l pteridophytes. The pollen spore bursts its
outer coat, and sends out a tube which penetrates for some distance
into the tissue of the ovule, acting very much as a parasitic fungus
would do, and growing at the expense of the tissue through which it
grows. After a time growth ceases, and is not resumed until the
development of the female prothallium and archegonia is nearly
complete, which does not occur until more than a year from the time
the pollen spore first reaches the ovule. Finally the pollen tube
penetrates down to and through the open neck of the archegonium, until
it comes in contact with the egg cell. These stages can only be seen
by careful sections through a number of ripe ovules, but the track of
the pollen tube is usually easy to follow, as the cells along it are
often brown and apparently dead (Fig. 77, _G_).

CLASSIFICATION OF THE GYMNOSPERMS.

There are three classes of the gymnosperms: I., cycads (_Cycadeae_);
II., conifers (_Coniferae_); III., joint firs (_Gnetaceae_). All of the
gymnosperms of the northern United States belong to the second order,
but representatives of the others are found in the southern and
southwestern states.

The cycads are palm-like forms having a single trunk crowned by a
circle of compound leaves. Several species are grown for ornament in
conservatories, and a few species occur native in Florida, but
otherwise do not occur within our limits.

[Illustration: FIG. 78.--Illustrations of gymnosperms. _A_, fruiting
leaf of a cycad (_Cycas_), with macrosporangia (ovules) (_ov._), x 1/4.
_B_, leaf of _Gingko_, x 1/2. _C_, branch of hemlock (_Tsuga_), with a
ripe cone, x 1. _D_, red cedar (_Juniperus_), x 1. _E_, _Arbor-vitae_
(_Thuja_), x 1.]

The spore-bearing leaves usually form cones, recalling somewhat in
structure those of the horse-tails, but one of the commonest
cultivated species (_Cycas revoluta_) bears the ovules, which are very
large, upon leaves that are in shape much like the ordinary ones
(Fig. 78, _A_).

Of the conifers, there are numerous familiar forms, including all our
common evergreen trees. There are two sub-orders,--the true conifers
and the yews. In the latter there is no true cone, but the ovules are
borne singly at the end of a branch, and the seed in the yew (_Taxus_)
is surrounded by a bright red, fleshy integument. One species of yew,
a low, straggling shrub, occurs sparingly in the northern states, and
is the only representative of the group