CHAPTER XXIII. 
INTRODUCTORY REMARKS ON PTERIDOPHYTA. 
In the comparative sketch of the sporophyte in the Bryophyta which has 
been given in the preceding chapters, it has been seen that for these plants 
a theory of sterilisation of potentially fertile cells accords well with the 
developmental facts. Numerous cases have been seen of cells, similar in 
origin to the sporogenous cells, being diverted to other uses than that 
of propagation: these form somatic tissue: there is indeed good reason to 
think that most, if not even the whole, of the somatic tissue of the 
sporogonium originated in this way. This is no new conception: it is a very 
natural corollary on the fundamental conclusions of Hofmeister: it was 
first clearly stated in the writings of Leitgeb on Liverworts, and was 
extended by him also to the Mosses: it was adopted by Goebel in his 
work on the Muscineae in Schenk’s Handbuch, and it is now more 
definitely formulated in his Organography, Eng. edn., vol. ii., pp. 93-167. 
It may be held as the generally accepted hypothesis underlying any 
comparative study of the sporogonia of the Bryophytes at the present 
time. 
But the hypothesis of sterilisation has not been extended with the 
same readiness to other Archegoniate forms. In treating the Pterido- 
phytes, notwithstanding that they have an essentially similar life-cycle, 
there is rarely any reference in the current literature to the effect which 
progressive sterilisation may have had in their evolution. A certain 
excuse for this want of consistency may be found in the fact that in the 
Pteridophytes the proportion of somatic to propagative tissue is very large: 
any hypothesis of sterilisation must therefore recognise the process as 
having extended much further in them than in the Bryophytes. The form 
of the sporophyte also is much more complicated than in the Bryophytes: 
consequently the difficulties of application of a theory of sterilisation to 
the Pteridophytes are much greater, and the results less secure. This is 
certainly true, but it does not appear to be a sufficient reason for a plain 
departure from a theoretical position wh’ch has illuminated the comparative 
