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 all 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 (_Cycadeæ_); II., conifers (_Coniferæ_); III., joint firs (_Gnetaceæ_). 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._), × ¼. _B_, leaf of _Gingko_, × ½. _C_, branch of hemlock (_Tsuga_), with a ripe cone, × 1. _D_, red cedar (_Juniperus_), × 1. _E_, _Arbor-vitæ_ (_Thuja_), × 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 at the north. The European yew and the curious Japanese _Gingko_ (Fig. 78, _B_) are sometimes met with in cultivation.