XII LEPTOSPORANGIAT^ HETEROSPORE^ 419 



line of each sorus, is a row of large tetrahedral cells, which form 

 three sets of segments, like any three-sided apical cell. Each of 

 these cells produces a group of sporangia. The terminal one, 

 derived directly from the apical cell, is a macrosporangium ; the 

 smaller lateral ones, derived from its earlier segments, the 

 microsporangia. 



Fossil Lcptosporangiate 



Sporangia of undoubted Leptosporangiatae are exceedingly 

 rare in the earlier geological formations. Solms-Laubach ^ 

 cites HynienopJiyllites as probably being a genuine lcptospor- 

 angiate Fern, and Zeiller - describes some isolated sporangia 

 that seem to be much like those of the modern Gleicheniaceae. 

 Forms like the Osmundacea; have also been described by 

 various writers, but no traces of Cyatheaceai or Polypodiaceae 

 have been yet detected in Palaeozoic formations. In the 

 Jurassic, undoubted evidences of Gleicheniaceae, Osmundaceae, 

 and Schizaeaceae are found,^ but the Polypodiaceae do not 

 seem to have appeared until still later. The existence of the 

 Hydropterides below the Tertiary is doubtful, but in the latter 

 formation occur undoubted remains of the living genera 

 Salvinia, Pilularia, and Marsilia. 



Affinities of the Leptosporangiatce 



The Osmundaceae undoubtedly are intermediate between 

 the Eusporangiatae and Leptosporangiatae, but with which 

 order of the former their affinities are closest is difficult to say. 

 Among the Ophioglossaceae, the larger species of BotrycJiiuDi 

 and HehnintJiostacJiys show apparent close structural similarity ; 

 but, on the other hand, in the distinctly circinate leaves and 

 the character of the sporangia, as well as the histology, the 

 Marattiaceae are certainly quite as nearly related. Apparently 

 all of these forms are generalised types, springing from a 

 common stock, but no two of them directly related. 



Among the Leptosporangiatae themselves the relationships 

 are evidently much closer. A common type of prothallium and 

 sporangium prevails throughout, even in the heterosporous forms. 

 The four families, Osmundaceae, Gleicheniacea;, Cyatheaceae, and 



^ Solms-Laubach (2). - Zeiller (i) ; Bower (12), p. 126. •* Raciborski (i). 



