The Power of Movement in Plants - Charles Darwin (top reads txt) 📗

61 h.; and its course is shown in the preceding figure (Fig. 98). During the first day and night the leaf moved downwards, but circumnutated 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 circumnutated on an extremely small scale. On the third day the circumnutation 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 circumnutation, though plain enough as it was, would probably have been far more conspicuous.

 

Fig. 99. Cissus discolor: circumnutation 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 glass.

 

(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 circumnutated 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 glass. During the latter part of the night or early morning it again fell in the same manner as before.

[page 234]

 

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: circumnutation 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 glass. 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 analogous 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: circumnutation 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 glass.

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 circumnutate round the same small space.

[page 236]

 

(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 circumnutation 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: circumnutation 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 glass; 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 circumnutating.

 

Fig. 103. Lupinus speciosus: circumnutation of leaf, traced on vertical glass, from 10.15 A.M. to 5.45 P.M.; i.e., during 6 h. 30 m.

[page 237]

 

(12.) Echeveria stolonifera (Crassulaceae, 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 circumnutation 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 circumnutation, though on a very small scale.

 

Fig. 104. Echeveria stolonifera: circumnutation 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 glass, so that the movement was much magnified; temp. 23o - 24 1/2o C.

(13.) Bryophyllum (vel Calanchae) calycinum (Crassulaceae).—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 circumnutated 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: circumnutation 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 circumnutated 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]

circumnutates 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 circumnutation. Whether the rising of the glass 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 circumnutate, the back of a young leaf, with the innermost tentacles as yet incurved, was firmly cemented with shellac 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 circumnutation. 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 circumnutate; 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