9 of an inch.

In order to ascertain the effect of darkness, a filament was fixed to a leaf 5 inches in length, borne by a plant which after forming a head had produced a stem. The leaf was inclined 44o above the horizon, and its movements were traced on a vertical gla.s.s every hour by the aid of a taper.

During the first day the leaf rose from 8 A.M. to 10.40 P.M. in a slightly zigzag course, the actual distance travelled by the apex being .67 of an inch. During the night the leaf fell, whereas it ought to have risen; and by 7 A.M. on the following morning it had fallen .23 of an inch, and it continued falling until 9.40 A.M. It then rose until 10.50 P.M., but the rise was interrupted by one considerable oscillation, that is, by a fall and re-ascent. During the second night it again fell, but only to a very short distance, and on the following morning re-ascended to a very short distance. Thus the normal course of the leaf was greatly disturbed, or rather completely inverted, by the absence of light; and the movements were likewise greatly diminished in amplitude.

We may add that, according to Mr. A. Stephen Wilson,* the young leaves of the Swedish turnip, which is a hybrid between B. oleracea and rapa, draw together in the evening so much "that the horizontal breadth diminishes about 30 per cent. of the daylight breadth." Therefore the leaves must rise considerably at night.

(5.) Dianthus caryophyllus (Caryophylleae, Fam. 26).--The

* 'Trans. Bot. Soc. Edinburgh,' vol. xiii. p. 32. With respect to the origin of the Swedish turnip, see Darwin, 'Animals and Plants under Domestication,' 2nd edit. vol. i. p. 344.

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terminal shoot of a young plant, growing very vigorously, was selected for observation. The young leaves at first stand up vertically and close together, but they soon bend outwards and downwards, so as to become horizontal, and often at the same time a little to one side. A filament was fixed to the tip of a young leaf whilst still highly inclined, and the first dot was made on the vertical gla.s.s at 8.30 A.M. June 13th, but it curved downwards so quickly that by 6.40 A.M. on the following morning it stood only a little above the horizon. In Fig. 96

Fig. 96. Dianthus caryophyllus: circ.u.mnutation of young leaf, traced from 10.15 P.M. June 13th to 10.35 P.M. 16th. Apex of leaf stood, at the close of our observations, 8 3/4 inches from the vertical gla.s.s, so tracing not greatly magnified. The leaf was 5 1/4 inches long. Temp. 15 1/2o - 17 1/2o C.

the long, slightly zigzag line representing this rapid downward course, which was somewhat inclined to the left, is not given; but the figure shows the highly tortuous and zigzag course, together with some loops, pursued during the next 2 days. As the leaf continued to move all the time to the left, it is evident that the zigzag line represents many circ.u.mnutations.

(6.) Camellia j.a.ponica (Camelliaceae, Fam. 32).--A youngish leaf, which together with its petiole was 2 3/4 inches in length and which arose from a side branch on a tall bush, had a filament attached to its apex. This leaf sloped downwards at an angle of 40o beneath the horizon. As it was thick and rigid, and its [page 232]

petiole very short, much movement could not be expected. Nevertheless, the apex changed its course completely seven times in the course of 11 h., but moved to only a very small distance. On the next day the movement of the apex was traced during 26 h. 20 m. (as shown in Fig. 97), and was nearly of the same nature, but rather less complex. The movement seems to be periodical, for on both days the leaf circ.u.mnutated in the forenoon, fell in the afternoon (on the first day until between 3 and 4 P.M., and on the second day until 6 P.M.), and then rose, falling again during the night or early morning.

Fig. 97. Camellia j.a.ponica: circ.u.mnutation of leaf, traced from 6.40 A.M.

June 14th to 6.50 A.M. 15th. Apex of leaf 12 inches from the vertical gla.s.s, so figure considerably magnified. Temp. 16o - 16 1/2o C.