(3.) Crambe maritima (Cruciferae, Fam. 14).—A leaf 9½ 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 was deflected, so that the filament projected almost horizontally, and its movements were traced during 48 h. on a vertical glass as shown in the accompanying figure (Fig. 95). We here plainly see that the leaf was continually circumnutating; 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 9° 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: circumnutation 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 glass, so that the tracing was much magnified, but is here reduced to one-fourth of original scale.
(4.) Brassica oleracea (Cruciferae).—Hofmeister and Batalin[[9]] 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-glass, placing it in the same position with respect to the 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 11° in the course of 24 h., and the distance travelled by the apex, up and down, was between .8 and .9 of an inch.
[9] ‘Flora,’ 1873, p. 437.
In order to ascertain the effect of darkness, a filament was fixed to a leaf 5½ inches in length, borne by a plant which after forming a head had produced a stem. The leaf was inclined 44° above the horizon, and its movements were traced on a vertical glass 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,[[10]] 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.
[10] ‘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.
(5.) Dianthus caryophyllus (Caryophylleae, Fam. 26).—The 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 glass 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 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 2½ days. As the leaf continued to move all the time to the left, it is evident that the zigzag line represents many circumnutations.
Fig. 96. Dianthus caryophyllus: circumnutation 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 glass, so tracing not greatly magnified. The leaf was 5 1/4 inches long. Temp. 15½°–17½° C.
(6.) Camellia Japonica (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 40° beneath the horizon. As it was thick and rigid, and its petiole very short, much movement could not be expected. Nevertheless, the apex changed its course completely seven times in the course of 11½ 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 circumnutated 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.