PTRRIDOPHYTA 307 



them and do not emerge except, in some cases, to a very slight extent. 

 Each lives on food stored within the spore. The advantage of heteros- 

 pory lies in the fact that, since the gametophytes derive their nourish- 

 ment from food made by the sporophyte, they are independent of such 

 external conditions as might interfere with the growth of a free-living 

 gametophyte. 



Except in the Anthocerotales, the sex organs of bryophytes are super- 

 ficial structures, but in the pteridophytes they are embedded, either 

 wholly or in part. Moreover, as compared with the sex organs of bryo- 

 phytes, both the antheridia and archegonia of pteridophytes are reduced. 

 The greatest reduction of spermatogenous tissue occurs in the heterospo- 

 rous forms, reaching an extreme in Isoetes, where each antheridium pro- 

 duces only four sperms. The most primitive archegonia, those with the 

 greatest number of neck canal cells, are found in Lycopodium. In nearly 

 all the other homosporous pteridophytes there are either two neck canal 

 cells or, more commonly, only one, this being usually binucleate. In the 

 heterosporous forms there is a single neck canal cell that may be either 

 binucleate or uninucleate, according to the genus. Nearly all pterido- 

 phytes have multiciliate sperms. Biciliate sperms, resembling those of 

 bryophytes, are confined to Lycopodium, Phylloglossum, and Selaginella. 



Interrelationships. The Psilophy tales are the oldest known and most 

 primitive group of vascular plants. Whether they were derived from 

 ancestors resembling bryophytes or directly from alga-like forms is a mat- 

 ter of difference of opinion. It is rather generally agreed, however, that 

 the Psilophytales gave rise to the other pteridophytes, since transitional 

 forms have been found. The Lycopodiinae, Equisetinae, and Filicinae 

 separated early from the Psilophytales and each has subsequently pursued 

 an independent course of evolution. The Psilophytales also gave rise to 

 the Psilotales, a group that has made relatively little progress and one 

 that stands apart from the other existing groups. 



The Lycopodiinae are a relatively primitive group in spite of the fact 

 that some members have advanced to the condition of heterospory. 

 They reached their climax in the Paleozoic and are now relatively unim- 

 portant members of the flora. There is no evidence that they have given 

 rise to any of the higher groups. The Equisetinae, more advanced than 

 the Lycopodiinae, also made their greatest display during the Paleozoic. 

 They are a peculiar group with many features not seen in any other vascu- 

 lar plants. They also represent a line of evolution that ends blindly. 



The Filicinae are the most highly developed of all pteridophytes and 

 show much progress among themselves. It is generally believed, on the 

 basis of much morphological and paleobotanical evidence, that the ferns 

 have given rise to the seed plants. The leptosporangiate ferns are essen- 

 tially modern, while the eusporangiate ferns are more ancient and more 



