i] REACTIONS TO STIMULI 33 



or Polystictus grow out as horizontal plates. Hasselbring found that the fruit 

 bodies of Polystictus cinnabarinus , when rotated on a klinostat during their 

 development, were no longer differentiated into a dorsal sterile and a ventral 

 fertile region, but that hymenial tubes were formed over the whole surface. 

 Alike in the Hydnaceae, Agaricaceae and Polyporaceae the trama plates 

 of the fertile region are positively geotropic. In stemless forms this reaction 

 is responsible for the orientation of the hymenium ; it may be particularly well 

 seen in some of the Hydnaceae where the spines grow downward whatever 

 the orientation of the sporophore, and it could doubtless be demonstrated 

 also in Tremellodon among the Tremellaceae. In stipitate species it appears 

 as a supplementary reaction coming into play if further adjustment is needed 

 when the stipe is fully grown. The limitations of the method in the gill- 

 bearing fungi are obvious for, if the pileus is oblique and the gills undergo much 

 curvature, they become crowded together and interfere with spore dispersal. 



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