D4 College of Forestry 
the surface singly or in loose strands. The upward extend- 
ing surface hyphwe become a part of the pileus tissue as 
growth proceeds and do not differ materially from the hyphee 
of the context. The outer hyphe terminating in many loose 
ends often give a villous appearance to the surface. Not 
infrequently minute particles of dirt, bacteria, and oceasional 
spores of other fungi are to be seen among these loosely 
interwoven ends of the surface hyphe. Such foreign matter 
is readily apparent in stained microtome sections. In this 
and similar forms where no surface growth’ oceurs the 
exposed hyphee become darker than those in the tissues 
below, and, as they become dry, become somewhat appressed. 
There is no change, however, in the hyphee themselves or in 
their arrangement (Plate XII, Fig. 2). 
The surface generally is marked by several more or less 
distinct suleations. Bayliss (1908), im experiments on 
Polystictus versicolor (.) Fr., has shown that this sulea- 
tion of the pileus is due to an alternate checking and _ pro- 
motion of growth caused by changes in the amount of 
moisture which, in turn, is dependent upon varying atmos- 
pheric conditions. Where pilei were allowed to “develop 
under uniform temperature and moisture conditions hardly 
any signs of zoning could be observed and the velvety sur- 
face, instead of having the usual ribbed appearance, was 
quite even. The mere changing of conditions of growth so 
as to secure a drier atmosphere was sufficient to cause a 
check in the growth and hence a marked zone in the pileus. 
The writer observed the natural growth of Polyporus 
pargamenus in the field, making measurements at regular 
intervals. After a period of drought, during which growth 
was almost if not entirely arrested, a distinct furrow or zone 
marked the end of the old zone and the beginning of the 
new. When the period of drought had been prolonged for 
a week or more the division line could be seen, in a vertical, 
radial section through the pileus, extending down to the 
hymenial surface. When atmospheric eonditions were un- 
favorable to growth the short hairs forming the velvety 
surface were little or not at all developed. . The concentric 
