I 7 4 TROPIC MOVEMENTS 



mycelium here and in other fungi appears to possess but little heliotropic 

 irritability. The rhizoids of Marchantia l , of the prothallia of Ferns 2 , and 

 of Equisetum 3 afford, however, instances of unicellular organs which show 

 a negatively heliotropic reaction even to weak illumination, while a similar 

 reaction is shown by Vaucheria and by the sporangiophores of Phycomyces 

 when the light is intense. The non-cellular fronds of Caulerpa and Bryopsis *, 

 as well as the internodes of Char a and Nitella 5 , react in the same way as 

 Vaucheria, and show positive heliotropism in ordinary light. Algae in 

 general, which are not adapted to high intensities of illumination, show 

 orienting heliotropic movements of this character 6 . 



Without doubt the heliotropic irritability is more or less dependent 

 upon the stage of development and upon the general external conditions. 

 The peduncle of Linaria cymbalaria is, for instance, positively heliotropic 

 when the flower opens, but later becomes negatively heliotropic, and hence 

 curves so as to press the ripe capsule against the wall, or into a crevice of 

 the rock or wall on which the plant may be growing 7 . In addition, the 

 young internodes of Tropaeolum mafus 8 and of other plants are either 

 positively heliotropic or indifferent, whereas the older internodes assume 

 a positive or negative klinotropic position. We may still term a reaction 

 heliotropic when the change of position is due to the induction of dorsi- 

 ventrality, or to a related modification produced by the unilateral illumina- 

 tion. The change to the klinotropic position of the older internodes of the 

 Ivy indicates, therefore, an alteration of heliotropic irritability ; but, since it 

 may also be produced by changes in other properties, direct experiment 

 is necessary to determine the exact causation of an altered power of 

 response. It is only in a few cases, however, that these requirements have 

 been properly fulfilled. 



SECTION 37. The Heliotropic Action of Rays of Different Wave-length. 



The more refrangible rays are not only more effective in inducing 

 heliotropic curvature, but also influence growth, formative activity, and 



Hefte 3, 6, 7; Zopf, Pilze, 1890, p. 204; Elfving, Einwirkung d. Lichtes auf Pilze, 1890, p. 19; 

 Eidam, Cohn's Beitrage zur Biologic, 1886, Bd. IV, p. aia; Klebs, Jahrb. f. wiss. Bot., 1898, 

 Bd. xxxii, p. 55 (Sporodinia) ; Neger, Flora, 1902, p. 228 (Erysiphe). 



Pfeffer, Arb. d. bot. Inst. in Wiirzburg, 1871, Bd. I, p. 88. 



Leitgeb, Studien iiber d. Entwickelung d. Fame, 1879, p. 7 (reprint from Sitzungsb. d. Wien. 



Ak( 



., Bd. LXXX, Abth. i) ; Prantl, Flora, 1879, P- 6 79- 



Stahl, Ber. d. bot. Ges., 1885, p. 338 ; Buchtien, Bibliotheca botanica, 1887, Heft 8, p. 28. 

 Klemm, Flora, 1893, p. 472 ; Noll, Arb. d. bot. Inst. in Wiirzburg, 1888, p. 467. 

 Hofmeister, Pflanzenzelle, 1867, p. 289 ; J. Richter, Flora, 1894, p. 400. 

 Cf. Oltmanns, 1. c., and Berthold, Jahrb. f. wiss. Bot., 1882, Bd. xn, pp. 573, 581 ; E. Winkler, 

 Kriimmungsbewegungen von Spirogyra, 1902, p. 20. 



7 Hofmeister, 1. c., p. 292. According to Wiesner (Die heliotropischen Erscheinungen, n, 

 p. 72) the peduncles of Helianthemum vulgare behave similarly at flowering and fruiting. 



8 Sachs, Experimentalphysiol., 1865, p. 41 ; Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. n > P- 2 7 r> 



