340 ATKINSON— MORPHOLOGY AND [April 23, 



the pileus, while in Amanita the blematogen is finally delimited from 

 the surface of the pileus by a cleavage layer. A double annulus 

 homologous with that of Agarictis rodmCini is often present in cer- 

 tain other species of Agaricus. 



2. The primordium of the basidiocarp is oval in form, and homo- 

 geneous in structure, consisting of intricately interwoven hyphse. 



3. The four primary parts of the basidiocarp, pileus, stem, mar- 

 ginal veil and hymenophore, are first differentiated by the origin of 

 the hymenophore fundament. 



4. The hymenophore primordium arises as an internal, annular 

 zone of new growth toward the upper part of the young basidiocarp. 

 It consists of slender hyphse rich in protoplasm, parallel, and di- 

 rected obliquely downward. The lower outer surface is at first 

 more or less open and uneven, presenting a frayed or fimbriate ap- 

 pearance. By continued growth and m^ultiplication of these hyphse 

 the hymenophore primordium becomes more compact and the under 

 surface becomes even, forming a level palisade zone. Growth of 

 the hymenophore proceeds in a centrifugal direction, the older por- 

 tions being next the stem fundament. By the epinastic growth of 

 the pileus margin the hymenophore takes on the form of an annular 

 arch. 



5. The increase in number and diameter of the elements of the 

 hymenophore fundament produce a tension upon the ground tissue 

 beneath, which lags behind in growth and is torn away from the 

 under surface of the hymenophore, thus forming an annular, pre- 

 lamellar cavity. This cavity may later be nearly filled by the ground 

 tissue of the inner portion of the veil which increases in bulk, and 

 is often crowded up against the young gills by the involute margin 

 of the pileus. 



6. The lamellse originate as downward growing radial salients 

 of the level palisade zone, beginning next, or on the stem, according 

 as the hymenophore primordium is free from or extends down on 

 the upper portion of the stem fundament. They progress in a cen- 

 trifugal direction. In an intermediate stage of development or the 

 basidiocarp, all three stages of the hymenophore may be present, 

 the zone of gill salients next the stem, then the level palisade zone, 

 and beyond this the primordial zone. 



