se manure, or a mixture of one or two kinds of manure with some soil,
and with the threads of the mycelium growing in it. The mycelium, as is
well known, is the growing or vegetative part of the mushroom.
Sometimes the word "fiber" is used by the mushroom growers in referring
to the mycelium which appears in the spawn, or in the mushroom bed. The
mycelium is that portion of the plant which, in the case of the wild
varieties, grows in the soil, or in the leaf mold, in the tree trunk or
other material from which the mushroom derives its food. The threads of
mycelium, as we know, first originated from the spore of the mushroom.
The spore germinates and produces delicate threads, which branch and
increase by growth in extent, and form the mycelium. So the term spawn
is rarely applied to the pure mycelium, but is applied to the substratum
or material in which spawn is growing; that is, the substratum and
mycelium together constitute the spawn.

=Natural spawn or virgin spawn.=--This is termed natural spawn because
it occurs under natural conditions of environment. The original natural
spawn was to be found in the fields. In the early history of mushroom
culture the spawn from the pastures and meadows where mushrooms grew was
one of the sources of the spawn used in planting. The earth containing
the spawn underneath clumps of mushrooms was collected and used.

It occurs more abundantly, however, in piles of horse manure which have
stood for some time in barn yards, or very often in stalls where the
manure is allowed to accumulate, has been thoroughly tramped down and
then has been left in this condition for some time. It occurs also in
composts, hothouse beds, or wherever accumulations of horse manure are
likely to occur, if other conditions are congenial. The origin of the
natural spawn under these conditions of environment is probably
accounted for in many cases by the presence of the spores which have
been in the food eaten by the horse, have passed through the alimentary
canal and are thus distributed through the dung.

The spores present in the food of the horse may be due to various
conditions. Horses which go out to pasture are likely to take in with
the food obtained in grazing the spores scattered around on the grass,
and in the upper part of the sod, coming from mushrooms which grew in
the field. In other cases, the spores may be present in the hay, having
been carried by the wind from adjacent fields, if not from