1906] ATKINSON—AGARICUS CAMPESTRIS 259 
that of Agaricus arvensis (figs. 21, 22 in Studies Am. fungi, etc.), 
while in Agaricus silvicola and others the lower half of the marginal 
veil is often separated into patches (l. c. fig. 20). In the pasture 
or field forms of Agaricus campestris the marginal veil is thinner, 
but even here its double character is often manifest (fig. 7, /. c.). 
The growth of the pileus which at first is strongly hyponastic 
becomes less so as the pileus expands. The upper surface gradually 
ceases to grow and the extension of the underlying part often tears 
the pileus cuticle into fibrous scales. The growth of the pileus 
gradually becomes epinastic, as the lower area and the hymenophore 
with the gills now grow more rapidly than the middle and upper 
portions, This causes the pileus to become plane, or in old speci- 
mens the margin itself becomes upturned. This peculiarity in the 
growth of the Agaricaceae during the period of elongation was sup- 
posed by some of the earlier botanists?? to be due to the influence 
of light, for it was thought by them to be necessary that the hymenium 
should be turned up to the light. We now know that light is not 
necessary for the growth and ripening of many species. This partial 
eversion of the pileus in many species unquestionably serves a useful 
purpose in providing for the wider distribution of the spores, for 
they are more easily caught by currents of air as they leave the 
hymenium, 
The order in which elongation of the different parts takes 
place is thus different from the order of their initiation in the young 
primordium, As has been shown in these cultivated forms of A gari- 
cus campestris, the organization of the primordia of pileus, stem, 
hymenium, and marginal veil is practically simultaneous by the 
appearance of the hymenial primordium as an internal annular area; 
while the organization of the parts gradually proceeds and is also 
simultaneous to a certain degree. But the period of elongation of 
the parts after they have become organized, while overlapping to a 
certain extent, follows in succession. The marginal veil completes 
its period of elongation first, then the stem, followed by the pileus, 
and finally the hymenium. 
_ One striking feature of the hymenium of these cultivated forms 
is that, so far as I have examined (the varieties Columbia, Alaska, 
27 See NEES VON ESENBECK, Das System der Pilze und Schwamme 179-187. 1816. 
