Conducting Tissue- System in Bryophyta. 3 
increasing adaptation of a simple leafy form to terrestrial life. 
And the final result, as seen in the highest Polytrichaceae, is 
so strikingly like, even in many details, the state of things 
obtaining in the true vascular plant, as to furnish probably 
one of the completest and most interesting cases of homo- 
plastic development in the plant-kingdom. It can hardly 
therefore be denied that the study of the conducting system 
in Mosses is calculated to throw most valuable sidelights 
on the question of the evolution of the vascular systems of 
the higher plants. 
Vaizey 1 has already investigated the conducting tissues of 
the sporogonium of Mosses with the object of throwing light 
on the evolution of these tissues among the vascular plants, 
but he was under the influence, as it seems to us, of curiously 
narrow and misleading morphological doctrine. He sets on 
one side the tissues of the gametophyte of the Bryophyta, 
since according to the antithetic theory of alternation of gene- 
ration the plant-body in which they are developed is not 
homologous with the plant of the Vasculares, and goes on to 
claim that ‘ every one will admit that the tissues of the 
sporophyte of the Muscineae are homologous with those of 
the sporophyte of the Vasculares ’ (p. 275). He thus implicitly 
assumes not only that the sporophyte of the Pteridophytes 
has been evolved from a Bryophytic sporogonium, but that 
its differentiated tissue-systems have been evolved from the 
differentiated tissue-systems of the highly specialized moss- 
sporogonium, an assumption so extremely improbable that 
it may be dismissed without discussion. In our opinion, 
while there can be no question of homology in either case, 
the conducting tissues of the Bryophytic gametophyte are 
at least as instructive as those of the sporogonium from 
this point of view of comparison, first because foliage-leaf 
structures are involved, and secondly because in the Poly- 
trichaceae at least, there is a rhizome which is physiologically 
and histologically a root, directly continuous with the aerial 
1 Journ. Linn. Soc. Bot., vol. xxiv, 1887-8. 
B 2 
