246 TROPIC MOVEMENTS 



removed by plasmolysing solutions was a sure indication that the heliotropic and geo- 

 tropic curvatures were produced by unequal growth. De Vries l then found that 

 curvatures which had just begun could be partially removed by the action of 20 per 

 cent, solutions of salt, so that the primary curvature is due to elastic stretching, which 

 is rapidly followed up by growth. De Vries, however, erroneously assumed that this 

 was due to a rise of turgor on the convex side, whereas direct observation affords no 

 evidence of any such rise. According to Noll 2 , the primary curvature is due to an 

 increased extensibility of the cells of the convex side, but at the same time it is 

 possible that the thickening of the wall on the concave side may render this part less 

 capable of extension although the energy of turgor may be the same throughout. 

 In parts which had undergone positively heliotropic curvature, Weinzierl 3 found 

 that the epidermis of the concave side possessed a higher breaking strain and limit 

 of elasticity than the epidermis on the convex side, but it is uncertain whether this is 

 due to changes in the properties or to unequal thickening of the respective cell-walls. 

 Wiesner * supposed that positive heliotropism is due to a rise of elasticity on the 

 shaded side, and to an increase of ductility in the cell-walls, and of turgor in the cells of 

 the illuminated side. Hofmeister 5 also seems to have considered that changes of elastic 

 extensibility took part in the production of heliotropic and geotropic curvature, but 

 his conclusions are vitiated by physical errors and by his inability to discriminate 

 between the results of growth and of strain. 



In some cases no straightening of the curvature can be produced in young organs, 

 and it is not known whether the same effect is given by suddenly killing the geo- 

 tropically or heliotropically curving organs as by plasmolysis. To produce the 

 latter often requires a considerable time, during which readjustment may occur 6 . 

 Slowly curving hooks or tendrils, when suddenly killed during curvature, show no 

 perceptible straightening 7 . Both plasmolysis and death by heat or the action of 

 alcohol may cause a withdrawal of water from the cell-wall, and so produce contrac- 

 tion or compression which does not exist in the intact organ. Correns 8 found, in 

 fact, that when curving tendrils were dropped into alcohol the curvature increased, 

 whereas straightening was shown when the dead dehydrated tendril was returned to 

 water. 



Noll 9 found that the same force produced a greater bending when applied to 

 a stem in the direction of an incipient geotropic curvature than when opposed to it, 



1 De Vries, Landw. Jahrb., 1880, Bd. ix, p. 302. Cf. also Wiesner, Die heliotropischen 

 Erscheinungen, 1880, Bd. II, p. 3 ; Noll, Arb. d. hot. Inst. in Wiirzburg, 1888, Bd. in, p. 516 ; Flora, 

 1895, Ergzbd., p. 82 ; Barth, Die geotropischen Wachsthumskriimmungen der Knoten, 1894, p. 12 ; 

 Kohl, Mechanik der Reizkriimmungen, 1894, p. 67. 



8 Noll, I.e., 1 888 and 1895. 



8 Weinzierl, Sitzungsb. d. Wien. Akad., 1877, Bd. LXXVI, Abth. i, p. 434. 



4 Wiesner (1. c., 1880, Bd. II, p. 20). 



5 Hofmeister, Jahrb. f. wiss. Bot., 1860, Bd. II, p. 265; 1863, Bd. in, p. 88; Pflanzenzelle, 

 1867, p. 287. 



6 Cf. Fitting, Ber. d. bot. Ges., 1902, p. 380. 



7 Cf. Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. xv, pp. 210, 221. 



8 Cf. Ewart, 1. c., p. 221. 



9 Noll, 1. c., 1888, p. 514; 1825, p. 56. Cf. also Pfeffer, Druck- u. Arbeitsleistungen, 1893, 

 p. 417; Kohl, 1. c., p. 73. 



