12 
GEORGE F. ATKINSON 
early organization, stand out so distinctly from the enveloping tissue 
as is shown in the illustration of Agaricus ruhellus by Fayod,^^ Al- 
though, in some cases it appears that the primordium of the pileus, in 
the form of an internal area, central in the upper part of the carpo- 
phore primordium, may be distinguished from the surrounding tissue, 
slightly before or simultaneous with the earliest appearance of the 
hymenophore primordium (see figure 4) by the more active growth of 
the hyphae, and consequently the deeper staining of this area, the 
hyphae are intricately interwoven and show no organization into a 
definite structure. The earliest differentiation which indicates the 
organization of a structure, in the three species which I have studied 
{A. campestris, arvensis and comtulus), is marginal, and is the primor- 
dium of the hymenophore. Since it is impossible in very young 
stages to clearly distinguish between the hymenophore and pileus 
margin, this early marginal growth may share in the organization 
of both structures. Epinastic growth is first manifested in this mar- 
ginal region, and, simultaneous with the progressive development of 
the hymenophore, epinasty is more and more marked. 
Organization of the primordial surface of the pileus. — The organi- 
zation of the pileus structure, judged by the radial and epinastic 
growth of the hyphae, and the clearness with which it can be differ- 
entiated from the external fundamental tissue, is marginal also. While 
it is not separated from the external fundamental tissue, the parallel,, 
radial and epinastic direction of the hyphae enables one to distinguish 
it quite clearly from the interwoven texture of the external funda- 
mental tissue. From this point the organization of the pileus cortex 
seems to proceed to some extent in a centripetal direction, but the 
region over the center is very limited in such young specimens and 
the hyphae of course do not grow from the marginal area toward the 
center. Successive areas of the primorial tissue of the young pileus 
surface, in a centripetal direction, organize new and abundant hyphae 
which grow in a radial direction, and become curved downward by 
epinasty. The subsequent development of the pileus is for the most 
part centrifugal. 
Fundament of the universal veil. — In figures 1-4 of Agaricus arvensis, 
and figures 7 and 10 of Ag. comtulus, external to the fundaments of 
12 Also the structure of the universal veil as figured by Fayod for Agaricus 
ruhellus is very different from anything which I have seen in the species of Agaricus- 
which I have studied. 
