318 ATKINSON— MORPHOLOGY AND [April 23, 



upper outer portion of this new zone of growth, i. e., the hymeno- 

 phore primordium, represents the early stage of the organization of 

 the pileus margin : in other words, the annular internal zone of new 

 growth is to be interpreted as the young primordium of hymeno- 

 phore and pileus margin, the latter including the area from which 

 the new hyphal branches arise as well as the basal area of these 

 branches. Not only is there interstitial growth in the increase of 

 these hyphal branches, the new ones crowding in between the older 

 ones forming a more compact zone, but there is also a centrifugal 

 increase in the periphery of the annular zone. The centrifugal 

 growth of the pileus margin and hymenophore primordium is very 

 characteristic. 



The position and direction of the hyphse of the young hymeno- 

 phore primordium, as well as the increasing density of this area, is 

 well shown in Figs. 10-16. The stem axis of all the figures is 

 parallel with the long axis of the Plate. Several of these figures 

 are highly magnified views of the hymenophore primordium shown 

 in Figs. 5-7; Figs. 10 and 15 being highly magnified views of the 

 hymenophore of Figs. 5 and 6, while Figs. 12 and 16 are highly mag- 

 nified views of that in Figs. 7 and 8. Figs. 10 to 14 are from 

 median longitudinal sections of the basidiocarps. Fig. 10 is from 

 the right-hand side of the stem axis, i. e., the stem axis is at the left. 

 Figs. 11-14 are from the left-hand side of the stem axis, the stem 

 axis therefore being on the right-hand of the figures. The increas- 

 ing density of the elements of the young hymenophore is progres- 

 sively shown in Figs. 10 to 13. With the increasing density the 

 ends of the hyphze reach more and more to the same level and 

 thus tend to form an even surface which forms the transition to 

 the palisade layer. 



Origin of the General Annular Gill Cavity. — A striking feature 

 in all these radial transections of the hymenophore zone and pileus 

 margin is the curved outline of the zone as seen in transection. 

 This is remarkably strong in Figs. 11 and 12 because the young 

 hymenophore primordium extends for a considerable distance down 

 around the apex of the stem fundament. This arched form of the 

 young annular hymenophore zone is the result of epinastic growth 

 of the pileus margin, which is very marked even in this very early 

 stage in the organization. The rapid increase in the number of 



