476 TRANSFORMATION OF ENERGY 



Oltmanns. 1897. Flora, 83, i. 



Pfeffer. 1884. Unters. a. d, bot, Institut Tubingen, i, 407. 



RoTHERT. 1894. Cohn's Beitr. z. Biol. 7, i. 



Sachs. 1879. Arb. bot. Inst. Wiirzburg, 2, 226. 



Sachs. 1882. Vorlesungen iiber Pflanzenphysiologie. Leipzig. 



ScHWENDENER and Krabbe. 1892. Abh. Berliner Akademie. 



Stahl. 1880. Bot. Ztg. 38, 412. 



Stahl. 1 88 1. Ueber sog. Kompasspflanzen. Jena. 



Vochting. 1888. Bot. Ztg. 46, 501. 



WiESNER. 1878-80. Die heliotr. Ersch. i. Pflanzenreich (Denkschriften d. K.K. 



Akad. Wien, 39 and 43). 

 WiESNER. 1899. Biol. Centrbl. 19, i. 

 WiESNER. 1 90 1. Ibid. 21, 801. 



LECTURE XXXVII 



COMBINED ACTION OF GEOTROPISM AND HELIOTROPISM. 

 THERMOTROPISM AND OTHER TROPISMS 



Having now discussed separately heliotropic and geotropic movements, we 

 have still to glance at the movements which result from the simultaneous or 

 rapidly consecutive stimuli of light and gravity. We will confine ourselves to 

 a discussion of the data which Czapek's (1895) recent researches have given us 

 access to. 



CzAPEK experimented with plants which, like the seedlings of Avena and 

 Lepidium, exhibited a heliotropic curvature under optimum conditions in time 

 and degree, like that of geotropism. He convinced himself that the plants 

 mentioned when placed on the klinostat began to exhibit a curvature, when 

 they were subjected to unilateral illumination, simultaneously with other seed- 

 lings placed horizontally and kept in the dark ; further that the heliotropic 

 curvature progressed in the same manner as the geotropic, and that the 

 maximum amount of curvature (90°) was reached in both cases at the same 

 time. The influence of consecutive stimuli was next investigated. The seed- 

 lings were kept horizontally in the dark until the first trace of geotropic 

 curvature became apparent, when (about an hour after the beginning of the 

 experiment) they were placed vertically and illuminated from one side, so 

 that the ensuing heliotropic curvature might operate antagonistically to the 

 geotropic curvature. Under these conditions a reduction in the geotropic 

 curvature took place exactly at the same time that control plants which had 

 not previously been subjected to geotropic stimulus began to show heliotropic 

 curvature. This experiment, according to Czapek, teaches us that a primary 

 geotropic induction has no effect on a subsequent heliotropic stimulation. 



The case was entirely different if the seedling was first of all heliotropically 

 stimulated and then laid horizontally in the dark m such a way that the side 

 which was more brightly illuminated faced downwards. There was a very marked 

 delay in the initiation of the geotropic reaction as compared with control 

 plants which had not been heliotropically stimulated, and this delay was all 

 the greater the longer the heliotropic stimulus was allowed to influence the plant ; 

 after a stimulus of ten minutes' duration it amounted to a quarter of an hour, 

 after sixty minutes' stimulation the delay amounted to two hours. It would 

 appear, therefore, that the geotropic reaction is affected by heliotropism, 

 although as far as we are concerned the matter must not be taken as proved. 

 Czapek's supposition that the two stimulus reactions, in the case of a solitary 

 induction, take place in exactly the same time has yet to be more definitely 



