CHAP. X FILICINE.E LEPTOSPORANGIAT^i 303 



out by the facts in the case. Any one who has seen the 

 wonderful profusion of Ferns in a tropical forest, and the 

 enormous size to which many of them i^nrjw, is very quickly 

 disabused of any such notion. 



The fossil record is also extremely instructive as bcarin<^ on 

 this point. According to Solms-Laubach ^ there is but one 

 certainly authentic case from the Carboniferous rock which can 

 be regarded certainly as a leptosporangiate form, all of the 

 other sporangia discovered being of the eusporangiate type. 

 In the later formations the Leptosporangiates increase in 

 number, but according to Luerssen '^ undoubted Polypodiacea; 

 are not found before the Tertiary, where a number of living 

 genera are represented. That is, so far as we can judge from 

 the fossil record, the Leptosporangiatae, instead of being a left- 

 over type, are essentially a modern one. 



Except in the few heterosporous forms there is, on the 

 whole, great uniformity in the prothallium. The most marked 

 exception to this is the well-known filamentous protonema-like 

 prothallium of some species of Trichovianes . Except in these, 

 however, the germinating spore, either directly or after forming 

 a short filament, produces normally a flat, heart-shaped pro- 

 thallium, growing at first by a two-sided apical cell, the pro- 

 thallium being at first one cell thick, but later producing a 

 similar cushion to that found in Marattia but less prominent, 

 and the wangs always remain one cell thick. Upon the lower 

 side of the cushion are produced the archegonia, which have 

 always a projecting neck, sometimes straight, but more com- 

 monly bent backward. The antheridia are produced upon the 

 same prothallium as the archegonia in most forms, but a few 

 species of Ferns are dioecious, and usually there are small male 

 prothallia in addition to the large hermaphrodite ones. The 

 antheridia, like the archegonia, always project above the pro- 

 thallium. 



The heterosporous genera, as in Isoetes, produce two sorts 

 of prothallia, but the male prothallium is not so much reduced, 

 and the female is formed by successive cell divisions and not 

 by free cell formation. 



The first divisions in the embryo always divide it into 

 regular quadrants, and the young members always grow from 

 a definite apical cell, which, with the possible exception of some 

 1 Solms-Laubach (2), p. 153. - Luerssen (7), vol. ii. p. 574. 



