Gingko biloba, diagrammatic representation of the relation of pollen tube to the archegonium in the end of the nucellus. pt, pollen tube; o, archegonium. (After drawing by Hirase and Ikeno.)
636. Gymnosperms are naked seed plants.—The pine, as we have seen, has naked seeds. That is, the seeds are not enclosed within the carpel, but are exposed on the outer surface. All the plants of the great group to which the pine belongs have naked seeds. For this reason the name “gymnosperms” has been given to this great group.
Fig. 376.
Spermatozoids of zamia
in pollen tube;
pg, pollen grain;
a, a, spermatozoids.
(After Webber.)
Fig. 377.
Spermatozoid of zamia
showing spiral row of
cilia. (After Webber.)
637. Classification of gymnosperms.—The gingko tree has until recently been placed with the pines, yew, etc., in the order Pinales, but the discovery of the spermatozoids in the pollen tube suggests that it is not closely allied with the Pinales, and that it represents an order coordinate with them. Engler arranges the living gymnosperms somewhat as follows:
Class Gymnospermæ.
| Order 1. | Cycadales; | family Cycadaceæ. | Cycas, Zamia, etc. |
| Order 2. | Gingkoales; | family Gingkoaceæ. | Gingko. |
| Order 3. | Pinales (or Coniferæ); | family 1. Taxaceæ. | Taxus, the common yew in the eastern United States, and Torreya, in the western United States, are examples. |
| family 2. Pinaceæ. | Sequoia (redwood of California), firs, spruces, pines, cedars, cypress, etc. | ||
| Order 4. | Gnetales. | Welwitschia mirabilis, deserts of southwest Africa; Ephedra, deserts of the Mediterranean and of West Asia. Gnetum, climbers (Lianas), from tropical Asia and America. | |