478 TRANSFORMATION OF ENERGY 



4. If, finally, light from beneath be projected on an inverted plant, e. g., 

 Helianthus, it takes up a rest position at an angle of 45° downwards, while other 

 plants keep on growing vertically downwards. 



It is obvious, therefore, that in certain plants, such as Avena, heliotropism 

 is always the dominant factor, whilst in other cases it is always geotropism that 

 is predominant and such plants are less sensitive to heliotropism. Czapek has 

 shown that, after Avena has taken up its rest position after stimulation by 

 horizontal illumination, first of all an obvious geotropic upward curvature follows. 

 Further he has found the same initial geotropic curvature occurring in hori- 

 zontally laid plants which have been illuminated vertically from below, and 

 this indeed occurs at the normal time and delays the heliotropic downward 

 curvature. In this case one cannot assume that the geotropic curvature 

 is finally overcome by heliotropism, and we must agree with Czapek in 

 believing that the geotropic perception cannot be obliterated. The antago- 

 nism expresses itself in the reaction, although we are still entirely unable to 

 show where it makes itself apparent in particular. Again it is quite possible 

 that in other cases also the perception may be destroyed. 



The observations which have been recorded on the combined action 

 of geotropism and heliotropism do not as yet lead to any satisfactory general 

 conclusions ; they require revision and correction in many respects, because 

 possibly the individual variants in Czapek' s experiments have not been sufficiently 

 excluded. The chief difficulty in all such experiments lies in obtaining equaUy 

 great excitations by different stimuli. The goal aimed at is perhaps quite 

 unrealizable, if different and non-comparable excitations correspond to different 

 stimuli. 



If the problem as to the combined influence of geo- and helio-tropism 

 in orthotropic organs presents great difficulties these are greatly increased 

 when we turn to plagiotropic, and more especially dorsiventral organs ; so much 

 so, indeed, that we will not attempt to enter on a discussion of the phenomena 

 presented by them. 



We have by no means exhausted the movements leading to orientation 

 of plant organs, for there are still quite a number of stimuli which lead to move- 

 ments of this character. That most closely related to light is heat, which 

 also induces a special kind of movement. Since Wiesner (1878) established 

 the occurrence of heliotropic curvatures due to the action of red and ultra-red 

 light, so we may with equal justice speak of thermotropic curvature, for rays 

 which pass through a solution of iodine in bisulphide of carbon are known 

 as dark heat rays. Positive curvatures induced by such rays have been observed 

 by Wiesner in Vicia sativa and in the cress. Wortmann (1883), in a lengthy 

 series of experiments, endeavoured to prove thermotropic curvatures in the 

 sporangiophores of Phycomyces and in seedlings of Lepidium, Linum, Zea, &c. 

 He employed as a source of heat a large vertically placed metal plate, which 

 had been warmed and which gave off the heat rays from a darkened surface 

 facing the plant ; but in repeating Wortmann's experiments Steyer (1901) 

 showed that Wortmann's apparatus was not a suitable one. This latter investi- 

 gator by careful experimental means showed that in the case of Phycomyces 

 no sign of thermotropism was exhibited, and that a seedling of Lepidium 

 was positively thermotropic at high temperatures and negatively so at low. 

 Steyer' s statements as to seedlings are also, however, wanting in detail, 

 without which the subject cannot be considered as presented in a clearly 

 intelligible form. At the same time we know for certain that shoots are 

 positively thermotropic, since Vochting (1890) has proved that the peduncle 

 of Anemone stellata follows the course of the sun only on account of its positive 

 thermotropism. That heliotropism has nothing to do with this reaction is 

 shown by the fact that these movements go on in darkened chambers where 

 the peduncle bends towards that part of the wall of the vessel which is most 



