eaf ends in a long point (_D_). The ovules are still very small.
As the growth of the tree is resumed in the spring, the flower (cone)
increases rapidly in size and becomes decidedly green in color, the
ovules increasing also very much in size. If a scale from such a cone
is examined about the first of June, the ovules will probably be
nearly full-grown, oval, whitish bodies two to three millimetres in
length. A careful longitudinal section of the scale through the ovule
will show the general structure. Such a section is shown in Figure 77,
_G_. Comparing this with the sporangia of the pteridophytes, the first
difference that strikes us is the presence of an outer coat or
integument (_in._), which is absent in the latter. The single
macrospore (_sp._) is very large and does not lie free in the cavity
of the sporangium, but is in close contact with its wall. It is filled
with a colorless tissue, the prothallium, and if mature, with care it
is possible to see, even with a hand lens, two or more denser oval
bodies (_ar._), the egg cells of the archegonia, which here are very
large. The integument is not entirely closed at the top, but leaves a
little opening through which the pollen spores entered when the flower
was first formed.

After the archegonia are fertilized the outer parts of the ovule
become hard and brown, and serve to protect the embryo plant, which
reaches a considerable size before the sporangium falls off. As the
walls of the ovule harden, the carpel or leaf bearing it undergoes a
similar change, becoming extremely hard and woody, and as each one
ends in a sharp spine, and they are tightly packed together, it is
almost impossible to separate them. The ripe cone (Fig. 75, _A_)
remains closed during the winter, but in the spring, about the time
the flowers are mature, the scales open spontaneously and discharge
the ripened ovules, now called seeds. Each seed (_E_, _s_) is
surrounded by a membranous envelope derived from the scale to which it
is attached, which becomes easily separated from the seed. The opening
of the cones is caused by drying, and if a number of ripe cones are
gathered in the winter or early spring, and allowed to dry in an
ordinary room, they will in a day or two open, often with a sharp,
crackling sound, and scatter the ripe seeds.

A section of a ripe seed (_F_) shows the embryo (_em._) surrounded by
a dense, white, starch-bearing tissue derived from the prothallium
cells, and called th