In the chapter on the Sleep of Plants we shall see that the leaves in several Malvaceous genera sink
Fig. 98. Pelargonium zonale: circ.u.mnutation and downward movement of young leaf, traced from 9.30 A.M. June 14th to 6.30 P.M. 16th. Apex of leaf 9 1.4 inches from the vertical gla.s.s, so figure moderately magnified. Temp. 15o - 16 1/2o C.
at night; and as they often do not then occupy a vertical position, especially if they have not been well illuminated during [page 233]
the day, it is doubtful whether some of these cases ought not to have been included in the present chapter.
(7.) Pelargonium zonale (Geraniaceae, Fam. 47).--A young leaf, 1 1/4 inch in breadth, with its petiole 1 inch long, borne on a young plant, was observed in the usual manner during 61 h.; and its course is shown in the preceding figure (Fig. 98). During the first day and night the leaf moved downwards, but circ.u.mnutated between 10 A.M. and 4.30 P.M. On the second day it sank and rose again, but between 10 A.M. and 6 P.M. it circ.u.mnutated on an extremely small scale. On the third day the circ.u.mnutation was more plainly marked.
(8.) Cissus discolor (Ampelideae, Fam. 67).--A leaf, not nearly full-grown, the third from the apex of a shoot on a cut-down plant, was observed during 31 h. 30 m. (see Fig. 99). The day was cold (15o - 16o C.), and if the plant had been observed in the hot-house, the circ.u.mnutation, though plain enough as it was, would probably have been far more conspicuous.
Fig. 99. Cissus discolor: circ.u.mnutation of leaf, traced from 10.35 A.M.
May 28th to 6 P.M. 29th. Apex of leaf 8 3/4 inches from the vertical gla.s.s.
(9.) Vicia faba (Leguminosae, Fam. 75).--A young leaf, 3.1 inches in length, measured from base of petiole to end of leaflets, had a filament affixed to the midrib of one of the two terminal leaflets, and its movements were traced during 51 h. The filament fell all morning (July 2nd) till 3 P.M., and then rose greatly till 10.35 P.M.; but the rise this day was so great, compared with that which subsequently occurred, that it was probably due in part to the plant being illuminated from above. The latter part of the course on July 2nd is alone given in the following figure (Fig. 100). On the next day (July 3rd) the leaf again fell in the morning, then circ.u.mnutated in a conspicuous manner, and rose till late at night; but the movement was not traced after 7.15 P.M., as by that time the filament pointed towards the upper edge of the gla.s.s. During the latter part of the night or early morning it again fell in the same manner as before.
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As the evening rise and the early morning fall were unusually large, the angle of the petiole above the horizon was measured at the two periods, and the leaf was found to have risen 19o
Fig. 100. Vicia faba: circ.u.mnutation of leaf, traced from 7.15 P.M. July 2nd to 10.15 A.M. 4th. Apex of the two terminal leaflets 7 1/4 inches from the vertical gla.s.s. Figure here reduced to two-thirds of original scale.
Temp. 17o - 18o C.
between 12.20 P.M. and 10.45 P.M., and to have fallen 23o 30 seconds between the latter hour and 10.20 A.M. on the following morning.
The main petiole was now secured to a stick close to the base [page 235]
of the two terminal leaflets, which were 1.4 inch in length; and the movements of one of them were traced during 48 h. (see Fig. 101). The course pursued is closely a.n.a.logous to that of the whole leaf. The zigzag line between 8.30 A.M. and 3.30 P.M. on the second day represents 5 very small ellipses, with their Fig 101. Vicia faba: circ.u.mnutation of one of the two terminal leaflets, the main petiole having been secured, traced from 10.40 A.M. July 4th to 10.30 A.M. 6th. Apex of leaflet 6 5/8 inches from the vertical gla.s.s.
Tracing here reduced to one-half of original scale. Temp. 16o - 18o C.
longer axes differently directed. From these observations it follows that both the whole leaf and the terminal leaflets undergo a well-marked daily periodical movement, rising in the evening and falling during the latter part of the night or early morning; whilst in the middle of the day they generally circ.u.mnutate round the same small s.p.a.ce.
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(10.) Acacia retinoides (Leguminosae).--The movement of a young phyllode, 2 3/8 inches in length, and inclined at a considerable angle above the horizon, was traced during 45 h. 30 m.; but in the figure here given (Fig.
102), its circ.u.mnutation is shown during only 21 h. 30 m. During part of this time (viz., 14 h. 30 m.) the phyllode described a figure representing 5 or 6 small ellipses. The actual amount of movement in a vertical direction was .3 inch. The phyllode rose considerably between 1.30 P.M. and 4 P.M., but there was no evidence on either day of a regular periodic movement.
Fig. 102. Acacia retinoides: circ.u.mnutation of a young phyllode, traced from 10.45 A.M. July 18th to 8.15 A.M. 19th. Apex of phyllode 9 inches from the vertical gla.s.s; temp. 16 1/2o - 17 1/2o C.
(11.) Lupinus speciosus (Leguminosae).--Plants were raised from seed purchased under this name. This is one of the species in this large genus, the leaves of which do not sleep at night. The petioles rise direct from the ground, and are from 5 to 7 inches in length. A filament was fixed to the midrib of one of the longer leaflets, and the movement of the whole leaf was traced, as shown in Fig. 103. In the course of 6 h. 30 m. the filament went four times up and three times down. A new tracing was then begun (not here given), and during 12 h. the leaf moved eight times up and seven times down; so that it described 7 ellipses in this time, and this is an extraordinary rate of movement. The summit of the petiole was then secured to a stick, and the separate leaflets were found to be continually circ.u.mnutating.
Fig. 103. Lupinus speciosus: circ.u.mnutation of leaf, traced on vertical gla.s.s, from 10.15 A.M. to 5.45 P.M.; i.e., during 6 h. 30 m.
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(12.) Echeveria stolonifera (Cra.s.sulaceae, Fam. 84).--The older leaves of this plant are so thick and fleshy, and the young ones so short and broad, that it seemed very improbable that any circ.u.mnutation could be detected. A filament was fixed to a young upwardly inclined leaf, .75 inch in length and .28 in breadth, which stood on the outside of a terminal rosette of leaves, produced by a plant growing very vigorously. Its movement was traced during 3 days, as here shown (Fig. 104). The course was chiefly in an upward direction, and this may be attributed to the elongation of the leaf through growth; but we see that the lines are strongly zigzag, and that occasionally there was distinct circ.u.mnutation, though on a very small scale.
Fig. 104. Echeveria stolonifera: circ.u.mnutation of leaf, traced from 8.20 A.M. June 25th to 8.45 A.M. 28th. Apex of leaf 12 1/4 inches from the gla.s.s, so that the movement was much magnified; temp. 23o - 24 1/2o C.
(13.) Bryophyllum (vel Calanchae) calycinum (Cra.s.sulaceae).--Duval-Jouve ("Bull. Soc. Bot. de France," Feb. 14th, 1868) measured the distance between the tips of the upper pair of leaves on this plant, with the result shown in the following Table. It should be noted that the measurements on Dec. 2nd were made on a different pair of leaves: --
8 A.M. 2 P.M. 7 P.M.
Nov. 16. . . . . . . . . . . . . . . . . . .15 mm.. . . . . .25 mm. . . ..
. . .(?) " 19. . . . . . . . . . . . . . . . . . .48 " . . . . . . . 60 ". .
. . . . . 48 mm.
Dec. 2. . . . . . . . . . . . . . . . . . .22 ". . . . . . . . 43 ". . .
. . . . .28 "
We see from this Table that the leaves stood considerably further apart at 2 P.M. than at either 8 A.M. or 7 P.M.; and this shows that they rise a little in the evening and fall or open in the forenoon.
(14.) Drosera rotundifolia (Droseraceae, Fam. 85).--The movements of a young leaf, having a long petiole but with its tentacles (or gland-bearing hairs) as yet unfolded, were traced during 47 h. 15 m. The figure (Fig.
105) shows that it circ.u.mnutated largely, chiefly in a vertical direction, making two ellipses each [page 238]
day. On both days the leaf began to descend after 12 or 1 o"clock, and continued to do so all night, though to a very unequal distance on the two occasions. We therefore thought that the movement was periodic; but on observing three other leaves during several successive days and nights, we found this to be an error; and the case is given merely as a caution. On the third morning the above leaf occupied almost exactly the same position as on the first morning; and the tentacles by this time had unfolded sufficiently to project at right angles to the blade or disc.
Fig. 105. Drosera rotundifolia: circ.u.mnutation of young leaf, with filament fixed to back of blade, traced from 9.15 A.M. June 7th to 8.30 A.M. June 9th. Figure here reduced to one-half original scale.
The leaves as they grow older generally sink more and more downwards. The movement of an oldish leaf, the glands of which were still secreting freely, was traced for 24 h., during which time it continued to sink a little in a slightly zigzag line. On the following morning, at 7 A.M., a drop of a solution of carbonate of ammonia (2 gr. to 1 oz. of water) was placed on the disc, and this blackened the glands and induced inflection of many of the tentacles. The weight of the drop caused the leaf at first to sink a little; but immediately afterwards it began to rise in a somewhat zigzag course, and continued to do so till 3 P.M. It then circ.u.mnutated about the same spot on a very small scale for 21 h.; and during the next 21 h. it sank in a zigzag line to nearly the same level which it had held when the ammonia was first administered. By this time the tentacles had re-expanded, and the glands had recovered their proper colour. We thus learn that an old leaf [page 239]
circ.u.mnutates on a small scale, at least whilst absorbing carbonate of ammonia; for it is probable that this absorption may stimulate growth and thus re-excite circ.u.mnutation. Whether the rising of the gla.s.s filament which was attached to the back of the leaf, resulted from its margin becoming slightly inflected (as generally occurs), or from the rising of the petiole, was not ascertained.
In order to learn whether the tentacles or gland-bearing hairs circ.u.mnutate, the back of a young leaf, with the innermost tentacles as yet incurved, was firmly cemented with sh.e.l.lac to a flat stick driven into compact damp argillaceous sand. The plant was placed under a microscope with the stage removed and with an eye-piece micrometer, of which each division equalled 1/500 of an inch. It should be stated that as the leaves grow older the tentacles of the exterior rows bend outwards and downwards, so as ultimately to become deflected considerably beneath the horizon. A tentacle in the second row from the margin was selected for observation, and was found to be moving outwards at a rate of 1/500 of an inch in 20 m., or 1/100 of inch in 1 h. 40 m.; but as it likewise moved from side to side to an extent of above 1/500 of inch, the movement was probably one of modified circ.u.mnutation. A tentacle on an old leaf was next observed in the same manner. In 15 m. after being placed under the microscope it had moved about 1/1000 of an inch. During the next 7 h. it was looked at repeatedly, and during this whole time it moved only another 1/1000 of an inch; and this small movement may have been due to the settling of the damp sand (on which the plant rested), though the sand had been firmly pressed down. We may therefore conclude that the tentacles when old do not circ.u.mnutate; yet this tentacle was so sensitive, that in 23 seconds after its gland had been merely touched with a bit of raw meat, it began to curl inwards. This fact is of some importance, as it apparently shows that the inflection of the tentacles from the stimulus of absorbed animal matter (and no doubt from that of contact with any object) is not due to modified circ.u.mnutation.
(15.) Dionoea muscipula (Droseraceae).--It should be premised that the leaves at an early stage of their development have the two lobes pressed closely together. These are at first directed back towards the centre of the plant; but they gradually rise up and soon stand at right angles to the petiole, and ultimately in nearly a straight line with it. A young leaf, which with the [page 240]
petiole was only 1.2 inch in length, had a filament fixed externally along the midrib of the still closed lobes, which projected at right angles to the petiole. In the evening this leaf completed an ellipse in the course of 2 h. On the following day (Sept. 25th) its movements were traced during 22 h.; and we see in Fig. 106 that it moved in the same general direction, due to the straightening of the leaf, but in an extremely zigzag line. This line represents several drawn-out or modified ellipses. There can therefore be no doubt that this young leaf circ.u.mnutated.
Fig. 106. Dionaea muscipula: circ.u.mnutation of a young and expanding leaf, traced on a horizontal gla.s.s in darkness, from noon Sept. 24th to 10 A.M.
25th. Apex of leaf 13 inches from the gla.s.s, so tracing considerably magnified.
A rather old, horizontally extended leaf, with a filament attached along the under side of the midrib, was next observed during 7 h. It hardly moved, but when one of its sensitive hairs was touched, the blades closed, though not very quickly. A new dot was now made on the gla.s.s, but in the course of 14 h. 20 m. there was hardly any change in the position of the filament. We may therefore infer that an old and only moderately sensitive leaf does not circ.u.mnutate plainly; but we shall soon see that it by no means follows that such a leaf is absolutely motionless. We may further infer that the stimulus from a touch does not re-excite plain circ.u.mnutation.
Another full-grown leaf had a filament attached externally along one side of the midrib and parallel to it, so that the filament would move if the lobes closed. It should be first stated that, although a touch on one of the sensitive hairs of a vigorous leaf causes it to close quickly, often almost instantly, yet when a bit of damp meat or some solution of carbonate of ammonia is placed on the lobes, they close so slowly that generally 24 h. is required for the completion of the act. The above leaf was first observed for 2 h. 30 m., and did not circ.u.mnutate, but it ought to have been observed for a [page 241]
longer period; although, as we have seen, a young leaf completed a fairly large ellipse in 2 h. A drop of an infusion of raw meat was then placed on the leaf, and within 2 h. the gla.s.s filament rose a little; and this implies that the lobes had begun to close, and perhaps the petiole to rise.
It continued to rise with extreme slowness for the next 8 h. 30 m. The position of the pot was then (7.15 P.M., Sept. 24th) slightly changed and an additional drop of the infusion given, and a new tracing was begun (Fig.
107). By 10.50 P.M. the filament had risen only a little more, and it fell during the night. On the following morning the lobes were closing more quickly, and by 5 P.M. it was evident to the eye that they had closed considerably; by 8.48. P.M. this was still plainer, and by 10.45 P.M. the marginal spikes were interlocked. The leaf fell a little during the night, and next morning (25th) at 7 A.M. the lobes were completely shut. The course pursued, as may be seen in the figure, was
Fig. 107. Dionoea muscipula: closure of the lobes and circ.u.mnutation of a full-grown leaf, whilst absorbing an infusion of raw meat, traced in darkness, from 7.15 P.M. Sept. 24th to 9 A.M. 26th. Apex of leaf 8 inches from the vertical gla.s.s. Figure here reduced to two-thirds of original scale.
strongly zigzag, and this indicates that the closing of the lobes was combined with the circ.u.mnutation of the whole leaf; and there cannot be much doubt, considering how motionless the leaf was during 2 h. 30 m.
before it received the infusion, that the absorption of the animal matter had excited it to circ.u.mnutate. The leaf was occasionally observed for the next four days, but was kept in rather too cool a place; nevertheless, it continued to circ.u.mnutate to a small extent, and the lobes remained closed.
It is sometimes stated in botanical works that the lobes close or sleep at night; but this is an error. To test the statement, very long gla.s.s filaments were fixed inside the two lobes of three leaves, and the distances between their tips were measured in the middle of the day and at night; but no difference could be detected.
The previous observations relate to the movements of the whole leaf, but the lobes move independently of the petiole, and [page 242]
seem to be continually opening and shutting to a very small extent. A nearly full-grown leaf (afterwards proved to be highly sensitive to contact) stood almost horizontally, so that by driving a long thin pin through the foliaceous petiole close to the blade, it was rendered motionless. The plant, with a little triangle of paper attached to one of the marginal spikes, was placed under a microscope with an eye-piece micrometer, each division of which equalled 1/500 of an inch. The apex of the paper-triangle was now seen to be in constant slight movement; for in 4 h. it crossed nine divisions, or 9/500 of an inch, and after ten additional hours it moved back and had crossed 5/500 in an opposite direction. The plant was kept in rather too cool a place, and on the following day it moved rather less, namely, 1/500 in 3 h., and 2/500 in an opposite direction during the next 6 h. The two lobes, therefore, seem to be constantly closing or opening, though to a very small distance; for we must remember that the little triangle of paper affixed to the marginal spike increased its length, and thus exaggerated somewhat the movement. Similar observations, with the important difference that the petiole was left free and the plant kept under a high temperature, were made on a leaf, which was healthy, but so old that it did not close when its sensitive hairs were repeatedly touched, though judging from other cases it would have slowly closed if it had been stimulated by animal matter. The apex of the triangle was in almost, though not quite, constant movement, sometimes in one direction and sometimes in an opposite one; and it thrice crossed five divisions of the micrometer (i.e. 1/100 of an inch) in 30 m. This movement on so small a scale is hardly comparable with ordinary circ.u.mnutation; but it may perhaps be compared with the zigzag lines and little loops, by which the larger ellipses made by other plants are often interrupted.
In the first chapter of this volume, the remarkable oscillatory movements of the circ.u.mnutating hypocotyl of the cabbage have been described. The leaves of Dionaea present the same phenomenon, which is a wonderful one, as viewed under a low power (2-inch object-gla.s.s), with an eye-piece micrometer of which each division (1/500 of an inch) appeared as a rather wide s.p.a.ce. The young unexpanded leaf, of which the circ.u.mnutating movements were traced (Fig. 106), had a gla.s.s filament fixed perpendicularly to it; and the movement of the apex was observed in the hot-house (temp. 84o to 86o F.), with light admitted only from above, and with any lateral currents of air [page 243]
excluded. The apex sometimes crossed one or two divisions of the micrometer at an imperceptibly slow rate, but generally it moved onwards by rapid starts or jerks of 2/1000 or 3/1000, and in one instance of 4/1000 of an inch. After each jerk forwards, the apex drew itself backwards with comparative slowness for part of the distance which had just been gained; and then after a very short time made another jerk forwards. Four conspicuous jerks forwards, with slower retreats, were seen on one occasion to occur in exactly one minute, besides some minor oscillations. As far as we could judge, the advancing and retreating lines did not coincide, and if so, extremely minute ellipses were each time described. Sometimes the apex remained quite motionless for a short period. Its general course during the several hours of observation was in two opposite directions, so that the leaf was probably circ.u.mnutating.
An older leaf with the lobes fully expanded, and which was afterwards proved to be highly sensitive to contact, was next observed in a similar manner, except that the plant was exposed to a lower temperature in a room.
The apex oscillated forwards and backwards in the same manner as before; but the jerks forward were less in extent, viz. about 1/1000 inch; and there were longer motionless periods. As it appeared possible that the movements might be due to currents of air, a wax taper was held close to the leaf during one of the motionless periods, but no oscillations were thus caused. After 10 m., however, vigorous oscillations commenced, perhaps owing to the plant having been warmed and thus stimulated. The candle was then removed and before long the oscillations ceased; nevertheless, when looked at again after an interval of 1 h. 30 m., it was again oscillating.
The plant was taken back into the hot-house, and on the following morning was seen to be oscillating, though not very vigorously. Another old but healthy leaf, which was not in the least sensitive to a touch, was likewise observed during two days in the hot-house, and the attached filament made many little jerks forwards of about 2/1000 or only 1/1000 of an inch.
Finally, to ascertain whether the lobes independently of the petiole oscillated, the petiole of an old leaf was cemented close to the blade with sh.e.l.lac to the top of a little stick driven into the soil. But before this was done the leaf was observed, and found to be vigorously oscillating or jerking; and after it had been cemented to the stick, the oscillations of about 2/1000 of an inch still continued. On the following day a little infusion [page 244]
of raw meat was placed on the leaf, which caused the lobes to close together very slowly in the course of two days; and the oscillations continued during this whole time and for the next two days. After nine additional days the leaf began to open and the margins were a little everted, and now the apex of the gla.s.s filament remained for long periods motionless, and then moved backwards and forwards for a distance of about 1/1000 of an inch slowly, without any jerks. Nevertheless, after warming the leaf with a taper held close to it, the jerking movement recommenced.
This same leaf had been observed 2 months previously, and was then found to be oscillating or jerking. We may therefore infer that this kind of movement goes on night and day for a very long period; and it is common to young unexpanded leaves and to leaves so old as to have lost their sensitiveness to a touch, but which were still capable of absorbing nitrogenous matter. The phenomenon when well displayed, as in the young leaf just described, is a very interesting one. It often brought before our minds the idea of effort, or of a small animal struggling to escape from some constraint.