HELIOTROPISM 171 



tactically when the light passes beyond a certain intensity. A similar 

 change is also shown by the radial organs of various plants, for the 

 filaments of Vaucheria and Phycomyces^ the seedling stems of Lepidium and 

 other plants grow towards the light when of moderate intensity, but as the 

 latter increases pass through positively plagiotropic, diatropic, and negatively 

 plagiotropic positions, finally assuming a negatively parallelotropic position 

 of equilibrium 1 . Changes of this kind appear in certain plants and in 

 swarm-spores when the light is of moderate intensity, but in other plants 

 only when the intensity is considerable, and they may not be shown if the 

 light has to be so concentrated that the plant is rapidly injured. This 

 applies to many plants, and in others the deviation from the positively 

 heliotropic position of equilibrium is only produced by light of an intensity 

 which is not reached under natural conditions. The tendrils of Vitis and 

 AmpelopsiS) on the other hand, react positively heliotropically, according to 

 Wiesner 2 , only when the light is feeble, and negatively heliotropically even 

 when only moderately strongly illuminated on one side. It does not follow, 

 however, that every negatively heliotropic organ will show positive helio- 

 tropism when the light is weak enough. Nor is it surprising that the state- 

 ments as to phototropic reactions and the phototropic positions of equilibrium 

 should not always agree, for the tropic condition of tone varies according 

 to the stage of development and the other external conditions 3 . 



After it had been shown by N. J. C. Miiller 4 in the case of seedlings of Lepidium, 

 by Stahl 5 in that of Vaucheria, and by Berthold 6 in certain marine algae that the 

 positive heliotropic position was changed to a plagiotropic or negatively phototropic 

 one under strong illumination, Oltmanns carried out more extended researches on this 

 phenomenon, using at first sunlight 7 and later employing a strong arc- light as a source 

 of illumination 8 . These experiments showed that the sporangiophore of Phycomyces 



1 The positive movement is that towards the source of illumination, the negative the one away 

 from it. Oltmanns (Flora, 1897, p. 7) regards the transversal position as an indifferent one, but 

 there can be no doubt that it is as much the result of a stimulatory reaction as any other. 



2 Wiesner, Die heliotropischen Erscheinungen, 1880, II, p. 38. 



3 According to Oltmanns (1. c.), the young sporangiophores of Phycomyces react positively 

 heliotropically to light of an intensity that causes the old sporangiophores to assume a transverse 

 or negatively heliotropic position. In other words, the old sporangiophores are adapted to light of 

 feeble intensity. 



* N. J. C. Miiller, Bot. Unters., 1872, Bd. I, p. 57. 



5 Stahl, Bot. Ztg., 1880, p. 412; Bot. Centralbl., 1882, Bd. xn, p. 142. Cf. Oltmanns, Flora, 

 1892, p. 214. 



6 Berthold, Jahrb. f. wiss. Bot., 1882, Bd. xin, pp. 574 ff. 



7 Oltmanns, Flora, 1892, p. 214. On the gradation of the intensity of the light by the inter- 

 position of cells containing diluted indian ink cf. Oltmanns, I.e., p. 183, and Jahrb. f. wiss. Bot., 

 1892, Bd. xxin, p. 416. 



* Oltmanns, Flora, 1897, p. i. For details of the methods cf. Oltmanns, 1. c. On the removal 

 of the heat-rays see also Pfeffer, Jahrb. f. wiss. Bot., 1900, Bd. XXXV, p. 711. On the use of lamps 

 and gas-flames cf. Wiesner, 1. c., 1878, I, p. 35. A heliotropic curvature is readily produced by 

 covering the plant with a black cover having a slit or hole on one side. Cf. Sachs, Flora, 1895, 

 p. 293- 



