he shape of the ovules, and the degree
to which its coverings or integuments are developed.

The pollen spores of many angiosperms will germinate very easily in
a solution of common sugar in water: about fifteen per cent of sugar
is the best. A very good plant for this purpose is the sweet pea,
whose pollen germinates very rapidly, especially in warm weather.
The spores may be sown in a little of the sugar solution in any
convenient vessel, or in a hanging drop suspended in a moist
chamber, as described for germinating the spores of the slime
moulds. The tube begins to develop within a few minutes after the
spores are placed in the solution, and within an hour or so will
have reached a considerable length. Each spore has two nuclei, but
they are less evident here than in some other forms (Fig. 79).

[Illustration: FIG. 80.--Germinating pollen spores of the sweet pea,
x 200.]

The upper part of the pistil is variously modified, having either
little papillae which hold the pollen spores, or are viscid. In either
case the spores germinate when placed upon this receptive part
(stigma) of the pistil, and send their tubes down through the tissues
of the pistil until they reach the ovules, which are fertilized much
as in the gymnosperms.

The effect of fertilization extends beyond the ovule, the ovary and
often other parts of the flower being affected, enlarging and often
becoming bright-colored and juicy, forming the various fruits of the
angiosperms. These fruits when ripe may be either dry, as in the case
of grains of various kinds, beans, peas, etc.; or the ripe fruit may
be juicy, serving in this way to attract animals of many kinds which
feed on the juicy pulp, and leave the hard seeds uninjured, thus
helping to distribute them. Common examples of these fleshy fruits are
offered by the berries of many plants; apples, melons, cherries, etc.,
are also familiar examples.

The seeds differ a good deal both in regard to size and the degree to
which the embryo is developed at the time the seed ripens.

CLASSIFICATION OF THE ANGIOSPERMS.

The angiosperms are divided into two sub-classes: I. _Monocotyledons_
and II. _Dicotyledons_.

The monocotyledons comprise many familiar plants, both ornamental and
useful. They have for the most part elongated, smooth-edged leaves
with parallel veins, and the parts of the flower are in threes in the
majority of them. As their name indicates, there is but on