Evolution of Plants 353 



suited to such existence, the sporophyte growth assumed in- 

 creasingly bulky proportions. In the process, the above named 

 thiee groups diverged apart from a common ancestry, prob- 

 ably during late silurian time. 



Of the three, the group that seems most closely and directly 

 to have survived till now in a comparatively lowly organized 

 condition is that of the Archseofilicinese, that has produced 

 derivative evolving fern families and genera, from the devonian 

 to the present day. The usual retention of the concentric 

 stem and leaf bundles, the tendency to formation of a root or 

 roots at each node, the wide expanse of the mature leaves, and 

 the structure of the sporangia, have all remained fairly con- 

 stant. But very early in their history two divisions were 

 produced, whose characters depend on the origin of the spor- 

 angia. These, that are often designated as the leptosporangi- 

 ate and the eusporangiate divisions, are both recognized in 

 carboniferous strata, and have persisted till now. Of the 

 two, we would regard the former as the simpler, and so prob- 

 ably the more ancient. For in it we can still trace simple 

 root, stem, and leaf conditions, as exemplified in Ceratopteris, 

 and the sporangia all arise as outgrowths each of a single epi- 

 dermal cell. In this they resemble the origin of the vegetative 

 buds formed on the leaf edges of Ceratopteris^ as elucidated 

 by Hofmeister. Such also is their origin in that fern. 



In the eusporangiate ferns the sporangiferous tissue includes 

 not merely the epidermis, but one or more layers of subepi- 

 dermal tissue, from the latter of wliich the spores are formed. 

 These however almost certainly originated as two distinct 

 and derivative series from distinct members of the Lepto- 

 sporangiatse, and are now recognized as the Ophioglossaceae 

 and Marattiacese. The two have few points in common, 

 except that the sori and their sporangia are greatly more com- 

 plex than in the Leptosporangiatse. 



Of all examples of Filicinese known to us, whether fossil or 

 recent, Ceratopteris conforms to the simplest conditions, as 

 has been indicated above. So the group Parkeriaceae, made 

 for its reception, we would regard as representing a family of 



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