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 Azeura (Figs. 127, 129), and in Fella 
(Fig. 128), the final end here being a more effective distribution of the spores: 
it is more completely organised in Azthoceros, where it probably serves for 
nutrition as well as for distribution (Fig. 130 £); 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 (Sp/achnum, Buxbaumtia, 
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 thgt 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? 
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. 
