Another plan was to observe in the evening plants which during the day had become greatly curved heliotropically; for their stems under the gradually waning light very slowly became upright through the action of apogeotropism; and in this case modified circumnutation was sometimes well displayed.
[Apogeotropism.--Plants were selected for observation almost by chance, excepting that they were taken from widely different families. If the stem of a plant which is even moderately sensitive to apogeotropism be placed horizontally, the upper growing part bends quickly upwards, so as to become perpendicular; and the line traced by joining the dots successively made on a glass-plate, is generally almost straight. For instance, a young Cytisus fragrans, 12 inches in height, was placed so that the stem projected 10o beneath the horizon, and its course was traced during 72 h. At first it bent a very little downwards (Fig. 182), owing no doubt to the weight of the stem, as this occurred with most of the other plants observed, though, as they were of course circumnutating, the short downward lines were often oblique. After three-quarters of an hour the stem began to curve upwards, quickly during the first two hours, but much more slowly during the afternoon and night, [page 495] and on the following day. During the second night it fell a little, and circumnutated during the following day; but it also moved a short distance to the right, which was caused by a little light having been accidentally admitted on this side. The stem was now inclined 60o above the horizon, and had therefore risen 70o. With time allowed it would probably have become upright, and no doubt would have continued circumnutating. The sole remarkable feature in the figure here given is the straightness of the course pursued. The stem, however, did not move upwards at an equable rate, and it sometimes stood almost or quite still. Such periods probably represent attempts to circumnutate in a direction opposite to apogeotropism.
Fig. 182. Cytisus fragrans: apogeotropic movement of stem from 10o beneath to 60o above horizon, traced on vertical glass, from 8.30 A.M. March 12th to 10.30 P.M. 13th. The subsequent circumnutating movement is likewise shown up to 6.45 A.M. on the 15th. Nocturnal course represented, as usual, by a broken line. Movement not greatly magnified, and tracing reduced to two-thirds of original scale.
The herbaceous stem of a Verbena melindres (?) laid horizontally, rose in 7 h. so much that it could no longer be observed on the vertical glass which stood in front of the plant. The long line which was traced was almost absolutely straight. After the 7 h. it still continued to rise, but now circumnutated slightly. On the following day it stood upright, and circumnutated regularly, as shown in Fig. 82, given in the fourth chapter. The stems of several other plants which were highly sensitive to apogeotropism rose up in almost straight lines, and [page 496] then suddenly began to circumnutate. A partially etiolated and somewhat old hypocotyl of a seedling cabbage (2 3/4 inches in height) was so sensitive that when placed at an angle of only 23o from the perpendicular, it became vertical in 33 minutes. As it could not have been strongly acted upon by apogeotropism in the above slightly inclined position, we expected that it would have circumnutated, or at least have moved in a zigzag course. Accordingly, dots were made every 3 minutes; but, when these were joined, the line was nearly straight. After this hypocotyl had become upright it still moved onwards for half an hour in the same general direction, but in a zigzag manner. During the succeeding 9 h. it circumnutated regularly, and described 3 large ellipses. In this case apogeotropism, although acting at a very unfavourable angle, quite overcame the ordinary circumnutating movement.
Fig. 183. Beta vulgaris: apogeotropic movement of hypocotyl from 19o beneath horizon to a vertical position, with subsequent circumnutation, traced on a vertical and on a horizontal glass-plate, from 8.28 A.M.