COMPARATIVE DISCUSSION 483 
Among the microphyllous Pteridophytes there is sometimes no strict 
relation between the spore-bearing members and the bract-leaves, but in 
the majority of them some constant relation is found. The common 
type is for the former to be in the median position relative to the latter, 
though the level of insertion may vary. In the Sphenophyllales this 
position is seen in the simpler forms, such as .S. majus and trichoma- 
tosum; but it is departed from in others where, probably owing to fission, 
the number of sporangiophores is larger than that of the subtending 
bracts. In the Psilotaceae the radial position is maintained, but the 
insertion is very close to the apex of the sporophyll. This local relation 
of the two parts is so constantly seen in these groups, which include 
some of the most ancient plants, that it suggests for them a fixed 
morphological character rather than a mere result of independent adap- 
tation. The existence of a like constant relation in another group 
compels the exact comparison of the parts thus similarly placed and 
functionally identical. The criterion whether this is a mere analogy, or 
whether some deeper significance underlies it, will be found in the 
degree of correspondence which the plants show in other characters than 
the one in question. To apply this test a comparison will be made 
between the Sphenophyllales and the Ophioglossales, first as regards their 
spore-producing members, and afterwards in respect of the other characters 
of the sporophyte. 
Both in’ Ophioglossum and Botrychium species occur with small spikes 
bearing few sporangia, and they are specially found in the young plants 
(0. Bergianum, B. simplex, and Lunaria): it is in these that the closest 
similarity exists between the two genera, while from them by gradual 
steps the two types diverge. These small spikes consist of a simple stalk 
with vascular supply, bearing towards its distal end a few sporangia 
marginally disposed: the insertion is median on the sporophyll: the 
essentials of form, position, and function are here similar to those of 
the sporangiophores of the Psilotaceae, and especially of those stalked 
sporangiophores of Zmesipteris described by Thomas.! The differences 
lie in the forked leaf of the Psilotaceae, while that in these simple types 
of the Ophioglossaceae is usually entire, and in the exact position of the 
sporangia. 
But in both groups there may be a departure from the exact numerical 
and local correspondence of sporangiophores and sporophylls: and these 
lead to an interesting comparison. The case of O. palmatum has been 
referred to chorisis of the single spike, which seems the only explanation 
of the plurality of irregularly branching spikes seen in an intra-marginal 
position in that species. In the Sphenophylls the frequent close juxta- 
position of the pedicels of the sporangiophores points to a similar chorisis, 
as accounting for the condition seen in S. Dawson, and possibly also in 
the imperfectly known S. Roemer. Moreover, the vascular connections of 
1 Proc. R.S., vol. 1xix., p. 345. 
