PALEOBOTANY BERRY. 



359 



western China. The accompanying map (fig. 23) shows the area 

 within which fossil Ginkgos have been found as well as the more 

 restricted areas in Greenland, North America, Siberia, and Europe, 

 where Tertiary representatives are known. The area in eastern 

 Asia, included in a broken line, indicates the restricted natural 

 habitat of the existing species, although it has been widely planted 

 and thrives throughout the temperate zones, there being large num- 

 bers of magnificent trees in the Washington parks. Some representa- 

 tive fossils of both Ginkgo and Baiera are shown in the accom- 

 panying figures. 



The order Gnetales includes but three existing genera, which differ 

 widely in habit, habitat, anatomy, gametophytic structures, and in 



Fig. 23.— Sketch map illustrating the geologic history of Ginkgo and Baiera. 



endosperm and embryo formation, and yet have a thread of common 

 characters denoting their community of origin. Ephedra, with about 

 30 species of low, straggling shrubs with jointed ribbed steins and 

 reduced leaves, inhabits the more arid parts of both the Eastern and 

 Western Hemispheres. It is the least specialized of the three genera 

 and more like the Coniferales. Gnetum, with about 26 species of both 

 the eastern and western Tropics, comprises small trees or woody lianas 

 with opposite, netted- veined, dicotyledonous-like leaves, and is most 

 advanced in the direction of the angiosperms. The third genus, 

 Tumboa (Welwitschia), is a remarkable monotypic form of western 

 South Africa with a shortened, swollen axis and but two excessively 

 long parallel veined leaves, and is obviously most specialized (aber- 

 rant). All of these modern forms are evidently the end products of 



