662 CONCLUSION 



Another point of interest in the Bryophytes for comparison is the 

 establishment of a central sterile tract the columella. In the Liverworts 

 this is incompletely carried out in Aneura (Figs. 127, 129), and in Pellia 

 (Fig. 128), the final end here being a more effective distribution of the spores: 

 it is more completely organised in Anthoceros, where it probably serves for 

 nutrition as well as for distribution (Fig. 130 E) ; . but its more definite 

 character is established in the Mosses, where it is probably effective in 

 water-storage as well as in nutrition. However different these several parts 

 may be in origin or in function, they all illustrate that process of relegation 

 of the spore-production, originally central, to a more superficial position. 

 It has been pointed out above (p. 286) that in sporogonia of no great 

 bulk, which dehisce by apical pores or by lateral slits, the superficial 

 position of spore-production is not a point of biological moment in the 

 same way as it is in larger plants, with separate sporangia, and with a 

 larger proportion of sterile to propagative tissue ; doubtless here again the 

 tendency to a superficial position of the spores, so imperfectly carried out 

 in the Bryophytes, shows only a distant analogy to the more pronounced 

 condition in Vascular Plants, as seen in their superficial sporangia. 



So also with the assimilatory system, imperfectly represented in most 

 Bryophytes, though better developed in some few (Splachnum, Buxbaumia, 

 Anthoceros] ; however similar these tissues may be to the functionally cor- 

 responding tissues in Vascular Plants, the similarities cannot with certainty 

 be held as more than points of analogy. The facts point to a wide-spread 

 " homoplasy " as having been effective in the Bryophytes and Pteridophytes ; 

 at the same time the similarity of the consequent characters seen in the 

 simpler organisms, throws suggestive light upon the origin of those of the 

 more complex. Nevertheless the similarities cannot safely be held to lead 

 further than to the recognition of certain methods of morphological 

 advance : they indicate that the origin of the sporophyte was probably the 

 same in both classes ; it may be traced from a primitive body, initiated 

 by the post-sexual complications involving chromosome-reduction. The 

 requirements of both in respect of increasing spore-production, and con- 

 sequently of nutrition under subaerial conditions, were essentially alike ; 

 independently each has probably worked out its own evolution ; and they 

 have independently arrived at results which show points of analogy such as 

 those above recognised. The mere existence of those analogies, with the 

 differences both of general scheme and of detail which they show, appear 

 to lend probability to the recognition of the general biological conditions 

 under which they are believed to have arisen. They were briefly these 1 

 that in land-growing forms which maintained the aquatic type of fertilisation 

 by a spermatozoid motile in water, a premium was put upon multiplication 

 of germs : and that multiplication of germs necessitates increased facilities 

 for their nutrition and dissemination. It appears probable that these offices 

 were carried out by tissues which originated ultimately by sterilisation of a 

 proportion of the potential germs. 



1 Compare Chapter VI. where the biological aspect of alternation has been discussed. 



