had formed a true leaf a quarter of an inch in length, and the hypocotyl was 1⅜ inch in height. The figure traced was a very complex one, though the movement was not so great in extent as in the last case.
The hypocotyl of another seedling of the same age was secured to a little stick, and a filament having been fixed to the midrib of one of the cotyledons, the movement of the bead was traced during 14 h. 15 m. (see Fig. 10) in darkness. It should be noted that the chief movement of the cotyledons, namely, up and down, would be shown on a horizontal glass-plate only by the lines in the direction of the midrib (that is, [page 20] up and down, as Fig. 10 here stands) being a little lengthened or shortened; whereas any lateral movement would be well exhibited. The present tracing shows that the cotyledon did thus move laterally (that is, from side to side in the tracing) 12 times in the 14 h. 15 m. of observation. Therefore the cotyledons certainly circumnutated, though the chief movement was up and down in a vertical plane.
Fig 10. Brassica oleracea: circumnutation of a cotyledon, the hypocotyl having been secured to a stick, traced on a horizontal glass, in darkness, from 8.15 A.m. to 10.30 P.m. Movement of the bead of the filament magnified 13 times.
Rate of Movement.—The movements of the hypocotyls and cotyledons of seedling cabbages of different ages have now been sufficiently illustrated. With respect to the rate, seedlings were placed under the microscope with the stage removed, and with a micrometer eye-piece so adjusted that each division equalled ⅕00 inch; the plants were illuminated by light passing through a solution of bichromate of potassium so as to eliminate heliotropism. Under these circumstances it was interesting to observe how rapidly the circumnutating apex of a cotyledon passed across the divisions of the micrometer. Whilst travelling in any direction the apex generally oscillated backwards and forwards to the extent of ⅕00 and sometimes of nearly ½50 of an inch. These oscillations were quite different from the trembling caused by any disturbance in the same room or by the shutting of a distant door. The first seedling observed was nearly two inches in height and had been etiolated by having been grown in darkness. The tip of the cotyledon passed across 10 divisions of the micrometer, that is,⅕0 of an inch, in 6 m. 40 s. Short glass filaments were then fixed vertically to the hypocotyls of several seedlings so as to project a little above the cotyledons, thus exaggerating the rate of movement; but only a few of the observations thus made are worth giving. The most remarkable fact was the oscillatory movement above described, and the difference of rate at which the point crossed the divisions of the micrometer, after short intervals of time. For instance, a tall not-etiolated seedling had been kept for 14 h. in darkness; it was exposed before a north-east window for only [page 21] two or three minutes whilst a glass filament was fixed vertically to the hypocotyl; it was then again placed in darkness for half an hour and afterwards observed by light passing through bichromate of potassium. The point, oscillating as usual, crossed five divisions of the micrometer (i.e. 1/100 inch) in 1 m. 30 s. The seedling was then left in darkness for an hour, and now it required 3 m. 6 s. to cross one division, that is, 15 m. 30 s. to have crossed five divisions. Another seedling, after being occasionally observed in the back part of a northern room with a very dull light, and left in complete darkness for intervals of half an hour, crossed five divisions in 5 m. in the direction of the window, so that we concluded that the movement was heliotropic. But this was probably not the case, for it was placed close to a north-east window and left there for 25 m., after which time, instead of moving still more quickly towards the light, as might have been expected, it travelled only at the rate of 12 m. 30 s. for five divisions. It was then again left in complete darkness for 1 h., and the point now travelled in the same direction as before, but at the rate of 3 m. 18 s. for five divisions.
We shall have to recur to the cotyledons of the cabbage in a future chapter, when we treat of their sleep-movements. The circumnutation, also, of the leaves of fully-developed plants will hereafter be described.
Fig. 11. Githago segetum: circumnutation of hypocotyl, traced on a horizontal glass, by means of a filament fixed transversely across its summit, from 8.15 A.m. to 12.15 P.m. on the following day. Movement of bead of filament magnified about 13 times, here reduced to one-half the original scale.
Githago segetum (Caryophylleae).—A young seedling was dimly illuminated from above, and the circumnutation of the hypo- [page 22] cotyl was observed during 28 h., as shown in Fig. 11. It moved in all directions; the lines from right and to left in the figure being parallel to the blades of the cotyledons. The actual distance travelled from side to side by the summit of the hypocotyl was about .2 of an inch; but it was impossible to be accurate on this head, as the more obliquely the plant was viewed, after it had moved for some time, the more the distances were exaggerated.
We endeavoured to observe the circumnutation of the cotyledons, but as they close together unless kept exposed to a moderately bright light, and as the hypocotyl is extremely heliotropic, the necessary arrangements were too troublesome. We shall recur to the nocturnal or sleep-movements of the cotyledons in a future chapter.
Fig. 12. Gossypium: circumnutation of hypocotyl, traced on a horizontal glass, from 10.30 A.m. to 9.30 A.m. on following morning, by means of a filament fixed across its summit. Movement of bead of filament magnified about twice; seedling illuminated from above.
Gossypium (var. Nankin cotton) (Malvaceae).—The circumnutation of a
hypocotyl was observed in the hot-house, but the movement was so much
exaggerated that the bead twice passed for a time out of view. It was,
however, manifest that two somewhat irregular ellipses were nearly
completed in 9 h. Another seedling, 1½ in. in height, was then observed
during 23 h.; but the observations were not made at sufficiently short
intervals, as shown by the few dots in Fig. 12, and the tracing was not now
sufficiently enlarged. Nevertheless there could be no doubt about the
circumnutation of the hypocotyl, which described in 12 h. a figure
representing three irregular ellipses of unequal sizes.
The cotyledons are in constant movement up and down during the whole day,
and as they offer the unusual case of moving downwards late in the evening
and in the early part of the night, many observations were made on them. A
filament was fixed along the middle of one, and its movement traced on a
vertical glass; but the tracing is not given, as the hypocotyl was not
secured, so that it was impossible to distinguish clearly between its
movement and that of the cotyledon. The cotyledons rose from 10.30 A.m. to
about 3 P.m.; they then sank till 10 P.m., rising, however, greatly in the
latter part of the night.
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The angles above the horizon at which the cotyledons of another seedling
stood at different hours is recorded in the following short table:—
Oct. 20 2.50 P.M … 25o above horizon. Oct. 20 4.20 P.M … 22o above horizon. Oct. 20 5.20 P.M … 15o above horizon. Oct. 20 10.40 P.M … 8o above horizon. Oct. 21 8.40 A.M … 28o above horizon. Oct. 21 11.15 A.M … 35o above horizon. Oct. 21 9.11 P.M … 10o below horizon.
The position of the two cotyledons was roughly sketched at various hours with the same general result.
In
