accumulation of manure trodden down in these places and
sometimes mixed with sawdust or earth, provided a congenial place for
the growth of the mycelium. The spawn was likely introduced here through
spores taken in with the food of the horse, or brought there from
highways, if they were not already in the soil from mushrooms grown
there. It would be then multiplied by the growth of the spawn, and from
spores of mushrooms which might appear and ripen. The well tramped
material in which the mycelium grew here, when broken up, formed
convenient blocks of spawn for storage and transportation, and probably
led to the manufacture of brick spawn.

=Manufactured spawn.=--The manufactured spawn, on the other hand, is
that which is propagated artificially by the special preparation of the
substratum or material in which the mycelium is to grow. This material
is inoculated either with a piece of natural spawn, or with pieces of
previously manufactured spawn. It is put upon the market in two
different forms; the brick spawn, and the flake spawn. The latter is
sometimes known as the French spawn, while the former, being largely
manufactured in England, is sometimes spoken of as the English spawn.

[Illustration: FIGURE 233.--Brick spawn. Three "bricks," one marked to
show into how many pieces one brick may be broken.]

=Brick spawn.=--The brick spawn is so called because the material in
which the mycelium is present is in the form of bricks. These bricks are
about 5 by 8 inches by 1-1/2 inches in thickness, and weigh about 1-1/4
pounds each when dried. The proportions of different kinds of material
used in the manufacture of brick spawn probably vary with different
manufacturers, since there is a difference in the size and texture of
bricks from different sources. One method of making the brick spawn is
as follows: Equal parts of horse dung, and cow dung, and loam soil are
thoroughly mixed together to a consistency of mortar. This is pressed
into the form of bricks and stood on edge to dry. When partly dry, a
piece of spawn about an inch in diameter is pressed into one side of
each brick. The bricks are then stood up again until thoroughly dried.
They are then piled upon a layer of fresh horse manure about 8 inches
deep, the pile of bricks being about 3 feet high. This pile is then
covered over loosely with fresh horse manure, a sufficient amount to
produce, when heating, a temperature of about 100 deg. F. They are left in
this c