The movements of a young plant bearing a few leaves and of a mature plant, will hereafter be described.
Fig. 16. Tropaeolum minus (?): circumnutation of buried and arched epicotyl, traced on a horizontal glass, from 9.20 A.M. to 8.15 P.M. Movement of bead of filament magnified 27 times. [page 28]
Citrus aurantium (Orange) (Aurantiaceae).--The cotyledons are hypogean. The circumnutation of an epicotyl, which at the close of our observations was .59 of an inch (15 mm.) in height above the ground, is shown in the annexed figure (Fig. 17), as observed during a period of 44 h. 40 m.
Fig. 17. Citrus aurantium: circumnutation of epicotyl with a filament fixed transversely near its apex, traced on a horizontal glass, from 12.13 P.M. on Feb. 20th to 8.55 A.M. on 22nd. The movement of the bead of the filament was at first magnified 21 times, or 10 1/2, in figure here given, and afterwards 36 times, or 18 as here given; seedling illuminated from above.
Aesculus hippocastanum (Hippocastaneae).--Germinating seeds were placed in a tin box, kept moist internally, with a sloping bank of damp argillaceous sand, on which four smoked glass-plates rested, inclined at angles of 70o and 65o with the horizon. The tips of the radicles were placed so as just to touch the upper end of the glass-plates, and, as they grew downwards they pressed lightly, owing to geotropism, on the smoked surfaces, and left tracks of their course. In the middle part of each track the glass was swept clean, but the margins were much blurred and irregular. Copies of two of these tracks (all four being nearly alike) were made on tracing paper placed over the glass-plates after they had been varnished; and they are as exact as possible considering the nature of the margins (Fig. 18). They suffice to show that there was some lateral, almost serpentine movement, and that the tips in their downward course pressed with unequal force on the plates, as [page 29] the tracks varied in breadth. The more perfectly serpentine tracks made by the radicles of Phaseolus multiflorus and Vicia faba (presently to be described), render it almost certain that the radicles of the present plant circumnutated.
Fig. 18. Aesculus hippocastanum: outlines of tracks left on inclined glass-plates by tips of radicles. In A the plate was inclined at 70o with the horizon, and the radicle was 1.9 inch in length, and .23 inch in diameter at base. In B the plate was inclined 65o with the horizon, and the radicle was a trifle larger.
Phaseolus multiflorus (Leguminosae).--Four smoked glass-plates were arranged in the same manner as described under Aesculus, and the tracks left by the tips of four radicles of the present plant, whilst growing downwards, were photographed as transparent objects. Three of them are here exactly copied (Fig. 19). Their serpentine courses show that the tips moved regularly from side to side; they also pressed alternately with greater or less force on the plates, sometimes rising up and leaving them altogether for a very short distance; but this was better seen on the original plates than in the copies. These radicles therefore were continually moving in all directions--that is, they circumnutated. The distance between the extreme right and left positions of the radicle A, in its lateral movement, was 2 mm., as ascertained by measurement with an eye-piece micrometer.
Fig. 19. Phaseolus multiflorus: tracks left on inclined smoked glass-plates by tips of radicles in growing downwards. A and C, plates inclined at 60o, B inclined at 68o with the horizon.
Vicia faba (Common Bean) (Leguminosae).--Radicle.--Some beans were allowed to germinate on bare sand, and after one had protruded its radicle to a length of .2 of an inch, it was turned upside down, so that the radicle, which was kept in damp air, now stood upright. A filament, nearly an inch in length, was affixed obliquely near its tip; and the movement of the terminal bead was traced from 8.30 A.M. to 10.30 P.M., as shown in Fig. 18. The radicle at first changed its course twice [page 30] abruptly, then made a small loop and then a larger zigzag curve.