508 MODIFIED CIEOUMNUTAT1ON. CHAP. X. 



tinually circumnutating, we may conclude that even 

 a rectilinear course is merely an extremely modified 

 form of circumnutation. It is remarkable that a stem 

 or other organ which is highly sensitive to apogeo- 

 tropism, and which has bowed itself rapidly upwaids 

 in a straight line, is often carried beyond the vertical, 

 as if by momentum. It then bends a little backwards 

 to a point round which it finally circuinnutates. Two 

 instances of this were observed with the hypocotyls of 

 Beta vulgaris, one of which is shown in Fig. 183, and 

 two other instances with the hypocotyls of Brassica. 

 This momentum-like movement probably results from 

 the accumulated effects of apogeotropism. For the 

 sake of observing how long such after-effects lasted, 

 a pot with seedlings of Beta was laid on its side in the 

 dark, and the hypocotyls in 3 h. 15 in. became highly 

 inclined. The pot, still in the dark, was then placed 

 upright, and the movements of the two hypocotyls were 

 traced ; one continued to bend in its former direction, 

 now in opposition to apogeotropism, for about 37m., 

 perhaps for 48 in. ; but after 61 m. it moved in an 

 opposite direction. The other hypocotyl continued 

 to move in its former course, after being placed 

 upright, for at least 37 in. 



Different species and different parts of the same 

 species are acted on by apogeotropism in very dif- 

 ferent degrees. Young seedlings, most of which cir- 

 cumnutate quickly and largely, bend upwards and 

 become vertical in much less time than do any older 

 plants observed by us; but whether this is due to 

 their greater sensitiveness to apogeotropism, or merely 

 to their greater flexibility we do not know. A hypo- 

 cotyl of Beta traversed an angle of 109 in 3 h. 8 m., 

 and a cotyledon of Phalaris an angle of 130 in 4 h. 

 30 m. On the other hand, the stem of a herbaceous 



