Page:Popular Science Monthly Volume 80.djvu/206

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and Japan. Of course Ginkgoales continued all the features of Cordaitales that looked towards Conifers, such as the branching habit, relatively simple leaves, and thick vascular cylinder. The monosporangiate cones were also continued, but the most notable feature is the continuation of the swimming sperms ©f Cordaitales. Cordaitales had continued the swimming sperms of Cycadofilicales and ferns; in fact all the vascular plants of the Paleozoic had swimming sperms. One of the most primitive features of the Cycadophyte branch is that it retained throughout this primitive, fern type of male cell. In the present Gymnosperm flora, therefore, Cycads and Ginkgos are distinguished by having swimming sperms, the former having continued them directly from the Cycadofilicales, the latter obtaining them indirectly from the same source, and directly through the Cordaitales. To state the situation in other terms, it may be said that the Cycads have continued primitive vegetative and reproductive structures, while the Ginkgos have retained primitive reproductive structures and have changed the vegetative structures, a change initiated by the Cordaitales.

All of this serves to emphasize the position of the Coniferales, the fourth Mesozoic group, and the dominant Gymnosperm group to-day. It not only continued to change the vegetative structures, but it also abandoned the primitive features of reproduction in abandoning the swimming sperm, which became a relatively passive cell conducted to the egg by a pollen tube. Few persons realize that Gymnosperms in general, in terms of great groups, have swimming sperms, which the pollen tubes do not conduct; for this situation is overshadowed by the fact that the single overwhelming group of Gymnosperms to-day has passive sperms conducted by pollen tubes.

In connection with this change of reproductive habit, the Coniferales during the Mesozoic differentiated into the six great families or tribes recognized to-day, so that it is the one great group of Gymnosperms that developed an extensive range of forms. The historical interest connected with Conifers, therefore, is not so much the origin of the group as a whole, for that seems to be traced clearly to the Paleozoic Cordaitales, as the origin and relative antiquity of its tribes. This question has been answered by the study of vascular anatomy, chiefly by Jeffrey of Harvard, supported by the morphology of the reproductive structures, and by history. The conclusion is that the tribe containing the pines is to be regarded as including the modern representatives of the most ancient Conifers. The only possible contestant for this honor is the tribe comprising the araucarians of the southern hemisphere. The four other tribes (podocarps, taxads, taxodiums and cypresses) are clearly relatively modern. The general conclusion, therefore, is that the Conifer stock became differentiated from the Cordaitales with features characteristic of the pine tribe, and that from