HELIOTROPISM 173 



endowed with a negative or transversal heliotropism, whereas the longer 

 nutritive roots which descend into the soil show a lessened power of 

 heliotropic reaction 1 . Furthermore, the penetration of the root of Viscum 

 into a host plant is brought about by the negatively heliotropic curvature 

 of the hypocotyl 2 , whereas the adult stem of Viscum, in accordance with 

 its special habit, shows neither geotropic nor heliotropic irritability. In 

 much the same way the heliotropic irritability decreases as we pass 

 outwards from the main trunks of many trees and shrubs to the successive 

 lateral branches. If the heliotropic irritability of a branch increases when 

 its neighbours are removed, it is evident that the dormant irritability 

 was suppressed or partially inhibited by the correlative and autotropic 

 stimuli radiating from the surrounding organs. Very many subaerial 

 runners are almost devoid of heliotropic irritability, changes in their 

 direction of growth produced by alterations in the intensity of the illumina- 

 tion being due to the fact that their geotropic irritability is modified by 

 the action of light. 



Roots which grow normally in the soil are either without any 

 heliotropic irritability or show feeble negative heliotropism, as in the cases 

 of Sinapis alba, Lepidium sativum, and HeliantJius annuus. The roots of 

 A Ilium sativum and Hyacinthus orientalis are, however, feebly positively 

 heliotropic 3 . 



Numerous instances of heliotropism in non-chlorophyllous organs are 

 afforded by fungi. Thus the stalks of the fructifications of Coprinus 

 stercorarius*, of C. niveus 5 , and of Pezisa fuckeliana*, the young stipes 

 of Lentinus lepidens, the perithecia of Sordaria fimiseda 7 , and the stalks of 

 the perithecium-heads of Claviceps microcephala * are positively heliotropic. 

 The same applies to the sporangiophores of Phycomyces nitens, Mucor 

 mucedo, Pilobolus crystallinus, and various other Mucorineae 9 , whereas the 



1 Dutrochet, Ann. sci. nat., 1833, Bd. xxix, p. 413; Wiesner, I.e., 1880, n, p. 76; H. Miiller, 

 Flora, 1876, p. 93 ; Schimper, Bot. Centralbl., 1884, Bd. xvn, p. 274; Die epiphytische Vegetation 

 Amerikas, 1888, p. 53 ; Went, Ann. d. Jard. hot. de Buitenzorg, 1894, Vol. XII, p. 24; Massart, Sur 

 I'lrritabilite" d. plantes superieures, 1902, p. 60 (Fzcus). 



2 Dutrochet, Rech. s. la structure intime, &c., 1824^.115; Wiesner, Sitzungsb. d. Wiener Akad., 

 1894, Bd. cm, Abth. i, p. 436. Keeble, Trans, of the Linnean Soc., 1896, p. 112 (Loranthus). 



3 For the literature and numerous observations see Wiesner, Die heliotropischen Erscheinungen, 

 1880, II, p. 79 ; also F. G. Kohl, Mechanik der Reizkriimmungen, 1894, p. 26. 



* Brefeld, Unters. iiber Schimmelpilze, 1877, Heft 3, p. 96. 



5 Hofmeister, Pflanzenzelle, 1867, p. 289; Wiesner, I.e., 1880, II, p. 89. 



6 Winter, Bot. Ztg., 1874, P- ' 



7 De Bary and Woronin, Beitrage z. Morphol. u. Physiol. d. Pilze, 1870, 3. Reihe, p. 10. 



8 G. Kraus, Bot. Ztg., 1876, p. 505 ; Duchartre, Compt. rend., 1870, T. LXX, p. 779. 



9 Hofmeister, Pflanzenzelle, 1867, p. 289; Vines, Arb. d. bot. Inst. in Wiirzburg, 1878, Bd. II, 

 P- : 33; Wiesner, I.e., n, p. 85; K. Steyer, Reizkriimmungen bei Phycomyces nitens, 1901. Since 

 Pilobolus curves towards the light during development, its sporangia will be thrown in this direction, 

 and can be collected on a glass plate. Noll, Flora, 1893, p. 32. See also Sorokin, Bot. Jahresb., 

 1874, p. 214; Fischer v. Waldheim, ibid., 1875, p. 779; Brefeld, Bot. Unters. iiber Schimmelpilze, 



