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 1/500 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 1/500 and sometimes of nearly 1/250 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, 1/50 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.

Charles Darwin

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