FUNGI. 



31 



the rudimentary pileus (p l ) which is here nothing 

 more than the upper, somewhat flattened portion of 

 the stipe. Also the case in the same plant, shown in 

 PI. II, fig. 4, where gills are arising from the lateral 

 surface of the upper part of the stipe, is very like a 

 reversion to the ancestral condition. 



All the cases of invagination of the hymenial tissue 

 on to the upper surface, or the congenital outgrowth 

 of inverted sessile caps, probably represent partial 

 attempts to reproduce, in the midst of, and against 

 the tendency of, the modern structure, the ancestral 

 form. There is too great a tendency nowadays, 

 especially among Continental botanists, to ascribe 

 abnormal structures either to pure accident or to the 

 direct action of the environment without regard to 

 any phylogenetic or hereditary causation. This atti- 

 tude it seems desirable to oppose. 



It is one aptly illustrated by the case of Magnus, 

 whose explanation of the phenomena under discussion 

 was that they represent new formations due to the 

 abnormal habitat. He says that they are not due 

 to splittings of the cap or to invaginations ; on the 

 other hand, cases like that of Boudier's Cortinarius 

 are to be attributed to modifications of the inner 

 structure : a mutation. In any case, he will have 

 nothing to do with phylogenetic origins. 



It is true that in the cases of Polyporus cited above 

 the appearance of hymenium on the upper surface 

 was due to the direct action of the environment ; but 

 the very ease with which such a change took place 

 points to the fact that the normally sterile tissue is 

 potentially fertile and has once been normally so. 



Another ancestral feature which reappears when 

 the recently-acquired sterile tissue of the upper surface 

 becomes changed, by means of the two methods above 

 described, into fertile hymenial tissue, is that in the 

 case of the Agarics the gill-tissue becomes, in that 

 position and in the vast majority of cases, alveolar or 

 pore-producing. PI. I, fig. 5, shows this very well, 



