326 ATKINSON— MORPHOLOGY AND [April 23, 



ture is not very distinctly shown in in this figure, due to the difficulty 

 of illumination which will produce on the photographic plate the 

 same degree of resolution which can be detected by the eye, still 

 the palisade character is evident. A similar situation is seen in 

 Fig. 18, but the progression in the origin and growth of the salients 

 is to be read from right to left. A somewhat later stage is shown 

 in Fig. 19. Here the hyphal structure is well shown. The palisade 

 character of the exposed surface of the hymenophore is very clearly 

 shown. This figure gives us some suggestion of the factors operat- 

 ing in the formation of the gill salients. The elements of the pali- 

 sade layer increase by interstitial growth, i. e., by new branches 

 which crowd in between the older ones. At the same time the elon- 

 gate cells composing the palisade layer increase in diameter. In 

 the primordial stage they passed from the terete tapering condition 

 to the cylindrical form. Now they pass from the cylindrical to 

 the clavate form, as well as increasing somewhat in diameter 

 throughout. This produces a great pressure on the level palisade 

 zone, which if continued, must result in throwing the level palisade 

 layer into folds. 



Another factor now comes into play which prevents the palisade 

 layer from being thrown into a series of irregular folds. This is the 

 downward growth, by elongation, of the subadjacent tramal hyphse, 

 along regularly spaced radial areas, beginning next the stem and 

 proceeding in a centrifugal direction toward the margm or the 

 pileus. These radial areas of subadjacent tramal hyphse, elongating 

 downwards, push the palisade area downward into corresponding 

 radial salients. These salients are the first evidence of folds or 

 ridges which appear in the young hymenophore. They are the 

 gill salients, and by continued growth form the lamellae themselves. 



Fig. 19 presents another very interesting situation. This is the 

 flaring, or fantailing, of the gill salients very soon after their emer- 

 gence below the level of the general palisade surface. This is very 

 clearly one of the first results of the release from the pressure to 

 which the elongate cells were subject in the level palisade condition. 

 Another still more interesting feature at this stage is the pressure 

 to which the neutral portion of the level palisade is subjected as a 

 result of this fantailing of the gill origins. The flanks of the young 



