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 t 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 which has illuminated the comparative 



