218 TROPIC MOVEMENTS 



Thus roots slowly responded to a displacement of as little as 2 C., and 

 more rapidly to one of 20 C. An increased divergence beyond this did 

 not further accelerate the reaction, but nevertheless, after equally long 

 periods of geotropic induction, the most pronounced after-effects were 

 shown by roots placed at angles of 140 to 160 C. to their normal 

 positions. Using the same method, Pertz 1 was able to show that the 

 node of a grass-haulm experienced a negatively geotropic excitation 

 when the stem was inverted and reverted for equal lengths of time, while 

 maintaining the same angle with the horizontal. Czapek found that 

 beyond angles of 140 to 160 C. the excitation again decreased, until 

 a labile position of equilibrium was reached in a precisely inverted position, 

 so that when placed on a klinostat no geotropic after-effect was shown 

 if the root or stem had been prevented from diverging from the vertical 

 position during its exposure to the action of gravity 2 . Under natural 

 conditions an inverted root always makes slight autonomic curvatures 

 from the vertical, which render possible a geotropic excitation leading to 

 the return to the normal direction of growth. 



If the apex of a shoot is fixed in a horizontal position, and the base 

 left free to move, the negatively geotropic reaction of the active zones 

 causes it to curve upwards, but no reaction is shown if the apex is bent 

 upwards into a vertical position. When the apex is fixed, however, in an 

 inverted vertical position, the circumnutation of the free portion renders 

 geotropic excitation possible, so that the free end bends upwards. If 

 the apical segment of a horizontally-placed shoot is fixed at the middle 

 of the active zones both the free ends curve upwards 3 . If, however, 

 the apex of a root is fixed in a normal vertical position, the free basal 

 portion performs no curvature since the apex alone is capable of perception, 

 whereas when the apex is fixed in a horizontal position the free portion 

 curves continually owing to the continuous excitation, just as when the 

 tip of a cotyledon of Panicum is held in a horizontal position. 



Diageotropic rhizomes behave in a similar way, but respond more 

 rapidly to an upward displacement than to a downward one of similar 

 extent 4 . The radial lateral roots of the first order behave similarly, and 

 hence if a lateral root is displaced and then slowly rotated, it assumes its 

 proper position, owing to the fact that it is more strongly excited during 

 the upper phase than during the lower one 5 . The excitation increases 



1 Pertz, Annals of Botany, 1899, Vol. xm, p. 620. 



2 Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvii, p. 291 ; Ricome, Compt. rend., 1903, 

 T. cxxxvu, cciv. 



3 Cf. Frank, Beitrage z. Pflanzenphysiologie, 1868, p. 80; Noll, Heterogene Induction, 1892, 

 p. 22 ; Hochreutiner, Actes du Congres Botanique de Paris, 1900, p. 39; Massart, 1. c., 1902, p. 31. 



* Czapek, Sitzungsb. d. Wien. Akad., 1895, Bd. civ, I, p. 1231. 



5 Czapek, 1895, 1. c., p. 1213; Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 244. Cf. also Schober, 

 Bot. Ztg., 1897, p. 7. 



