54 STIMULATIVE INFLUENCES. 



independently as Rhizomorpha, but subsequently assigned to the 

 cycle of development of Aijaiicus meJhus, turned away from 

 the light, though Brefeld (III.) was unable to confirm this 

 behaviour. E. Chr. Hansen (XXII.) made us acquainted, in 

 1897, with three new examples of negative heliotropism, in 

 species of the families Coprinus and Atjaricus. With regard to 

 all the remaining fungi examined for their sensitiveness to 

 light and found to be exclusively positive in their heliotropism — 

 such, for example, as the conidiophores of Pezi'^M FurJieliana 

 examined by G. Winter (I.), the sporangial hyphte of Muror 

 mucedo, Phycomyces nitens, and a species of Pilobolus, examined 

 by Kraus (I.) and Vines (I.), and the stalk of CopriJius lapojms 

 examined by Brefeld (III.) — an advance was then made by 

 separately examining the influence of the different colours of 

 the spectrum on growth. No uniform results, however, were 

 obtained ; for, whereas Fischer von Waldheim (I. ) found that 

 Pilobohis cridaUinus was only heliotropic under blue light, both 

 its congener, PliiluhoJus micwsporiis, and Muror mucedo are also 

 sensitive to yellow light, according to Brefeld (IY.) and Regel 

 (I.). The further researches of Wiesner (I.), in contradiction 

 to those of Fischer von Waldheim, show that PiJobolux cridal- 

 lirms and Coprinus niveus still continue to turn helioti'opically, 

 even in the ultra-red rays. We are indebted to Friedr. Olt- 

 MANNS (I.) for the settlement of this discrepancy, and also for 

 raising the considei-ation of this phenomenon to a higher plane 

 than before. His researches were performed on Phijcomyces 

 nitens ; that is to say, the very fungus that had hitherto been 

 regarded as decidedly positive in its heliotropism, and the one 

 chiefly used in lecture demonstrations on account of the unusual 

 sensitiveness of its long sporangial hyphse to light. By using a 

 very powerful electric arc light (up to 5300 Hefner units), 

 Oltmanns found that the fungus in question behaved posi- 

 tively phototropie under weak illumination, but negatively so 

 under a powerful light, whilst at an intermediate stage of 

 illumination it remained aphototropic. The universal law of 

 stimulants thus applies also to the phototropy of fungi, the 

 sign — to speak mathematically — of the stimulative effect being 

 determined by the strength of the infliience. However, the 

 degree of stimulation necessary to the production of a given 

 effect is also dependent on the actual condition of vitality of the 

 individual under examination, age, in particular, being an 

 important factor. Thus, in the case of Phyromijces nitens, a given 

 degree of illumination causes attraction in the young sporangial 

 hyphre (with just grey sporangia), whereas in the older ones 

 (with already blackened sporangia) it induces repulsion. The 

 applicability, to the phototropism of fungi, of Weber's law 

 (§ 233) of the I'atio between the degree of stimulation and the 

 effect, has been demonstrated by J. Massart (III.) in the case 



