AGARICUS ARVENSIS AND A. COMTULUS 
21 
DESCRIPTION OF PLATES I AND II 
The photomicrographs were made as follows: Figs. 1-6 with an extension 
camera and Zeiss lenses, X 15 diameters. Figs. 8-1 1 were made with a Zeiss 
microscope, the object being 370 mm. from the sensitive plate; figs. 7, 10 and 16 
with ocular no. 4 and objective no. 16 mm.; fig. 8 with ocular no. 8 and objective 
no. 3 mm.; fig. 9 with ocular no. 12 and objective no. 16 mm. 
Fig. I. Longitudinal section of very young carpophore of Ag. arvensis showing 
earliest origin of gill slits as two symmetrically disposed light spots, separating 
pileus fundament above from the stem fundament below, indicating a constriction 
between them. External to the fundament of the pileus and stem is the fundament 
of the "universal veil," or the blematogen layer. It is easily recognized in this 
figure by the more open mesh of its tissue compared with the denser tissue of the 
pileus and stem fundaments, and stains darker because the thick walls of the hyphae 
take up the stain readily. The base of the young carpophore is lighter colored than 
the stem fundament indicating that growth is more active in the latter. The rhizo- 
morph is attached to the base. 
Fig. 2. Same in a little older stage, the gill slits are evident, the hymenophore 
primordium is well organized as also the primordium of the pileus margin shown by 
the deeper stain over the gill slits. Note the oblique position of the gill slits rising 
outward and upward, also shown in fig. 3. 
Fig. 3. Same in a still more advanced stage. The pileus margin is more definite 
and the inner limit of the blematogen layer is more distinct. The outline of the stem 
is more distinct showing its present form to be shorter than broad. In figs. 2 and 3 
there is shown the exfoliation of a very thin layer from the carpophore. This may 
represent the primary universal veil, or protoblem, present sometimes on young 
carpophores of Ag. campestris in addition to the blematogen, or it may represent 
merely a dead outer layer of the blematogen which was in contact with the sub- 
stratum; it is difficult to determine this point on carpophores developed in the sub- 
stratum. A similar exfoliating layer is shown in figs. 6 and 10. 
Fig. 4 is a section of a young carpophore of Agaricus arvensis or a closely related 
species, collected in the edge of the forests south of Pontarlier in 1905. If it is not 
Ag. arvensis it is probably Ag. flavescens Gillet, as young carpophores of this species 
were collected, but the number became detached. It differs from fig. I chiefly in 
the very deep stain of the hymenophore primordium, and shows also a dome-shaped 
primordium of the pileus connecting with the primordium of the hymenophore and 
pileus margin, though not so deeply stained. The blematogen layer is very deeply 
stained due to the absorption of the stain by the thickened hypha walls. The section 
was not decolorized to the extent of that of fig. i, but if it were the primordium of the 
hymenophore would stand out strongly as compared to that in fig. i. The reactions 
here are more like those in specimens of Ag. campestris studied. 
Fig. 5. Sections of an older stage of Ag. arvensis than shown in fig. 3. The 
position of the gill "slit" is now reversed, sloping downward. The fundaments of 
the lamellae are beginning to show as low folds. The outline of the surface of the 
stem is very distinct as a downward and outward sloping dark area below the partial 
veil. The surface of the primordial pileus is nearly organized, its elements interlacing 
with the inner layer of the "universal veil," or blematogen which still shows the 
coarser mesh. The partial, or marginal veil shows a section of the blematogen or 
