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 JVotothylas, the spore-production 
is relegated even to the amphithecium. In Sflagnum the same isthe 
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 the 
deep-seated position of the archesporium of the Bryophytes, and _ its 
