PHOTOTROPISM 13$ 



these two forces. The explanation for this response occurs in 

 the accounts of Pringsheim and Czurda (1927) and van der Wey 

 (1929) and has been confirmed by Buller (1934). All are in 

 accord that this response may be explained by these assumptions: 

 (1) the subsporangium acts as an ocellus, and (2) the red annular 

 area in the base of the subsporangium is the region for light per- 

 ception. Then, when two spots of light become focussed along 

 the basal wall, the one nearest the annular area gives the greater 

 stimulus to the motor region of the stipe. In consequence the 

 stipe bends, and when the nearest spot comes to rest directly on 

 the annular area, heliotropic equilibrium becomes established. 



Among Ascomycetes. The position of the fruiting bodies of 

 many Discomycetes and Pyrenomycetes within or on the sub- 

 stratum and the orientation of their asci may be presumed to be 

 governed by phototropism or by geotropism. Some of these 

 fungi, for example, subterranean species, may be wholly unaf- 

 fected. In fact, little is known about the specific effect of either 

 of these tropic forces on Ascomycetes. This subject constitutes 

 a fertile field for study, especially in connection with leaf-inhabit- 

 ing species. 



As long ago as 1890 the tips of the asci of Ascobohis demidatits 

 were known to bend phototropically. The meager studies sub- 

 sequently made on phototropic responses among disk fungi are 

 assembled and interpreted by Buller (1934). Discomycetes pos- 

 sess hymenia that are plane, concave, or convex. If the hymenia 

 are plane, no structural adaptations are required to enable the 

 ascospores to be discharged without striking some part of the 

 hymenial surface. In certain species with concave or convex 

 hymenia it has been shown that the ends of the asci may be 

 curved phototropically, or else the opercula of the asci may be 

 asymmetrically situated near the apices. By this means ascospores 

 are ejected into the environment and do not lodge on the oppos- 

 ing walls of the fruit bodies. 



In Ascobohis magnificiis and A. stercorarius [Buller (1934)] 

 the tip of the ascus protrudes above the hymenium. This tip, 

 containing the ascospores, curves toward the source of light, and 

 eventually the ascospores are discharged toward the light. In 

 C Maria (Lachnea) scutellata, which normally is plane and nor- 

 mally possesses straight asci, curvature may be induced by uni- 

 lateral illumination. In Aleuria vesiculosa both asci and pa- 



