158 



visible is much less than 2 hours. It is évident from the 

 following figures which I found that in différent plants 

 the geotropic and phototropic stimuli often hâve widely 

 difFering values for definite strengths of curvature: when 

 seedlings of Avena are geotropically stimulated, by being 

 placed in a horizontal position for 15 minutes, then the 

 maximal strength of curvature amounts to 2 m. m. A 

 maximal phototropic reaction of 2 m. m. is obtained by 

 stimulation with 40 M.C.S. ^) seedlings of Sinapis must 

 be stimulated for 10 minutes in order to get a maximal 

 geotropic reaction of 3 m. m., whilst for a phototropic 

 reaction of only 2 m. m. a stimulation with 400 M.C.S. 

 is necessary. If one takes into considération that the 

 experiments with Avena took place at a température of 

 20° C and those with Sinapis at 23^ C, then according 

 to the law of Van 'tHoff the geotropic sensitiveness 

 of Avena and Sinapis are more or less equal: phototro- 

 pically, however, Avena is much more sensitive than 

 Sinapis. From this it is sufRciently évident that one must 

 be careful in making comparisons between geotropic and 

 phototropic reactions, particularly when working with a 

 stimulus of unknown strength. In my own experiments I 

 therefore followed a différent method to that of C o r r e n s, 

 as will be seen below. 



A few later papers hâve appeared on this subject. 

 Czapek^) in his paper on geotropism, published 1895, 

 devoted a chapter to the influence of the withdrawal of 

 oxygen on geotropic phenomena. From the observation 

 bij W ortmann ^) and by Correns*) that seedlings, 

 which had been stimulated geotropically for 6 hours in 



'l Metre-candlepower-seconds. 



^) F. Czapek. Untersuchungen ûber Geotropismus. Jahrb. f. wissensch. 

 Bot. 1895, Bd. XXVII, p. 274. 

 ^) Wortmann. Le, p. 711. 

 *) Correns. l.c, p. 132. 



