196 TROPIC MOVEMENTS 



The localization of the heliotropic irritability is most readily determined, since 

 the direction and point of application of the light is easily controlled. Thus Darwin 

 and Rothert found that unilateral illumination of the seedling of Panicum produced 

 no curvature when the cotyledon was covered with tinfoil, but that the full curvature 

 of the hypocotyl took place when the cotyledon was exposed, but the hypocotyl 

 wrapped round with tinfoil. These experiments can be performed without injury and 

 without placing the plant under abnormal conditions, and Rothert has shown that 

 the normal power of reaction is not affected by the enclosure in tinfoil. 



According to Vochting 1 , illumination of the lamina of Malva verticillata is able 

 to operate as a directive stimulus to the darkened petiole, causing the upper pulvinar 

 portion to move so that the leaf is placed in a diaphototropic position. Since, 

 however, the petiole is also capable of a heliotropic response, under normal circum- 

 stances its curvature is the result of direct and indirect heliotropic excitation. 

 Czapek 2 finds that darkening of the lamina of Cornus sangumea, Linaria cymba- 

 laria and Viola odorata prevents any phototropic orientation, whereas Rothert 3 was 

 unable to detect any phototropic direction of the leaf-stalk by the lamina of 

 Tropaeolum minus, and the same was found by Krabbe 4 to apply to the leaves 

 of Fuchsia and Phaseolus. Finally, Ewart 5 has shown that the folding together of 

 the leaflets of various Leguminosae in strong light takes place when the laminas are 

 darkened but the pulvini exposed, but not when the laminas are exposed to light 

 and the pulvini darkened. The various factors concerned in the orientation of leaves 

 are by no means clearly determined, and it is not certain whether Czapek 6 is correct 

 in ascribing to the laminas of certain leaves a power of perceiving geotropic stimuli 

 and transmitting them to the leaf-stalk. 



The perception of geotropic stimuli by the apex of the root. Darwin 7 found that 

 decapitated roots lost the power of reaction, whereas a curvature took place when 

 the decapitation followed previous geotropic induction. Although the geotropic 

 irritability is temporarily suspended as the result of injury, the opposition to Darwin's 

 views was largely unjustified, and Czapek 8 showed conclusively that the same results 

 could be obtained in the absence of an injury. The growing apex was caused to 

 grow in a bent glass tube closed at one end so that the apical region was kept 

 permanently at right angles to the growing zones behind, the segments derived from 

 the apical meristem expanding backwardly out of the tube. The seedlings were at 

 first rotated on a klinostat, and then arranged so that the apical region pointed 



1 Vochting, Bot. Ztg., 1888, p. 519. 



2 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxii, p! 274. Further research is needed in this 

 direction. 



3 Rothert, Cohn's Beitrage z. Biologic, 1896, Bd. vn, p. 121. 

 * Krabbe, Jahrb. f. wiss. Bot., 1889, Bd - x * P- 2 5^. 



5 Ewart, Annals of Botany, 1897, Vol. XI, p. 452 seq. The same was found by Oltmanns and 

 by Macfarlane (Flora, 1892, p. 234; Bot. Centralbl., 1895, i, p. 136) to apply to the pulvini of 

 Robinia pseudacacia. 



6 Czapek, 1. c., p. 274. 



7 Darwin, The Power of Movement in Plants, 1880, p. 523. 



8 Czapek, 1. c., 1895, Bd. xxvil, p. 243. The lateral roots behave similarly (1. c., p. 263). 



