172 



TROPIC MOVEMENTS 



nitens assumed a diaheliotropic position when exposed to light equivalent to 25,000 

 Hefner lamps, whereas an intensity of 500,000 to 600,000 Hefner lamps was required 

 to produce the same effect on seedlings of Lepidium sativum and of Hordeum 1 . 

 Observations under natural conditions show that the radial organs of many plants 

 assume a positively parallelotropic position when feebly illuminated from one side, but 

 in direct sunlight assume a more or less plagio-phototropic position 2 . The position 

 of many dorsiventral organs alters according to the intensity of the illumination, and 

 although the exact mode in which this altered reaction is produced is uncertain, there 

 can be no doubt that the light-position of leaves, of the prothallia of Ferns, of the 

 thallus of Marchantia, and of the plagiotropic shoots of the Ivy, are mainly the result 

 of a heliotropic reaction. Furthermore, the movement of the chlorophyll-plate of 

 Mesocarpus from the transverse to the profile position is produced as a direct response 

 to the stimulus of light. 



The positively heliotropic reaction of most seedling-stems, and of 

 subaerial stems in general under normal conditions of illumination, is 



obviously a purposeful biological adapta- 

 tion 3 . For in this way the leaves are 

 brought into brighter light and, when 

 endowed with a photometric power of 

 reaction, set their surfaces at right angles 

 to the direction in which the strongest 

 diffuse light falls upon them. Positive 

 heliotropism is also shown by the seed- 

 ling-stems of twiners, whereas the older 

 twining stem, in accordance with its habit, 

 shows only a feeble negative or positive 

 phototropic reaction. Most tendrils are 

 also comparatively indifferent, although 

 a few are aided in approaching and 

 .applying themselves to a support by 

 their negative heliotropism. 



There is also evidence of biological 

 adaption in the fact that attaching aerial 

 roots such as those of Aroids, Orchids, and Hartwegia are usually 



FlG. 37. Seedling of Sinapis alba. 

 The hypocotyl shows a positive, the 

 root in water a negative heliotropic 

 curvature. The arrows show the direc- 

 tion of the incident rays of light. 



1 The brightness of a Hefner- Altenach light corresponds to 1-162 German standard candles. 

 The spermaceti candle used by Wiesner (1. c.) is equivalent to a Hefner- Altenach lamp. Oltmanns, 

 1897, I.e., pp. 2, 20. The cessation of growth and of heliotropic curvature observed by Wiesner 

 with much feebler intensities of light is apparently the result of some accessory action of the gas- 

 flames employed. It must also be remembered that the greatest heliotropic action is exercised by 

 the more refrangible rays, so that the action of the light is not always proportional to its apparent 

 brightness. Cf. also Wiesner, Bot. Centralbl., 1897, Bd. LXIX, p. 305. 



3 Cf. Oltmanns, Flora, 1892, p. 225. 



8 A few facts concerning stems and other organs, as well as references to the literature, are given 

 by Wiesner, Die heliotropischen Erscheinungen im Pflanzenreich, I, 1878; II, 1880 (reprinted from 

 Denkschriften d. Wien. Akad., Bd. xxxix). 



