THE DEVELOPMENT OF SOME SPECIES OF AGARICS 223 
In Figure 2, a slightly older stage, the hyphae at the base are more 
loosely interwoven. This results largely from the increase in the 
length of the threads and contributes to the enlarging of the base of 
the young basidiocarp. As the plant develops the hyphal cells in- 
crease in size until in the stage represented by Figure 3, they average 
4 to 5m in diameter and 10 to 20/x in length. The hyphae near the 
surface are more closel}^ interwoven. 
Pileus Primordium. — In Figure 19 (a higher magnification of a 
portion of the apex of Fig. 2) the apex has increased in breadth by a 
slight spreading of the hyphae and by interstitial growth of its ele- 
ments. There is no differentiation in staining, but the whole structure 
has the appearance of very active growth. In a little later stage, 
Figure 20, the growth direction of these apical threads is out and up- 
ward, with a slight tendency, of the lateral ones, to epinasty. At 
the same time profuse branching takes place which supplies new ele- 
ments that interlace and ramify among the older ones. Thus, in a 
longitudinal section, a weft of closely interwoven hyphae is presented, 
except at the periphery where the terete ends of hyphae, advancing 
in growth, project (Figs. 20 and 21). This divergent growth of the 
hyphae at the apex of the young fruit body marks the origin of the 
pileus and differentiates it from the stipe primordium. The origin 
of the pileus primordium corresponds very closely to that described 
by DeBary in Nyctalis asterophora, N. parasitica (6), and CoUybia 
dryophila (8, 9). 
Further Differentiation of the Pileus. — By continued growth of the 
primordial elements the pileus is delimited from the stipe in the fol- 
lowing manner: The central hyphae continue to grow toward the 
surface, some curving outward more than others. This growth is 
accompanied with vigorous branching and interlacing of hyphae, 
which add new elements. In this way the inner structure of the pileus 
is formed (Figs. 3-6). At the same time the hyphae, by branching 
and growing in a radial direction, accompanied by strong epinasty, 
curve downward and form the margin of the young pileus (Fig. 22). 
By this radial and downward development of the margin of the pileus 
an annular groove, Figure 6, is formed on the surface of which is the 
hymenophore primordium. Figures 3-6 show the gradual development 
from the primordial condition of the pileus to that stage in which it is 
well differentiated from the stipe. 
The pileus continues to increase in thickness and diameter in a 
