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 Cheirostrobus. 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 defined 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. 
2T 
