Oct. 13, 1881] 



NATURE 



559 



other of these characters are the foundations of all the 

 Palaeozoic genera yet known. In the extinct Carboniferous 

 forms the fertile or sporangial whorls alternated with, and 

 were protected by, the overlapping whorls of barren leaves, 

 while in Equisetum the sporangial whorls are naked and 

 clustered in a terminal spike, an arrangement considered 

 by Saporta and Marion to be more favourable to the 

 dispersion of the spores. Annularia and Asterophyllites 

 were floating or procumbent plants. Calamites strongly 

 resembled the existing aquatic Equisitaceas, though ex- 



ceeding them twenty times in size, and surpassing them in 

 development by the possession of spores of two sexes. 

 Their more complex structure and consequent inadapta- 

 bility to changed conditions, favoured, the authors believe, 

 their early extinction in the Permian. In the Trias, and 

 until the Jurassic, several slightly modified genera coex- 

 isted with true Equisetum, and the survival of the latter, 

 one of the genera that have persisted almost unchanged 

 from the Carboniferous, is probably due to their simple 



Fig. 3. — Prothallus of an Osmunda aged eight months, slightly magnified to 

 show the double row of archegones down the centre. 



organisation, easy dispersion of the spores, and the 

 immense depths to which their rhizomes penetrate. 



The structure of ferns, unlike that of Equisitacete, 

 lends itself to infinite diversity. The fronds may be 

 simple or multipartite, without their form implying the 

 slightest degree of relationship, and supposed alliances 

 between fossil and recent ferns, such as Ettingshausen 

 has based upon the aspect and venation of the frond, 

 are declared by the authors to be valueless and mis- 

 leading. 



The earliest ferns had simple fronds, and probably 

 resembled in their vegetative organs the Hymenophylleae, 

 a group already well represented in the Carboniferous. 

 Next in order come the Osmundaceae, if the relative 



Fig. 4- — Under side of the proth.-iHus of another fern, more magnified, 

 showing the rhizoid radicles, the antherids dispersed over the surface, 

 and the archegones clustered at the terminal notch. 



complexity of their prothallus and simplicity of sporangia 

 are accepted as indications of inferiority. 



The relative perfection of the sporangium when taken 



Fig. 5. — Sporangium of HymcnophyUum, £ 



cellules which disrupt the spore-( 



ely by the ring 1 



as the essentially important organ, leads to a classifica- 

 tion coinciding approximately with the order in which 

 the groups made their appearance : — Hymenophyllea? 



Fig. 6. — Sporangia of Osmundacea:, showing dorsal dehiscence. 1 and 2, 

 sporangia of Todea A/ricana—i, ventral surface ; 2, dorsal surface ; 

 3, sporangium of Osmunda seen dorsally, and showing the jnfra-apic&l 

 group of cellules which disrupt the spore-case. 



Osmundacea, Schizeaceas, Gleicheniaceae, Marattiaceas, 

 Cyathew, Polypodiaceje. 

 From the simplest type of sporangium, two lines of 



