CHAP. X. APOGEOTROPISM. 497 



oblique During the next 3 h. 8 m. it rose in a nearly straight 

 line, passing through an angle of 109, and then (at 12.8 P.M.) 

 stood upright. It continued for 55 m. to move in the same 

 general direction beyond the perpendicular, but in a zigzag 

 course. It returned also in a zigzag line, and then circumnu- 

 tated regularly, describing three large ellipses during the 

 remainder of the day. It should be observed that the ellipses 

 in this figure are exaggerated in size, relatively to the length of 

 the upward straight line, owing to the position of the vertical 

 and horizontal glass-plates. Another and somewhat old hypo- 

 cotyl was placed so as to stand at only 31 from the perpen- 

 dicular, in which position apogeotropism acted on it with little 

 force, and its course accordingly was slightly zigzag. 



The sbeath-like cotyledons of P/talaris Canariensis are ex- 

 tremely sensitive to apogeotropism. One was placed so as to 

 project 40 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 130 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 pro- 

 trude, 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 inch in height) required 28 h. to become up- 

 right, and its course was distinctly zigzag ; whilst younger hypo- 

 cotyls 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 circumnu- 

 tation ; 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 movement had been replaced by one of a wholly distinct kind.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 circum- 

 nutate 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 occa- 

 sionally observed with Beta, Brassica, and Phalaris. 



We will now describe a few cases in which it may be 



