40 GENERAL CONSIDERATIONS 



found that these hyphae straighten in the drying process, and when 

 revived do not regain their original arrangement. The hyphal arrange- 

 ment in such circumstances appears to be parallel except for the 

 narrow, somewhat interwoven central strand. It seems to us that this 

 observation could be significant in postulating the origin of the diver- 

 gent arrangement in the first place. But regardless of significance, let 

 us return to the hyphal arrangement observed in H. colemannianus. 

 Here, all that is necessary for a primitive type of divergence to de- 

 velop is for the hyphae toward the exterior of the hymenopodial area 

 to grow out in a more orderly pattern through the subhymenium, and 

 thus reduce the distinctness of that layer. This condition would consti- 

 tute primitive divergence. The more this layer develops at the expense 

 of the layers interior to it, the more prominent the character of "diver- 

 gence" becomes. The more the interwoven central strand develops 

 the more the divergence is reduced. The operation of this last principle 

 is well exemplified by certain species of Catathelasma (Armillaria 

 ponderosa, and its relatives ) . In this genus the hyphae of the hymeno- 

 phoral trama is considered to be bilateral (divergent), but in C. ven- 

 tricosa this characteristic can be observed only on immature gills, and 

 the diverging hyphae are in the hymenopodial area. By the time the 

 cap is fully expanded the development of the interwoven central area 

 has progressed to the point of obliterating the original divergence and 

 from simply observing sections of mature gills one would be forced to 

 conclude that the hyphal arrangement of the gill trama was subparal- 

 lel to somewhat interwoven. Smith checked this on Collection 60672 

 of C. ventricosa from central Idaho. In Hygrophorus we assume that 

 the opposite tendency prevailed because in section Hygrophorus, in 

 fresh material, the hyphae can be demonstrated to be clearly diver- 

 gent, and the central strand of more or less interwoven hyphae is re- 

 duced in varying amounts depending on the species. Here again the 

 genetic mechanism for controlling the hyphal arrangement need not 

 undergo much of a change to produce the alterations in arrangement 

 postulated. 



Admittedly this subject of hyphal arrangement in the hymenophore 

 of Hygrophorus sensu lato needs to be studied further in order to eluci- 

 date more fully the arrangement in the species which might be con- 

 sidered intermediate. It is by means of this procedure that the manner 

 of origin of the bilateral type, with which we are here concerned, can 

 best be elucidated. When such a study has been made on a large series 

 of species, a more meaningful interpretation can be made relative to 

 the importance of the character in the recognition of genera. In the 

 meantime, and in view of the similarities of aspects in all three groups, 



