PTERIDOPHYTA 305 



These demands are met, in the pteridophytes, mainly by the vascular 

 system. Thus the sporophyte, in achieving independence, has developed 

 roots, stems, and leaves, with a system of conducting and supporting 

 tissues extending throughout the plant body. 



Among existing pteridophytes the Psilotales are unique in having a 

 sporophyte that shows little organization into vegetative organs. Their 

 poorly developed leaves (without veins in Psilotum) and lack of roots, 

 although probably related to a partially saprophytic existence, are to be 

 regarded as primitive features, since they are shared with the extinct 

 Psilophytales, a group which the Psilotales resemble in other respects as 

 well. 



The size of the leaves must be considered in relation to the size of the 

 stem. From this standpoint, small simple leaves, without a petiole, are 

 found in nearly all members of the three lower classes, while large leaves, 

 with a petiole and with a blade that is almost always divided into leaflets, 

 are characteristic of the ferns. Moreover, ferns are the only pterido- 

 phytes having leaf gaps, a feature setting them off in marked contrast to 

 the lower classes and indicating a relationship to the seed plants, where 

 leaf gaps are universally present. Except in the Equisetinae, where 

 they are cyclic, the leaves of pteridophytes are fundamentally spiral in 

 arrangement. 



The leaves of the Psilophytinae, except those of T7nesiptens, are vein- 

 less, while those of the Lycopodiinae have a single unbranched vein. 

 These groups are said to be microphyllous (small-leaved), for even the 

 leaves of the Lepidodendrales and Isoetales, though larger than those of 

 other lycopods, are narrow and have only one vein. The leaves of 

 microphyllous pteridophytes probably represent emergences, or simple 

 outgrowths from the stem. The Filicinae are megaphylloiis (large-leaved) 

 and their leaves have many branching veins. Such leaves apparently 

 have evolved from a lateral branch system that has become flattened and 

 limited in growth. Thus the leaves of lycopods and those of ferns have 

 probably had a different origin and so are not homologous. Most of the 

 Equisetinae have small simple leaves with a single vein, but some of the 

 fossil members have larger leaves with leaflets and branching veins. 

 This indicates that the group was originally megaphyllous, the small 

 leaves having been derived from larger ones by reduction. 



The Strobilus. A second contribution of the pteridophytes to the evolu- 

 tion of the plant kingdom has been the organization of a strobilus. Orig- 

 inally no distinction may have existed between sporophylls and foliage 

 leaves, a condition found in the simpler species of Lycopodium. But 

 gradually, as a result of "division of labor," sporophylls became less leaf- 

 like and were organized to form a compact strobilus. Although not pres- 

 ent in modern ferns, a strobilus is characteristic of nearly all the other 



