INFLUENCE OF EXTERNAL STIMULI. LIGHT 257 



plant itself has some bearing the points of strong growing shoots will not 

 become rhizoids, but bend in a negatively geotropic manner and continue 

 their growth as organs of assimilation. 



These last cases recall the fact stated above, that in the higher 

 plants failure of light or feeble illumination is favourable to the formation 

 of roots ; in cases where the differentiation of the organs is not yet a sharp 

 one, such illumination may result in the direct transformation of an organ 

 of assimilation into a root or rhizoid. 



(e) INFLUENCE OF LIGHT UPON THE RELATIONSHIPS OF CONFIGURATION OF 



THE FUNGI. 



The formation of organs in dicotyledonous parasites containing no 

 chlorophyll, for example, Orobancheae, Lathraea, Balanophoreae, may be 

 accomplished in the absence of light even to the formation of flower. 

 There is indeed but little room for doubt that the ' flower-forming material ' 

 of the parasites is formed for them by the host-plant. It would be of 

 interest were this to be proved experimentally, and the annual species 

 of Orobanche form specially favourable subjects for the investigation. Is 

 the formation of their flower dependent upon the same factors as is that of 

 the host-plant? It is quite possible that no such dependence exists, as 

 in many plants the course of development is quite independent of light, 

 whilst in others it is strongly influenced by it. It is well known that 

 Agaricus campestris is produced in forcing houses in more or less complete 

 absence from light, and that the Hypogaeae, which are partly Basidiomy- 

 cetes, partly Ascomycetes, in like manner complete their whole development 

 in darkness. Similarly Basidiobclos ranarum l forms its asexual organs 

 (gonidia), as well as its sexual organs, equally well whether it be in light or 

 in darkness. 



We must first of all consider the directive influence of light. 



In Polyporus fomentarius and Daedalea quercina, which live upon 

 trees, the spore-forming hymenium of the fructifications is found normally 

 upon their under side that turned away from the light. If the block upon 

 which the fungus grows be turned upside down 2 , hymenium begins to be 

 formed upon what was previously the uppermost side, whilst the hymenium 

 of the previously under side, now exposed to the light, gradually dies off. 

 There can be no doubt that the position of the hymenium upon the under 

 side, which in the fructifications of other Hymenomycetes is a consequence 

 of inner' causes, is of importance for the distribution of the spores, because 

 the rain cannot affect them there, and the heliotropic movement exhibited 



1 Raciborski, in Flora, Ixxxii (1896^, p. 117. 



2 St. Schulzer v. Miiggenburg, in Flora, 1878, p. 122. The effect of gravity is not considered in 

 this research. 



GOEBEL S 



