THE LYCOPSIDA AND PTEROPSIDA 249 



genetically megaphyllous, since their anatomical relations are 

 always characterized by the presence of foliar gaps. The repro- 

 ductive organs or sporangia in the case of the Pteropsida are on the 

 lower or abaxial (dorsal) surface of the leaf, and are often numerous 

 and complicated in structure. In the lower forms the dehiscence 

 of the spore sacs is ectokinetic, but in the higher representatives 

 of the phylum the opening mechanism is derived from modified 

 transfusion tissue (in turn derived from the centripetal wood of 

 the ectokinetic and lower forms). Those Pteropsida characterized 

 by an internal reproductive mechanism derived from transfusion 

 tissue are appropriately designated as endokinetic. The higher 

 members of the series have developed true seeds provided with an 

 integument and equipped with an apparatus either related to the 

 seed itself or to the seed leaf (or megasporophyll) for the reception 

 of microspores or pollen. Although developed in very early 

 geological times in forms resembling ferns, the Pteropsida are still 

 in full vigor; and in their highest manifestation, the angiosperms, 

 they constitute an overwhelmingly large proportion of the present 

 vegetable population of the earth. They have reached their 

 zenith of efficiency in the herbaceous angiosperms, which in all 

 probability will supplant arboreal angiospermous types in the not 

 very remote future. The gymnosperms, and in particular the 

 conifers, were the prevailing Pteropsida of the Mesozoic, while in 

 the Paleozoic age pteridosperms (Cycadofilicales) and other lower 

 gymnosperms and fern allies represented the group. 



The Pteropsida and Lycopsida are distinct as far back as the 

 geological record can be perused, and there seems to be little doubt 

 that they constitute two primitive stocks of vascular plants. 

 They are so clearly diverse even in their earliest manifestations 

 that it is difficult to picture how they may have been formerly 

 connected. It is obvious, however, that the lycopsid type has not 

 been able to cope with the changing conditions of environment and, 

 comparatively early in the periods recorded in the rocks, was 

 relegated to a position of relative inferiority. Whether this situa- 

 tion was the result of the failure to achieve true seeds or is to be 

 explained as the basis of some fundamental defect of internal 

 organization incapacitating the Lycopsida to succeed in competition 



