8 
GEORGE F. ATKINSON 
tissue, but by the parallel course of the crowded hyphae, which now 
form a more extensive area. The primordium of the pileus margin 
into which the hymenophore merges is also distinct. The hyphae are 
parallel and epinastic. It can be seen here, and in still older carpo- 
phores, that the radial and parallel arrangement of the hyphae, and 
their deeper stain, progresses centripetally, i. e., toward the center of 
the upper surface of the young pileus. This is brought about because 
successive areas in the same direction in the pileus primordium 
gradually assume active growth, new hyphae are formed in abundance 
which extend radially, and curve downward under the influence of 
epinasty. The advance hyphae penetrate through the fundamental 
tissue. Some of them can be seen to grow into the inner portion of 
the external fundamental tissue. 
Character of the hyphae in the young hymenophore primordium. — 
Here, as described for Agaricus campestris, many of the earlier hyphae 
of the young hymenophore primordium are very slender and sharp- 
pointed. This form permits them more easily to make their way 
through the tissue until they reach the annular cavity. When the 
sharp-pointed hyphae are exposed in the annular cavity, their form 
soon changes, or the new elements developed are different in form. 
The hyphae are now short, blunt at their free ends and form a very 
compact palisade layer. As centrifugal growth of the pileus con- 
tinues at the margin, the new elements arise in the same order as the 
gill cavity is extended. The newer elements growing into the cavity 
are sharp-pointed and often crowded into an uneven palisade layer, 
but the transition to the blunt palisade cells is soon effected. 
In figures 1-3, the youngest carpophores studied, the internal por- 
tion immediately below the constriction, and the annular cavity, is 
more deeply stained than the pileus primordium and the basal part 
of the stem. This activity of the hyphae in this region seems to be 
connected with the primordial differentiation of the stem. This can 
be traced in the older carpophores as seen in figures 2, 3 and 5, where 
the demarcation of the stem surface gradually appears. The stem in 
the very young fruit bodies is thus seen to be very short, its breadth 
exceeding its length for some time. The tissue marking the outline 
of the stem is thus more active and stains more deeply than that of 
the pileus, except in the immediate region of the hymenophore, and 
the margin of the pileus, where growth and the formation of new 
hyphal elements is necessarily active. 
