SUMMARY : 493 
some degree of real affinity, and strongly confirms the initial comparison 
of the spike with the sporangiophore of the Sphenophyllales. Lastly, the 
anatomical comparison of the Psilotaceae with the Ophioglossaceae has 
shown not only the interesting transition from the cladosiphonic to the 
‘phyllosiphonic structure, but also that in the upper region the wood of 
Tmesipteris is mesoxylic, as it is also in Aelminthostachys,. while feeble 
secondary development, analogous to that in OpAsoglossum and stronger in 
Botrychium, is seen both in Zmesipieris and in Psilotum. These several 
characters form a cumulative body of evidence, confirming the comparison 
of the shoot and of the sporangiophore in the Sphenophyllales with those 
of the Ophioglossales: the nearest approach among living plants being 
between the Psilotaceae and Ophzoglossum.! 
It would thus seem probable that the Ophioglossaceae sprang from 
some offshoot of the sporangiophoric Pteridophytes, allied in some degree 
to the Sphenophyllales, and possessing early a saprophytic habit of the 
underground prothallus. That this encouraged a peculiar specialisation of 
the sporophyte, which shared occasionally, though not generally, in the 
mycorhizic habit, but not so far as to lead to the cessation of self- 
nutrition. That the exigencies of the underground habit were met by an 
enlargement of the leaves, culminating finally to the monophyllous state. 
A parallel enlargement of the sporangiophore with that of the leaf was a 
natural consequence, since in homosporous forms, as comparison shows, 
the spore-output usually marches with the vegetative development. If this 
were so, then the spike would never in its descent have been anything 
other than it is now normally seen to be, viz. a spore-producing part, 
originally of the nature of a sporangiophore, and seated in a median position 
on the adaxial face of the sporophyll. 
Referring in conclusion to the theory of the strobilus, the Ophio- 
glossaceae readily conform to it. The shoot, with its rare dichotomous 
branching, appears as a simple strobilus, while the indeterminate position 
of the root in the embryo bespeaks the accessory nature of that part 
upon it. The axis bears leaves, which are of one order only. The 
spore-producing parts appéar earlier in the individual life than in any 
other group of Pteridophytes, and this indicates a probability that all the 
1] wish to state quite explicitly that the homology of the Psilotaceous synangium 
with the Ophioglossaceous spike is no new opinion on my part, though additional and 
more detailed evidence is here adduced to support it. It was accepted by me in 1891 
(Proc. Roy. Soc., p. 270) and more fully stated in 1893, on the basis of developmental 
evidence (Proc. Roy. Soc., vol. liii., p. 22): this view has never been relinquished. I 
emphasize this here because a passage recently published appears to suggest that I do 
not uphold that homology (Scott, Pragressus Rei Botanicae, i., p. 163). My position is 
unchanged, except in so far as I now include the Sphenophylleae also in the comparison: 
the suggestion of this came from Dr. Scott (On Chetrostrobus, Phil. Trans., vol. clxxxix., 
1897, p. 27), and it greatly strengthens the comparison originally drawn by Celakovsky. 
‘There may be differences of opinion as to what morphological rank these parts hold, 
‘or how ultimately they came into being: these are, however, separate questions from the 
tecognition of their homology. ; 
