COMBINED ACTION OF GEOTROPISM AND HELIOTROPISM 477 



established. In another memoir Czapek (1898) has shown that, in the case of 

 Avena and Lepidium, the latent geotropic period is twice as long as the latent 

 heliotropic period, the former being 15' the latter 7'. It may therefore be 

 affirmed that, under given conditions, these plants are less sensitive to geotropic 

 than to heliotropic stimuli. There is another important point which comes out 

 on a critical examination of these results, viz., ' the moment of the commence- 

 ment of the negative geotropic reaction in seedlings which had been curved 

 backwards heliotropicalty was naturally considered as that at which the angle 

 with the horizontal began to decrease.' Geotropic curvature, in other words, was 

 observed when it had overcome the heliotropic after-effect. Obviously it would 

 have been more scientifically correct to compare the experimental plants with 

 controls which had been heliotropically stimulated for the same length of time 

 and then rotated in the dark on a klinostat. The moment at which the 

 curvature of the seedhngs which had been geotropically stimulated took place 

 after the initiation of the same movement in the plants on the klinostat would 

 then have been the moment of the initiation of the geotropic movement. It is 

 very probable that no delay in the initiation of the geotropic curvature would 

 then have been noticed. 



We believe that only one conclusion can be drawn from these experiments, 

 a conclusion already suggested by the difference in the latent periods, viz. 

 that in Avena and Lepidium, of two stimuli of equal duration, one geotropic 

 and the other heliotropic, the latter has the greater effect. Hence a geotropic 

 reaction in full swing is soon overcome by a heliotropic, whilst geotropism 

 exerts only a gradual influence on heliotropic activity. Czapek' s assertion 

 that subsequent heliotropism influences geotropism appears to us to be entirely 

 without foundation. It must also be noted that Czapek himself did not observe 

 it to occur universally. In plants like Helianthus, which responds more 

 rapidly to geotropic than to heliotropic stimuli, no retardation of the geotropic 

 reaction in consequence of a previous unilateral illumination was noticeable. 

 In Helianthus geotropic curvature began at the same time as in Avena, only it 

 was much more vigorous and hence earlier observable. 



The behaviour of plants subjected to the antagonistic but simultaneous 

 influence of light and gravity is of great interest. Czapek has carried out 

 many experiments on this subject also, in continuation of those previously 

 instituted by Mohl (1851), Muller-Thurgau (1876), Vochting (1888 b), Noll 

 (1892), and others. As a detailed discussion of all these researches would carry 

 us too far, we will limit ourselves to a summary only. 



1. If unilateral light falls on normally orientated plants, many of them 

 place themselves directly in the line of the light rays {Phycomyces, Pilobolus, 

 Vicia sativa), others {Lepidium, Avena) place themselves at a small angle 

 with the path of the incident ray, others again {Phaseolus, Helianthus) bend 

 but slightly from the vertical. The beginning of the heliotropic curving 

 follows in all cases at the same time as it does in plants placed on the klino- 

 stat, but the heliotropic rest position is reached in general at a much later 

 period. 



2. If horizontally directed light falls on a plant already lying horizontally, 

 the final result is almost the same as in i. Phycomyces and Pilobolus continue 

 their horizontal growth, Avena seedlings and those of many other plants 

 form an angle of less than 20° with the horizontal, Helianthus and Ricinus 

 find their rest positions at an angle of under 45°, and Cucnrbita at an angle 

 under 50°. 



3. If light be projected from below in a vertical direction on the plant 

 lying horizontally, Avena and Phycomyces bend at right angles downwards ; 

 others remain horizontal, others again (Helianthus) curve upwards at an 

 angle of under 45°. 



