XI LEPTOSPORANGIAT^ HETEROSPORE^ 439 



and buries the ripe sporocarp more or less completely in the 

 earth. The statement^ that this species has but three cham- 

 bers is incorrect, and except for the longer pedicel of the fruit, 

 and a slightly thinner epispore in the upper part of the macro- 

 spore, it corresponds exactly to P. globulifera. The sporo- 

 carp splits into four parts, corresponding to the four lobes of 

 the young fruit, and the membranaceous margins of the leaf 

 form a tough indusium surrounding the sporangia. This in- 

 dusium is not, at least in P. globulifera, readily pervious to 

 water, and germination does not begin for a long time after the 

 valves separate, unless the indusium is artificially opened. 

 Except for the number and position of the sori, and the relative 

 position of the two sorts of sporangia, Marsilia agrees exactly 

 with Pilularia. The sorus canals form two longitudinal rows 

 along the sides of the elongated fruit rudiment, which may be 

 compared to a pinnate leaf. In Marsilia, occupying the middle 

 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 ter- 

 minal one, derived directly from the apical cell, is a macro- 

 sporangium; the smaller lateral ones, derived from its earlier 

 segments, the micros.porangia. 



Fossil Lcptosporangiatce 



Sporangia of undoubted Leptosporangiatas are exceedingly 

 rare in the earlier geological formations. Solms-Laubach (2) 

 cites Plymenophyllitcs as probably being a genuine leptospo- 

 rangiate Fern, and Zeiller (i) describes some isolated spo- 

 rangia that seem to be much like those of the modern Gleich- 

 eniaceae. Forms like the Osmundaceae have also been de- 

 scribed by various writers, but no traces of Cyatheaceae or 

 Polypodiaceae have been yet detected in Palaeozoic formations. 

 In the Jurassic, undoubted evidences of Gleicheniaceae, Os- 

 mundaceae, and Schizaeaceae are found (Raciborski (i)), 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. 



' Goebel (10), p. 240; Underwood (4), 2nd ed., p. 127; "Botany of Cali- 

 fornia," vol. ii. p. 352. 



