2i6 MORPHOLOGY OF GYMNOSPERMS 



Heer (7, 9). Jurassic remains of the group have been found in 

 every country, from the arctic regions to the south temperate regions, 

 being abundant in England, throughout Europe, Siberia, China, 

 Japan, North America, and Australia (37). The prominent genera 

 were Ginkgo and Baiera, the former becoming more abundant in 

 the more recent periods and in the more northern latitudes; the latter 

 including the majority of the older representatives of the group. 



6. Relationship to other gymnosperms 



From the previous account it is evident that the question as to the 

 origin of Ginkgoales is a choice between two alternatives : either they 

 were derived from the Cordaitales, or the two groups became differ- 

 entiated from a common stock. The historical evidence is all in favor 

 of the former view, and the testimony of the comparative structures 

 of the two groups seems equally convincing. The mesozoic Gink- 

 goales seem to hold the same relation to the paleozoic Cordaitales 

 that the mesozoic Bennettitales hold to the paleozoic Cycadofilicales. 

 In fact, the connections between the former pair seem closer and more 

 definite than those between the latter pair. Just as the whole cycado- 

 phyte phylum retained characters of seed structure and swimming 

 sperms that belonged to the ancient Cycadofilicales, so the Ginkgoales 

 have retained the primitive characters of the Cordaitales. Of course 

 one may imagine a primitive plexus of fernlike seed plants, related 

 in some way to the Botryopterideae, which differentiated at various 

 times into Cycadofilicales, Cordaitales, Ginkgoales, and perhaps 

 other gymnosperm phyla (47), but however true this may be of the 

 contemporaneous Cycadofilicales and Cordaitales, the historical 

 as well as the morphological relation of Ginkgoales to Cordaitales 

 indicates a more immediate connection between these two groups. 

 Scott (52) expresses the opinion that Ginkgo is "the one surviving 

 member of an ancient stock, derived from the same cycle of affinity 

 as the paleozoic Cordaiteae, once the dominant type of gymnosperms." 



Among living gymnosperms. Ginkgo is to be compared with cycads 

 and conifers. In the structure of the ovule and the seed, and in the 

 details of fertilization, the resemblance is to the cycads; while in 

 habit, stem structure, and vascular anatomy, the resemblance is to 

 conifers. There has recently been a tendency to discover, connections 



