(except _E_ and _F_). _A_, end of
a branch bearing a cluster of male flowers ([Male]), x 1/2. _B_, a similar
branch, with two young female flowers ([Female]), natural size. _C_, a
scale from a male flower, showing the two sporangia (_sp._); x 5. _D_,
a single ripe pollen spore (microspore), showing the vegetative cell
(_x_), x 150. _E_, a similar scale, from a female flower of the
Austrian pine, seen from within, x 4. _o_, the sporangium (ovule).
_F_, the same, seen from the back, showing the scale (_sc._) attached
to the back. _G_, longitudinal section through a full-grown ovule of
the Scotch pine. _p_, a pollen spore sending down its tube to the
archegonia (_ar._). _sp._ the prothallium (endosperm), filling up the
embryo sac, x 10. _H_, the neck of the archegonium, x 150.]

The ripe pollen spores (Fig. 77, _D_) are oval cells provided with
a double wall, the outer one giving rise to two peculiar
bladder-like appendages (_z_). Like the microspores of the smaller
club mosses, a small cell is cut off from the body of the spore
(_x_). These pollen spores are carried by the wind to the ovules,
where they germinate.

The wall of the ripe sporangium or pollen sac is composed of a
single layer of cells in most places, and these cells are provided
with thickened ridges which have to do with opening the pollen sac.

We have already examined in some detail the structure of the
macrosporangium or ovule. In the full-grown ovule the macrospore,
which in the seed plants is generally known as the "embryo sac," is
completely filled with the prothallium or "endosperm." In the upper
part of the prothallium several large archegonia are formed in much
the same way as in the pteridophytes. The egg cell is very large,
and appears of a yellowish color, and filled with large drops that
give it a peculiar aspect. There is a large nucleus, but it is not
always readily distinguished from the other contents of the egg
cell. The neck of the archegonium is quite long, but does not
project above the surface of the prothallium (Fig. 77, _H_).

The pollen spores are produced in great numbers, and many of them fall
upon the female flowers, which when ready for pollination have the
scales somewhat separated. The pollen spores now sift down to the base
of the scales, and finally reach the opening of the ovule, where they
germinate. No spermatozoids are produced, the seed plants differing in
this respect from al