HELIOTROPIC ACTION OF RA YS OF DIFFERENT WA VE-LENGTH 175 



movement in general more than the less refrangible rays. Hence a 

 heliotropic curvature is performed almost as rapidly beneath an ammoniacal 

 solution of copper hydrate, which allows mainly the blue and violet rays to 

 pass through, as in ordinary light, whereas beneath a solution of potassium 

 bichromate, which only allows the less refrangible rays to pass, little or no 

 heliotropic action can usually be excited. 



This applies to green and non-green cells, to cellular and non-cellular 

 or unicellular plants, and to positively and negatively heliotropic organs 1 . 

 The relative efficiency of the different rays is not, however, the same in all 

 plants, and, according to G. Kraus 2 , the positively heliotropic stalks of the 

 perithecial heads of Claviceps microcephala react almost as rapidly under 

 a solution of potassium bichromate as under one of cupr.-ammonia. 

 According to Brefeld 3 , Pilobolus microsporus behaves similarly, and the 

 mixture of yellow and red light exercises nearly as strong a heliotropic 

 action upon the sporangiophore of Pilobolus crystallinus 4 as that from the 

 more refrangible half of the spectrum. Specific differences of this kind are 

 known to exist in other forms of growth and movement, and the curves 

 showing the action of the different rays of the spectrum upon these forms 

 of vital activity need not necessarily coincide with the curve showing their 

 relative heliotropic action. Pilobolus and Coprinus stercorarius behave as 

 regards etiolation and formative activity similarly in yellowish-red light 

 and in darkness, whereas the yellowish-red rays are able to excite a strong 

 heliotropic response in them. On the other hand, both the more and 

 less refrangible halves of the spectrum exercise approximately the same 

 action upon the stalks of the perithecium-heads of Claviceps microcephala in 

 regard both to etiolation and heliotropism. 



In most cases, according to Wiesner 5 , the maximum point on the 

 curve showing the heliotropic action of different rays is reached between 

 the violet and ultra-violet rays. The curve, as measured by the rapidity 

 of the heliotropic response, falls gradually towards the green, sinks to 

 nothing in the yellow 6 , recommences in the orange, and rises to a small 



1 On negatively heliotropic organs cf. Wolkoff, communicated by Hofmeister, Pflanzenzelle, 

 1867, P- 2 99 (aerial roots) ; Sachs, Lehrbuch, 4. Aufl., p. 810 (Ivy) ; Kraus, Bot. Ztg., 1876, p. 505 

 (aerial roots) ; Prantl, Bot. Ztg., 1879, p. 699 (rhizoids of fern prothalli) ; Wiesner, Die helio- 

 tropischen Erscheinungen im Pflanzenreich, 1878, I, p. 53. Sorokin's statement (Bot. Jahresb., 

 1874, p. 214) that Mucor mucedo and a few other fungi are positively heliotropic in blue light 

 (cupr.-ammonia) and negatively heliotropic in yellow light (potassium bichromate) is incorrect 

 according to other observers. Cf. Wiesner, 1. c., II, p. 88'. 



2 G. Kraus, Bot. Ztg., 1876, p. 505. 



3 Brefeld, Unters. iiber Schimmelpilze, 1881, Heft 4, p. 77 ; Grantz, Ueber d. Einfluss d. Lichtes 

 auf d. Entwickelung einiger Pilze, 1898, p. 1 8. 



4 Wiesner, 1. c., II, p. 88. 

 6 Wiesner, 1. c., I, p. 50. 



6 It is worthy of note that the yellow rays exercise a certain influence on growth. Cf. Wiesner, 

 I.e., II, p. ii. 



