The sheath-like cotyledons of Phalaris Canariensis are extremely sensitive to apogeotropism.  One was placed so as to project 40o beneath the horizon.  Although it was rather old and 1.3 inch in height, it became vertical in 4 h. 30 m., having passed through an angle of 130o in a nearly straight line.  It then suddenly began to circumnutate in the ordinary manner.  The cotyledons of this plant, after the first leaf has begun to protrude, are but slightly apogeotropic, though they still continue to circumnutate.  One at this stage of development was placed horizontally, and did not become upright even after 13 h., and its course was slightly zigzag.  So, again, a rather old hypocotyl of Cassia tora (1 1/4 inch in height) required 28 h. to become upright, and its course was distinctly zigzag; whilst younger hypocotyls moved much more quickly and in a nearly straight line.

When a horizontally placed stem or other organ rises in a zigzag line, we may infer from the many cases given in our previous chapters, that we have a modified form of circumnutation; but when the course is straight, there is no evidence of circumnutation, and any one might maintain that this latter movement had been replaced by one of a wholly distinct kind.  This view seems the more probable when (as sometimes occurred with the hypocotyls of Brassica and Beta, the stems of Cucurbita, and the cotyledons of Phalaris) the part in question, after bending up in a straight course, suddenly begins to circumnutate to the full extent and in the usual manner.  A fairly good instance of a sudden change of this kind—­that is, from a nearly straight upward movement to one of circumnutation—­is shown in Fig. 183; but more striking instances were occasionally observed with Beta, Brassica, and Phalaris.

We will now describe a few cases in which it may be [page 498] seen how gradually circumnutation becomes changed into apogeotropism, under circumstances to be specified in each instance.

Rubus idaeus (hybrid).—­A young plant, 11 inches in height, growing in a pot, was placed horizontally; and the upward movement was traced during nearly 70 h.; but the plant, though growing vigorously, was not highly sensitive to apogeotropism, or it was not capable of quick movement, for during the above time it rose only 67o.  We may see in the diagram (Fig. 184) that during the first day of 12 h. it rose in a nearly straight line.  When placed horizontally, it was evidently circumnutating, for it rose at first a little, notwithstanding the weight of the stem, and then sank down; so that it did not start on its permanently upward course until 1 h. 25 m. had elapsed.  On the second day, by which time it had risen considerably, and when apogeotropism acted on it with somewhat less power, its course during 15 ½ h. was clearly zigzag, and the rate of the upward movement was not equable.  During the third day, also of 15 ½ h., when apogeotropism acted on it with still less power, the stem plainly