nd pores are all empty, the soil being
_perfectly dry_; and the canals communicating freely at the surface
with the surrounding atmosphere, the whole will of course be filled
with air. If in this condition, a seed be placed in the soil, as at
_a_, you at once perceive that it is freely supplied with air, _but
there is no moisture_; therefore, when soil is _perfectly dry_, a
seed cannot grow.

[Illustration: Fig. 91.]

[Illustration: Fig. 92.]

"Let us turn our attention now to Fig. 92. Here we perceive that
both the pores and canals are no longer represented white, but
black, this color being used to indicate water; in this instance,
therefore, water has taken the place of air, or, in other words,
the soil is _very wet_. If we observe our seed _a_ now, we find it
abundantly supplied with water, but _no air_. Here again,
therefore, germination cannot take place. It may be well to state
here, that this can never occur _exactly_ in nature, because water
having the power of dissolving air to a certain extent, the seed
_a_ in Fig. 92 is, in fact, supplied with a _certain_ amount of
this necessary substance; and, owing to this, germination does take
place, although by no means under such advantageous circumstances
as it would were the soil in a better condition.

[Illustration: Fig. 93.]

[Illustration: Fig. 94.]

"We pass on now to Fig. 93. Here we find a different state of
matters. The canals are open and freely supplied with air, while
the pores are filled with water; and consequently you perceive
that, while the seed _a_ has quite enough of air from the canals,
it can never be without moisture, as every particle of soil which
touches it, is well supplied with this necessary ingredient. This,
then, is the proper condition of soil for germination, and in fact
for every period of the plant's development; and this condition
occurs when soil is _moist_ but not _wet_--that is to say, when it
has the color and appearance of being well watered, but when it is
still capable of being crumbled to pieces by the hands, without any
of its particles adhering together in the familiar form of mud.

"Turning our eyes to Fig. 94, we observe still another condition of
soil. In this instance, as far as _water_ is concerned, the soil is
in its healt