HELIOTROPISM 473 



experiment, however, there can be no doubt that heliotropic curvature does 

 take place and that too, forward, i. e. in a plane at right angles to the incident 

 ray. According to the hypothesis which regards the inequality of illumination 

 on opposite flanks as the origin of the stimulus in heliotropism, this result is 

 quite intelligible, but we have yet to inquire whether this result really contra- 

 dicts SACHS'S hypothesis. Apparently it does not, since it is possible to imagine 

 that, owing to refraction and reflection in the interior of the cotyledon, the rays 

 may be deflected in a direction at right angles to the course followed outside. 



It is perhaps impossible to arrive at a decision as to which of the two 

 hypotheses is the correct one, because it is scarcely possible to obtain a definite 

 direction of light rays without inducing differences in intensity at the same time, 

 and because differences in intensity cannot be attained without light diffusing 

 from a brighter to a less bright region. It is a matter for individual judgement 

 which hypothesis is to be accepted ; on the ground of analogy (Lecture XXXVII) 

 we lean to the hypothesis based on difference of intensity of light. If that be 

 so, we must assume that the plant has the power of comparing the degree 

 of illumination on different regions. Orthotropic organs perform heliotropic 

 curvatures when opposite flanks are unequally illuminated, which curvatures 

 have the effect of cancelling this difference. Plagiotropic organs, on the other 

 hand, are attuned to unequal illumination. Looking at orthotropic organs 

 only, it is obvious that the liminal stimulus may be exceeded when a certain 

 difference is reached, but this can only be determined by experiment. Such 

 experiments have been carried out by MASS ART (1888). He exposed Phy- 

 comyces in appropriate ways for two hours to unequal illumination on opposite 

 sides, and found that a positive heliotropic reaction took place when the 

 relationship between the two light intensities was at least in the ratio of 

 100 to 118. This ratio was found to be constant for light of varying intensity. 

 Thus MASSART was enabled to prove in the case of heliotropism the validity 

 of WEBER'S law as to the relation subsisting between the amount of the 

 stimulus and sensitivity, a law which we shall have to refer to later on in 

 reference to other stimulus phenomena, and thus he was able to confirm an 

 earlier suggestion made by PFEFFER (1884) . Further investigations are urgently 

 needed, however, since the results we have already arrived at (p. 463) as to 

 this power of adjustment show most clearly that the law can apply only to 

 light of certain intensity. With light of higher intensity no reaction occurs 

 (in the indifferent condition), and obviously also when the degree of illumina- 

 tion on opposite sides reaches higher relative proportions than 100 to 118. 

 In opposition to MASSART'S results, we must expect also the difference in 

 illumination leading to a stimulus movement to vary with the absolute light 

 intensity, but the laws governing this change have yet to be discovered. The 

 answers to very many questions, which cannot even be indicated here, will 

 depend on such experimental researches. 



Experiments are certainly not wanting in modern literature which have 

 for their object the determination of the liminal intensity in heliotropism (com- 

 pare p. 462 ; FIGDOR, 1893). They all deal, however, with unilateral light alone 

 and only one special case is considered, viz. how weak the light may be which 

 still induces heliotropic curvature, if the opposite side be shaded as much as 

 possible. 



The latent period of the heliotropic stimulus has already been determined. 

 According to CZAPEK (1898 a) it amounts to 7 minutes in the cotyledons of Avena 

 and in Phycomyces ; 10 minutes in the hypocotyls of Sinapis alba and Beta vul- 

 garis, 20 minutes in the hypocotyl of Helianthus and 50 minutes in the epicotyl of 

 Phaseolus. If one of these organs be unilaterally illuminated for the specified 

 time, heliotropic curvature ensues afterwards in the dark, that is to say, we 

 meet with an after-effect in this case as in geotropism. We are quite ignorant, 



