47 
and Helmint host achy s zeylanica, 
beneath the surface of the soil or humus. They are normally 
devoid of chlorophyll, but this was developed when the pro- 
thallus of O . pedunculosum reached the surface and was 
exposed to light. In all the cases, in which its presence has 
been looked for, a symbiotic Fungus has been found in the 
tissues. Besides this physiological agreement, which might 
of course be due to independent adaptation, there are common 
characters in the general symmetry, the structure of the sexual 
organs and the embryogeny, to which a higher degree of 
morphological weight attaches. In all, the growth is localized 
in an apical meristem, behind which the new organs arise in 
acropetal succession. In Ophioglossum and Helmin thostachys 
the general form is that of a rather stout cylinder, which may 
be more or less branched ; in the distribution of its organs 
this exhibits strict radial symmetry. In both Botrychium 
Lunaria and B. virginianum , however, a tendency towards 
a thick-flattened form can be recognized ; this is less marked 
in the forms in which the sexual organs occur on all sides of 
the prothallus. In B. virginianum they are definitely localized 
on the upper surface, the antheridia occupying a median ridge, 
while the archegonia are found on the flanks. Whether the 
flattened form of prothallus in Botrychium was derived directly 
from the unknown autophytic ancestry of the Ophioglossaceae, 
or has arisen by modification of the cylindrical type, an 
adaptive significance may be recognized in it. For the 
position of the sexual organs on the upper side of a flattened 
subterranean prothallus would be of advantage in arresting 
the water percolating downwards through the soil and allowing 
it time to bathe the sexual organs and permit of fertilization. 
Support is lent to the idea of some such view by the flattening 
of some of the subterranean types of Lycopodium prothalli, 
which, on this view, would be regarded as presenting an 
analogous but independent adaptation. 
The sexual organs are also constructed on the same type 
throughout the Ophioglossaceae. The antheridia are large and 
sunken ; the outer wall may be one-layered and dehiscence 
take place by the breaking down of a triangular opercular cell 
