1 6 PLANT GEOGRAPHY 



possibly climbing, plants with jointed stems, long ribbed 

 internodes, and broadly wedge-shaped leaves in whorls 

 generally of six, sometimes so divided as to appear as 

 distinct leaves. Their fructification was, in general, 

 a cone resembling that of Catamites; and undoubtedly 

 the Sphenophyllales suggest a common ancestry with the 

 Equisetales; whilst Pseudobornia not only confirms this 

 community of descent, but also suggests that ferns 

 may have had a closely related origin. 



The Club-mosses are a more isolated class; and, while 

 at the present day they are divided into types, such as 

 Lycopodium, with only one kind of spore, and those 

 with two, such as Selaginella and Isoetes, in Palaeozoic 

 rocks every known form is like these last, " hetero- 

 sporous." The group as a whole seems not only to have 

 dwindled in the size of its members, but also to have 

 undergone structural degeneration. This, however, did 

 not begin until a later period. 



Other fern-like leaves occur in Devonian rocks, 

 described as Neuropteris, Alethopteris , etc. Recent 

 work, however, has shown not only that these leaves 

 were connected with large woody stems, formerly 

 believed to be coniferous, but now seen to resemble that 

 of the Cycads, but also that they bore true seeds and 

 pollen-grains approximating in structure to those of 

 Cycads. These plants are now, therefore, referred to 

 a group, known as Pteridospermece, which we must 

 apparently consider as gymnospermous rather than 

 pteridophytic. 



Lastly, we have in Devonian rocks leaves attributable 

 to the Covdaitece. These were lofty trees, sometimes 

 100 feet high, branching above and bearing simple 

 leaves, sometimes three feet in length and five inches 

 wide. In foliage and wood they resembled the broad- 

 leaved AraucaviecB of the Southern Hemisphere of to-day; 

 but they had a large discoid pith like that of the walnut. 

 They had male catkins of erect stamens bearing from 

 four to six pollen-sacs and protected by bracts, and 

 similar groups of fewer ovules resembling those of 

 Ptevidospevmece and Cycadece. Related as this promi- 

 nent group undoubtedly is both to these classes and to 

 Conifer &, it is most closely represented to-day by that 

 curiously isolated type the Maidenhair-tree (Ginkgo 

 biloba L.) of China. 



