and Helmint host achy s zeylanica . ' 51 
to ascertain whether the evidence from the gametophyte renders 
this view probable or not. The eusporangiate Marattiaceae 
must be specially considered, but the general similarity in all 
the homosporous Filicales justifies the use of evidence from 
the leptosporangiate forms also. In considering the general 
symmetry the difficulty presents itself that no holosaprophytic 
fern-prothalli are known. In the flat green prothalli the 
meristem is apical and the succession of the sexual organs 
acropetal. ^ The latter are developed on the under surface of 
the dorsiventral prothallus, but this distribution is well known 
to be due to an irritability to light. When, under cultivation 
or in nature, this and possibly other stimuli act more uniformly 
on all sides of the prothallus, the flattened form is lost and 
the prothallus may continue its growth as a cylindrical process 
with an apical meristem h This growth-form presents points 
of resemblance with the prothalli of Ophioglossum and Hel - 
minthostachys in its symmetry, its apical growth, and the 
uniform distribution of the sexual organs on its surface. 
Something similar is seen in Gymnogramme leptophylla 3 , in 
which the process which bears the archegonia buries itself in 
the soil ; this was early compared by Goebel to the Ophio- 
glossaceous prothallus, though his suggestion that a pre-‘ 
liminary green stage might occur in them also has not been 
borne out. These modifications of the common flat prothallus 
in the Filicales suggest that, were it possible to cultivate them 
below the surface of the ground, the form of a cylindrical 
structure with apical growth would probably be maintained 
throughout life. As regards form and symmetry the type of 
prothallus which might be expected to result is realized in the 
Ophioglossaceae, and it will be remembered that when the 
cylindrical process of O. pedunctdosum reached the light it 
became flattened and lobed. 
The structure of the sexual organs affords support to 
such an origin of the Ophioglossaceous gametophyte from 
1 Cf. Lang, On Apogamy, &c. ^Phil. Trans., 1898, Ser. B, p. 187, PI. VII, 
Figs. 1-5. 
2 Outlines of Classification and Special Morphology, p. 245. 
