286 BRYOPHYTA 



Pteridophytes, in which the apical growth is longer continued, and the 

 shifting of the fertile zone consequently more obvious. 



The results of progressive sterilisation should not only be studied in 

 their longitudinal aspect, but also in the transverse, as leading towards 

 decentralisation of the fertile residuum, and establishment of a central 

 sterile tract. There is reason to believe that the original type, both of 

 Liverworts and Mosses, had a solid core of sporogenous cells. In the 

 Liverworts a step toward decentralisation is seen in the partial elatero- 

 phores of the Jungermanniaceae, but it attains a greater completeness in 

 the Anthoceroteae, where, excepting in Notothylas, the spore-production 

 is relegated even to the amphithecium. In Sphagnum the same is the 

 case, though there is no guide as to the evolutionary steps which led to 

 it. In the Bryales also decentralisation has been effective, but has not 

 attained the length of relegating spore-production to the amphithecium. 

 The biological significance of decentralisation is plain, as the presence 

 of a central conducting column provides a means of better nutrition 

 for the increasing mass of spores than where these constitute a solid 

 core. In respect of the degree of decentralisation it may be said that 

 the Anthoceroteae and Sphagnaceae are the most advanced of the 

 Bryophytes ; but all Bryophytes stand far behind the Pteridophytes in 

 this respect, for as we shall see, in all the Pteridophytes the spore- 

 production is referable in origin, not to deeply seated, but to superficial 

 cells of the plant-body. 



The biological circumstances of dispersal of the ripe spores, as well 

 as those of nutrition, have doubtless affected the position of the 

 archesporium in Archegoniate Plants. Where, as in the Bryophytes 

 (excepting the Anthoceroteae), the spores are all produced simultaneously 

 in one capsule, which collapses at their maturity, a superficial position of 

 the archesporium is immaterial : indeed a relatively central position will be 

 advantageous as simplifying the problem of nutrition. Dispersal of the 

 ripe spores is then carried out by some drastic method of decay or of 

 dehiscence of the protective wall, and the whole sporogonium ceases its 

 functional activity with the liberation of the mature spores. The central 

 tissues can be sacrificed with impunity where, as in the Bryophytes, the 

 spore-production is simultaneous. But in Vascular Plants the spore- 

 production is in one way or another successive, and the succession, 

 acropetal as a rule, brings with it the great biological advantage of 

 spreading the physiological drain for nutrition over a longer period. In 

 this case the central tissue cannot be sacrificed, but must be maintained 

 as a nutritive core, in the interest of the later-formed spores of the 

 acropetal succession. A more superficial position of the archesporium 

 thus becomes necessary, while the projection of the separate sporangia 

 beyond the surface which bears them will increase the facility for 

 scattering the spores when mature. Thus the difference between t he- 

 deep-seated position of the archesporium of the Bryophytes, and its 



