ng) with moisture; _third_, proper
ventilation; _fourth_, a suitable medium or bed; _fifth_, good spawn. It
may be seen that in the open air, these conditions are rarely found
together for any length of time. It is therefore necessary, in order to
grow mushrooms on a commercial basis, that one or more of these elements
be artificially supplied or controlled. This is usually done in cellars,
caves, mines, greenhouses, or specially constructed mushroom houses. A
convenient disposition of the shelves in a cellar is shown in Figure
498. A large installation for commercial purposes is shown in Figure
500, and a specially constructed cellar is shown in Figure 499. Where
abandoned mines, natural or artificial caves are available, the required
atmospheric conditions are often found combined and may be uniformly
maintained throughout the year.

[Illustration: Figure 499.--Specially Constructed Mushroom Houses.]

[Illustration: Figure 500.--Mushroom Houses, Flat Beds.]

=TEMPERATURE.=--Within the limits prescribed, the temperature should be
uniform throughout the growth of the crop. When too cold, the
development of the spawn will be retarded or arrested. A high
temperature will favor the development of molds and bacteria which will
soon destroy the spawn or the growing crop. The cultivation of the
mushroom, as a summer crop, is therefore greatly restricted. As a fall,
winter or spring crop it may be grown wherever means are at hand to
raise the temperature to about 58 deg. F. Many florists are utilizing the
waste space under the benches for that purpose; they have the advantage
of being able to use the expended material of mushroom beds in growing
flowers.

=MOISTURE.=--Moisture is an important factor in the cultivation of the
mushroom, and demands intelligent application. The mushroom requires an
atmosphere nearly saturated with moisture, and yet the direct
application of water on the beds is more or less injurious to the
growing crop. It is therefore essential that the beds, when made,
contain the requisite amount of moisture, and that this moisture be not
lost by excessive evaporation. They should be protected from a dry
atmosphere or strong draughts. Where watering becomes necessary, it
should be applied in a fine spray around the beds with a view of
restoring the moisture to the atmosphere, and on the beds after the
mushrooms have been gathered.

=VENTILATION.=--Pure air is essential to a healthy crop. Provision