n of heat.
Moreover, the importance of maintaining a gentle bottom heat, at the roots
of forced plants, renders it necessary to avoid any application, which may
tend to lesson its effect, and submit the roots to any chilling influence.
The temperature of the soil is naturally above that of the atmosphere, and
as the application of moisture by exciting evaporation, has an abstract
tendency to lower the temperature, it should therefore, when applied, be
in a slight degree warmed, so as thus to increase rather than diminish the
heat contained in the soil.

As some moisture in the soil is necessary to render the food contained
therein, soluble, and available to the spongioles of the roots, so
moisture in the atmosphere is essentially necessary to assist in applying
the gaseous elements of that elastic compound fluid, to the nutrition of
plants by the action of the leaves: without moisture in the atmosphere,
the leaves and outer covering of plants would become dessicated, and the
stomatas shrivelled up and closed, so that neither the exhaling nor the
imbibing functions of the plants could then be carried on.

The moisture of the atmosphere, then must not be neglected; not only
because the healthy action of the vital organs of the plants depends on a
proper hygrometrical state of the atmosphere, but, inasmuch as it is the
readiest means both of avoiding, and when unhappily, they are present, of
destroying, many of the most destructive and troublesome insect enemies,
to whose depredations, plants are subject.

When a moist atmosphere is duly and regularly maintained, there is but
little fear need be entertained of the establishment of a colony of
insects--such as the thrip, and the red spider, which are perhaps the
greatest pests which have to be overcome in the forcing house; nor is
there a more effectual method of destroying them, than by applying a high
temperature in conjunction with an intense degree of moisture. To the want
of a balance of moisture in the composition of the atmosphere, and in the
soil, too, rather than as is commonly supposed, to an excess of it in the
former, is the appearance called mildew to be attributed; this it
occasions by checking the regular action of the perspiratory organs, and
thereby inducing an eruption of the cells of the tissue: the extravasated
sap lodging on the cuticle, affords a nidus for the germination of the
sporules of that particular fungus, which when grown, is the mildew: the