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 30o and 40o beneath the horizon.  The upper part alone became curved [page 501] 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 32o 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 [page 502] 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 20o 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.