red flowers,
to attract insects which, as we shall see, are important agents in
transferring pollen from one flower to another.

When we compare the embryo sac (macrospore) of the angiosperms with
that of the gymnosperms a great difference is noticed, there being
much more difference than between the latter and the higher
pteridophytes. Unfortunately there are very few plants where the
structure of the embryo sac can be readily seen without very skilful
manipulation.

There are, however, a few plants in which the ovules are very small
and transparent, so that they may be mounted whole and examined
alive. The best plant for this purpose is probably the "Indian pipe"
or "ghost flower," a curious plant growing in rich woods, blossoming
in late summer. It is a parasite or saprophyte, and entirely
destitute of chlorophyll, being pure white throughout. It bears a
single nodding flower at the summit of the stem. (Another species
much like it, but having several brownish flowers, is shown in
Figure 115, _L_.)

If this plant can be had, the structure of the ovule and embryo sac
may be easily studied, by simply stripping away the tissue bearing
the numerous minute ovules, and mounting a few of them in water, or
water to which a little sugar has been added.

[Illustration: FIG. 79.--_A_, ripe ovule of _Monotropa uniflora_, in
optical section, x 100. _m_, micropyle. _e_, embryo sac. _B_, the
embryo sac, x 300. At the top is the egg apparatus, consisting of the
two synergidae (_s_), and the egg cell (_o_). In the centre is the
"endosperm nucleus" (_k_). At the bottom, the "antipodal cells" (_g_).]

The ovules are attached to a stalk, and each consists of about two
layers of colorless cells enclosing a central, large, oblong cell
(Fig. 79, _A_, _E_), the embryo sac or macrospore. If the ovule is
from a flower that has been open for some time, we shall find in the
centre of the embryo sac a large nucleus (_k_) (or possibly two
which afterward unite into one), and at each end three cells. Those
at the base (_g_) probably represent the prothallium, and those at
the upper end a very rudimentary archegonium, here generally called
the "egg apparatus."

Of the three cells of the "egg apparatus" the lower (_o_) one is the
egg cell; the others are called "synergidae." The structure of the
embryo sac and ovules is quite constant among the angiosperms, the
differences being mainly in t