APPENDIX 417 



Geotropism. Figdor (Ber. d. D. bot. Ges., 1905, Bd. xxm, p. 182) finds that the 

 leaf-sheaths of various grasses perceive and respond to geotropic and heliotropic stimuli, 

 whereas the laminas (Rothert) are irresponsive. 



H. Fitting (Jahrb. f. wiss. Bot., 1905, p. 221) finds that in all cases the maximal 

 excitation is produced in the horizontal position, and not at an angular deviation of 

 135 from the equilibrium position, as is usually supposed. With angles less than 

 30 the excitation decreases somewhat more rapidly than the size of the angle of 

 divergence, but otherwise they are approximately proportional. Plants rotated 

 obliquely once every second show a geotropic curvature as the result of the sum- 

 mation of successive geotropic stimuli. 



Large angles of divergence must differ more to produce unequal results than 

 when the angles are small (Weber-Fechner's law). 



The presentation periods for epicotyls of Vicia Faba and Phaseolus are from 

 6 to 7 minutes, and for hypocotyls of Helianthus from 5 to 6 minutes. 



Rapid intermittent stimulation is not more active than constant stimulation, and 

 the duration of the individual periods of stimulation is almost immaterial. If they 

 bear a ratio of i : 5 to the periods of rest, the response is nearly as rapid as 

 with continuous stimulation. 



The length of the induction-period and the time of response afford no criterion as 

 to the geotropic irritability, which, in sensitive organs, involves a discriminatory power 

 equal to that possessed by heliotropic organs. 



Newcombe (Annals of Botany, 1905, Vol. xix, p. 313) now considers that 

 orthotropic stems and roots are equally stimulated geotropically by similar angular 

 divergences above and below the horizontal, whereas lateral roots when dis- 

 placed curve more readily downwards than upwards. 



Luxburg (Jahrb. f. wiss. Bot., 1905, p. 399) states that in shoots with apical 

 growth (excepting Hippuris] the average rate of growth is slightly retarded on 

 a klinostat, whereas in plants with nodes it is accelerated, and in Tradescantia flumi- 

 nensis more than when the nodes are placed horizontally so that they are under the 

 continuous unilateral action of gravity. In T. virginica, however, growth is unaltered 

 on the klinostat, but accelerated in the horizontal position. 



The action of unilateral geotropic stimuli is, therefore, twofold, involving : 



1. A change in the general rate of growth, 



2. A change in the distribution of the rate of growth ; 

 but the exact relationship between them is uncertain. 



Hering (Jahrb. f. wiss. Bot., Bd. XL, Heft 4) found that the growth of parallelo- 

 geotropic organs was directly retarded in the inverse position. 



Portheim (Sitzungsb. d. k. Akad. der Wiss. Wien, October, 1904) discusses the 

 influence of gravity upon the orientation of flowers. 



The Slatolith Theory of Geotropism. Tondera (Anz. d. Akad. d. Wiss. in 

 Krakau, 1903, p. 512) states that the youngest geotropic internodes of Cucurbitaceae 

 possess no movable starch-grains; but Jost (Bot. Ztg., 1904, p. 277) was unable 

 to confirm this observation. 



F. Darwin (Proc. Royal Soc., 1903, Vol. LXXI, p. 362) confirms Haberlandt's obser- 

 vation that in plants kept at high temperatures, as the starch disappears, the geotropic 



PFEFFER. HI 



