212 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



and somewhat downward, at an oblique angle from the stem axis. 

 These branches at first are more or less isolated, are slender, 

 and make their way through the loose mesh of the ground tissue. 

 As they increase in number they become more or less crowded, 

 and parallel. This internal annular zone of new growth is the 

 primordium of the hymenophore and pileus margin (figs. 3-6). 

 The hyphae are rich in protoplasm and stain deeply in contrast 

 to the hyphae of the ground tissue and of the pileus and stem 

 fundaments. In median longitudinal section of the young 

 basidiocarps, the hymenophore fundament appears as two symr 

 metrically disposed deeply stained areas near the margin of the 

 pileus above the weakly developed annular cavity as shown in 

 FIGURE 3. Its character is more clearly seen in longitudinal 

 "tangential" sections. 



Figure 4 is from a "tangential" section of the same basidio- 

 carp, parallel with the stem axis and through the hymenophore 

 primordium. The light area is the weak annular cavity with 

 shreds of the ground tissue extending across the cavity. The 

 slender hyphae above the cavity are those of the hymenophore 

 primordium. They are parallel, but not very crowded, their ends 

 do not form an even lower surface, but their different lengths and 

 loose arrangement indicate a fimbriate condition of the hymeno- 

 phore primordium. Figure 6 is from a similar section of another 

 basidiocarp. The annular cavity is not so well formed, but the 

 character of the hymenophore primordium is well shown. Figure 

 5 is from a section of the same basidiocarp (as represented in 

 FIGURE 6), also "tangential," but it runs through the stem surface 

 in the angle at the- junction of pileus and stem. The h^^menophore 

 is therefore not continuous in this section, but is shown as two 

 broad deeply stained areas one on either side of the central pileus 

 tissue. 



Origin of the general annular gill cavity. — The rapid increase 

 in the elements of the hymenophore, and their increase in diameter 

 as the primordial stage of the hymenophore changes to the palisade 

 stage, together with the centrifugal and epinastic growth of the 

 pileus margin,^ produces a tension on tlic ground tissue below 



' For a full discussion of the organization and development of the hymenophore 

 and its relation to the annular cavity see Atkinson, Geo. F. Morphology and develop- 

 ment of Agaricus Rodmani. Proc. Am. Phil. See. 54: 309-343. pi. 9-13. 1915. 



