ESSENTIALLY A STROBILOID TYPE 657 



which share many of the characters thus set down. The chief difference 

 lies in the extent of the development of the leaves, and the number and 

 position of the sori upon them. These are, however, matters of degree 

 rather than of kind. A dichotomous leaf like those seen in Sphenophyllum 

 or in Pseudobornia is in point of construction not unlike that type which 

 is found in certain primitive Ferns, where dichotomy was also prevalent. It 

 has been seen that the sporangiophores may be multiplied on the single leaf 

 of the Psilotaceae or on the leaf-sheath of the Sphenophylls, while a plurality 

 of them is a character of Chcirostrobus. But sporangiophores are held as 

 correlatives of Fern-sori, though probably not homogenetic with them : it 

 is thus seen that precedents are present for their plurality on the single 

 sporophyll in the strobiloid types. On the view of the Ophioglossaceae 

 given above (pp. 490-494), this family is held to represent a series in which 

 megaphylly has been achieved from a strobiloid origin : the spore-producing 

 members have there been shown to follow the leaf-enlargement, in size, 

 and sometimes even in number. This series, though probably a quite 

 separate megaphyllous phylum, shows an interesting parallel to the Ferns, and 

 suggests how spore-producing members may be spread over an enlarging part. 

 Lastly, the position of the sorus is seen to have varied in Ferns from the 

 margin to the lower surface, and occasionally to the upper surface : what is 

 thus liable to change within the Filicales as now denned may probably have 

 been equally liable to change at the inception of the phylum : therefore 

 the habitual position of the sori on the lower surface of the sporophyll 

 must not be held to be a vital point of difference from other Pteridophytes. 

 Accordingly, there appears to be reason for regarding the fundamental 

 plan of the sporophyte in the Filicales as being essentially strobiloid, like 

 that in the other phyla of Pteridophytes, but specialised to a greater extent 

 than in any of them in the direction of megaphylly, while a cognate 

 spread of the sori has followed the enlargement of the sporophylls. 



2 T 



