THE LYCOPSIDA AND PTEROPSIDA 247 



feature of organization in this instance related to reproduction. 

 In the phylum under discussion the sporangia or spore sacs are 

 invariably on the upper or adaxial (ventral) surface of the sporo- 

 phyll or reproductive leaf. The sporangia are single or at most 

 relatively few in number and are invariably ectokinetic in their 

 mode of dehiscence. True seeds seem never to have made their 

 appearance in the Lycopsida, although, as has been pointed out 

 hi an earlier chapter, structures somewhat simulating seeds have 

 been found in certain of the Paleozoic representatives of the 

 group. The organs in question, however, lacked a true integu- 

 ment and, so far as is known, were without arrangements for 

 receiving the microspores or pollen grains, a universal equipment 

 in the case of pteridospermous and gymnospermous seeds. Possibly 

 the failure to achieve true seminal structures was the cause of the 

 decline of the Lycopsida, which at the present time constitute an 

 insignificant proportion of the vegetation of the world. The group 

 is very ancient and goes back to the beginning of the geological 

 record. The primitive forms representing the Lycopsida were 

 often arboreal in their habit, and the alliance reached its culmina- 

 tion in the carboniferous forests of the Paleozoic. It became largely 

 reduced in the Mesozoic, and the Cenozoic saw its virtual extinction. 

 In the Pteropsida we have to do with forms in which the leaves 

 are relatively large in comparison to the stem and often extremely 

 complicated in structure. As is shown in the diagram (Fig. 177), 

 the transverse section of the stem reveals a central cylinder 

 which, when siphonostelic, as in the illustration, is characterized 

 by gaps corresponding to the traces departing to the leaves. This 

 feature of its anatomy is in marked contrast to that found univer- 

 sally in the Lycopsida, in which the leaves are not related to foliar 

 gaps. The traces of the branches in the Pteropsida likewise sub- 

 tend gaps in the central cylinder (which may be designated as 

 ramular gaps). The fibro vascular supply of the root makes its 

 exit from the stele or fibrovascular cylinder without causing any 

 gap or interruption. The roots are often, but not invariably, 

 related to the bases of the leaves. The foliar organs, usually much 

 larger in size than in the Lycopsida and often of great dimensions 

 and lobed in a complicated manner, are to be regarded as palin- 



