COMPARATIVE DISCUSSION 483 



Among the microphyllous Pterfdophytes 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- 

 tosurn ; 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 

 (O. 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 Tmesipteris described by Thomas. 1 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 ,5". Dawsoni, and possibly also in 

 the imperfectly known S. Roemeri. Moreover, the vascular connections of 



1 Proc. R.S., vol. Ixix., p. 345. 



