Fig. 77.—Scotch pine (except E and F). A, end of a branch bearing a cluster of male flowers (♂), × ½. B, a similar branch, with two young female flowers (♀), natural size. C, a scale from a male flower, showing the two sporangia (sp.); × 5. D, a single ripe pollen spore (microspore), showing the vegetative cell (x), × 150. E, a similar scale, from a female flower of the Austrian pine, seen from within, × 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, × 10. H, the neck of the archegonium, × 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 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.
