Fig 184: Rubus idæus (hybrid): apogeotropic movement of stem, traced on a vertical glass during 3 days and 3 nights, from 10.40 A.M. March 18th to 8 A.M. 21st. Figure reduced to one-half of the original scale.
Lilium auratum.—A plant 23 inches in height was placed horizontally, and the upper part of the stem rose 58° in 46 h., in the manner shown in the accompanying diagram (Fig. 185). We here see that during the whole of the second day of 15½ h., the stem plainly circumnutated whilst bending upwards through apogeotropism. It had still to rise considerably, for when the last dot in the figure was made, it stood 32° from an upright position.
Fig. 185. Lilium auratum: apogeotropic movement of stem, traced on a vertical glass during 2 days and 2 nights, from 10.40 A.M. March 18th to 8 A.M. 20th. Figure reduced to one-half of the original scale.
Phalaris Canariensis.—A cotyledon of this plant (1.3 inch in height) has already been described as rising in 4 h. 30 m. from 40° beneath the horizon into a vertical position, passing through an angle of 130° in a nearly straight line, and then abruptly beginning to circumnutate. Another somewhat old cotyledon of the same height (but from which a true leaf had not yet protruded), was similarly placed at 40° beneath the horizon. For the first 4 h. it rose in a nearly straight course (Fig. 186), so that by 1.10 P.M. it was highly inclined, and now apogeotropism acted on it with much less power than before, and it began to zigzag. At 4.15 P.M. (i.e. in 7 h. from the commencement) it stood vertically, and afterwards continued to circumnutate in the usual manner about the same spot. Here then we have a graduated change from a straight upward apogeotropic course into circumnutation, instead of an abrupt change, as in the former case.
Avena sativa.—The sheath-like cotyledons, whilst young, are strongly apogeotropic; and some which were placed at 45° beneath the horizon rose 90° in 7 or 8 h. in lines almost absolutely straight. An oldish cotyledon, from which the first leaf began to protrude whilst the following observations were being made, was placed at 10° beneath the horizon, and it rose only 59° in 24h. It behaved rather differently from any other plant, observed by us, for during the first 4½ h. it rose in a line not far from straight; during the next 6½ h. it circumnutated, that is, it descended and again ascended in a strongly marked zigzag course; it then resumed its upward movement in a moderately straight line, and, with time allowed, no doubt would have become upright. In this case, after the first 4½ h., ordinary circumnutation almost completely conquered for a time apogeotropism.
Fig 186. Phalaris Canariensis: apogeotropic movement of cotyledon, traced on a vertical and horizontal glass, from 9.10 A.M. Sept. 19th to 9 A.M. 20th. Figure here reduced to one-fifth of original scale.
Brassica oleracea.—The hypocotyls of several young seedlings placed horizontally, rose up vertically in the course of 6 or 7 h. in nearly straight lines. A seedling which had grown in darkness to a height of 2 1/4 inches, and was therefore rather old and not highly sensitive, was placed so that the hypocotyl projected at between 30° and 40° beneath the horizon. The upper part alone became curved upwards, and rose during the first 3 h. 10 m. in a nearly straight line (Fig. 187); but it was not possible to trace the upward movement on the vertical glass for the first 1 h. 10 m., so that the nearly straight line in the diagram ought to have been much longer. During the next 11 h. the hypocotyl circumnutated, describing irregular figures, each of which rose a little above the one previously formed. During the night and following early morning it continued to rise in a zigzag course, so that apogeotropism was still acting. At the close of our observations, after 23 h. (represented by the highest dot in the diagram) the hypocotyl was still 32° from the perpendicular. There can be little doubt that it would ultimately have become upright by describing an additional number of irregular ellipses, one above the other.
Fig 187. Brassica oleracea: apogeotropic movement of hypocotyl, traced on vertical glass, from 9.20 A.M., Sept. 12th to 8.30 A.M. 13th. The upper part of the figure is more magnified than the lower part. If the whole course had been traced, the straight upright line would have been much longer. Figure here reduced to one-third of the original scale.
Apogeotropism retarded by Heliotropism.—When the stem of any plant bends during the day towards a lateral light, the movement is opposed by apogeotropism; but as the light gradually wanes in the evening the latter power slowly gains the upper hand, and draws the stem back into a vertical position. Here then we have a good opportunity for observing how apogeotropism acts when very nearly balanced by an opposing force. For instance, the plumule of Tropaeolum majus (see former Fig. 175) moved towards the dim evening light in a slightly zigzag line until 6.45 P.M., it then returned on its course until 10.40 P.M., during which time it zigzagged and described an ellipse of considerable size. The hypocotyl of Brassica oleracea (see former Fig. 173) moved in a straight line to the light until 5.15 P.M., and then from the light, making in its backward course a great rectangular bend, and then returned for a short distance towards the former source of the light; no observations were made after 7.10 P.M., but during the night it recovered its vertical position. A hypocotyl of Cassia tora moved in the evening in a somewhat zigzag line towards the failing light until 6 P.M., and was now bowed 20° from the perpendicular; it then returned on its course, making before 10.30 P.M. four great, nearly rectangular bends and almost completing an ellipse. Several other analogous cases were casually observed, and in all of them the apogeotropic movement could be seen to consist of modified circumnutation.
Apogeotropic Movements effected by the aid of joints or pulvini.—Movements of this kind are well known to occur in the Gramineæ, and are effected by means of the thickened bases of their sheathing leaves; the stem within being in this part thinner than elsewhere.[[1]] According to the analogy of all other pulvini, such joints ought to continue circumnutating for a long period, after the adjoining parts have ceased to grow. We therefore wished to ascertain whether this was the case with the Gramineæ; for if so, the upward curvature of their stems, when extended horizontally or laid prostrate, would be explained in accordance with our view—namely, that apogeotropism results from modified circumnutation. After these joints have curved upwards, they are fixed in their new position by increased growth along their lower sides.