462 University of California PubUoations m Botrniy [Vol. 7 



Those "gills" which did arise under these conditions, however, showed 

 no departure from the process as described by the writer above. In 

 all cases observed the plates have been separated at the edges from 

 the very first. In later stages they may be separated only a short dis- 

 tance toward the pileus, or they may be split to any depth in the 

 tramal hyphae or pileus, or even completely through the flesh of the 

 pileus. Each "gill," after it unites with the pileus margin, is split 

 more deeply at the outer end, and the depth decreases toward the stipe 

 end, as does likewise the size of the ' ' gill. ' ' 



The splitting or loosening of the hymenium to permit the origin 

 of the paired hymenial plates is due to the same tensions which cause 

 the "gills" themselves later to be split to different depths. The addi- 

 tion of new elements to the hymenial layer does not keep pace with 

 the growth of the hyphae beneath the hymenium. When the hyme- 

 nium has attained a certain width the tension upon the, closely crowded 

 elements of the hymenial layer is so great that it is either split longi- 

 tudinally near the center of the area, or the palisade elements are 

 loosened, grow outward a short distance, and then split. Likewise 

 the gill plates are split apart to different depths due to the tensions 

 set up by these differences in the rate of growth at different regions 

 of the sporophore. The most rapidly growing region is at the margin 

 of the pileus, and, while growth may and actually does take place 

 throughout the sporophore, it decreases in rapidity from the periphery 

 to the place' of attachment of the stipe. Thus the gill plates are 

 longer at the periphery, but the difference between the rates of growth 

 of the hymenium and subhymenial and tramal layers is just as pro- 

 nounced. As a result the hymenial margins are incurved more at the 

 margin of the pileus and the gill plates are gradually drawn apart. 



In some young sporophores the two hymenial margins are separated 

 by a considerable layer of sterile surface which is level with the hyme- 

 nium (fig. 1, pi. 60). In this case there is not even a resemblance to 

 gills. 



From the evidence at hand it seems that the so-called "gills" of 

 Schizophyllum commune Fries are such by analogy only, being act- 

 ually two adjacent edges of hymenial areas which arise together, but 

 which become continuous with and are homologous to the margin of 

 the pileus. They increase the area of the hymenium as do gills, and 

 when in a moist condition, look much like typical gills ; but each plate 

 is independent of the other from the first, increasing in size by mar- 

 ginal growth. 



