202 THE POPULAR SCIENCE MONTHLY 



and Japan. Of course Ginkgoales continued all the features of Cor- 

 daitales that looked towards Conifers, such as the branching habit, 

 relatively simple leaves, and thick vascular .cylinder. The monospo- 

 rangiate 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 Gym- 

 nosperm flora, therefore, Cycads and Ginkgos are distinguished by hav- 

 ing 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, wliich 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 reproduc- 

 tive structures, and by history. The conclusion is that the tribe con- 

 taining the pines is to be regarded as including the modern representa- 

 tives 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) ase clearly relatively modern. The general conclusion, there- 

 fore, is that the Conifer stock became differentiated from the Cordai- 

 tales with features characteristic of the pine tribe, and that from this 



