CHAPTER XVIII 

 THE LYCOPSIDA AND PTEROPSIDA 



The discussion of tissues and organs and the elucidation of the 

 general principles applicable to these in the earlier chapters of the 

 present work bring us to the point where the higher plants may 

 profitably be discussed in regard to their general anatomy and 

 evolutionary affinities. Obviously, if it is possible to compass a 

 general grouping which will at the outset indicate the main lines 

 of evolution, the consideration of the particular groups will be 

 much facilitated. Some years ago the writer put forward a general 

 classification of vascular plants, based on the cardinal features of 

 the reproductive structures and the salient anatomical characters. 

 This attempt to group the higher plants in accordance with their 

 more important reproductive and anatomical features has met with 

 approval among competent judges and as a consequence may be 

 conveniently utilized in the present connection. 



A general survey of vascular plants, existing and extinct, 

 reveals the fact that there are certain features correlated in a 

 significant way. For example, a large number are characterized 

 by the possession of ventral or adaxial sporangia together with 

 usually small leaves. Another large assemblage presents relatively 

 large leaves and more numerous sporangia which are dorsal or 

 abaxial in position. The former group is characteristically repre- 

 sented by the lycopods or club mosses and their allies, while the 

 latter includes the ferns and the forms more nearly related to them, 

 the gymnosperms, and the angiosperms. The first aggregation 

 of types, known as the Lycopsida, is now practically extinct, but 

 played a large role in the Paleozoic and furnished a considerable 

 proportion of the raw material of that extremely important com- 

 bustible called coal, together with its derivative products, petroleum 

 and natural gas. The second alliance, to which the appellation 

 Pteropsida is appropriately given, although abundantly present 

 in the remote past, still prevails and has given rise to the seed plants 



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