2 00 THE POPULAR SCIENCE MONTHLY 



the Cycadophyte branch is not distinguishable until the Mesozoic. 



Historically, therefore, the Cordaitales must be considered as the 

 second group of Gymnosperms. Their connection with an ancient fern 

 stock is evident in their structure, but they have lost many of the fern 

 characters that were retained by Cycadofilicales. The fact that Cor- 

 daitales are much further from ferns than are the Cycadofilicales is 

 perhaps the best proof that they have come from the ferns by way of the 

 Cycadofilicales. The combination of changes involved in their structure 

 is all in the direction of the later Conifers, as, for example, the branch- 

 ing stem (constituting what is called "the habit") with its thick 

 cylinder of secondary wood, the narrow and entire leaves, and the cones 

 (strobili). It would not fit the purpose of this presentation to include 

 the changes in the more intimate structures, since their nature and 

 significance can be appreciated only by the special students of the group, 

 but they are just as striking as the more obvious changes mentioned. 



The Gymnosperm vegetation of the Paleozoic, therefore, comprised 

 two great genetic groups : the Cycadofilicales, representing the primitive 

 Gymnosperm stock that differentiated from the ferns; and the Cor- 

 daitales, representing the primitive Coniferophyte stock that differen- 

 tiated from Cycadofilicales more ancient than those we know. 



In the Mesozoic flora the Gymnosperms were represented by four 

 great groups, evidently derived from the two Paleozoic groups. As 

 stated above, the Cycadophyte branch became distinct, and for a long 

 time all of its representatives were thought to be Cycads. For this rea- 

 son, the Mesozoic has been called the " age of Cycads," so far as the 

 vegetation is concerned. It is one of the triumphs of American paleo- 

 botany that it has put on a firm basis our knowledge of the great 

 Cycadophyte group of the Mesozoic, and has shown that it is quite dif- 

 ferent from the modern Cycads. The group is called Bennettitales, and 

 although a few forms from foreign localities have been known for a 

 long time, it remained for Mesozoic deposits of the United States and 

 Mexico to reveal a remarkably rich display of forms in admirable 

 preservation. The investigation of this material has been carried on 

 chiefly by Dr. G. E. Wieland of the Yale Museum. 



The Bennettitales, therefore, are the so-called " fossil Cycads " of 

 the Mesozoic. So far as the records show, they are restricted to the 

 Mesozoic, so that they represent an extinct Mesozoic group, just as there 

 are two extinct Paleozoic groups. Of course it is not only conceivable, 

 but also probable that the Paleozoic Cycadofilicales, from which Ben- 

 nettitales were derived, continued into early Mesozoic; and that the 

 Mesozoic Bennettitales began to differentiate in late Paleozoic. The 

 external appearance of Bennettitales justifies their early assignment to 

 the Cycads, for the whole habit is Cycadean. The stems are either 

 tuberous or cylindrical, and crowned by a rosette of large, fern-like 



