2 Tansley and Chick . — Notes on the 
xylem and phloem. All the members of these two highest 
groups of plants, except perhaps a few doubtful cases among the 
lower Pteridophytes, possess such a differentiated conducting 
system, or can clearly be shown to have lost it by degeneration. 
In the Bryophyta this is not the case. Though (unlike the 
green Thallophyta) the great majority are land-plants, yet 
nearly all the Liverworts and several of the Mosses have no 
specialized conducting system, nor is there any reason to 
suppose that their ancestors ever had. But the exigencies 
of increasing bulk, and particularly of the erect habit, have 
led, in a few of the Liverworts and the majority of the Mosses, 
to the acquirement of a conducting system, which, while 
it is extremely simple in most cases, attains in the highest 
forms (Polytrichaceae) to a complexity not only distinctly 
surpassing that of the simplest true vascular plants but almost 
comparable with that found among highly developed Phanero- 
gams. Now it is almost as certain as any phylogenetic thesis 
is likely to be, that the conducting tissues of Bryophytes have 
nothing directly to do with the origin of the conducting tissues 
of the higher plants. The main seat of the development of 
these tissues in Bryophytes is the gametophyte generation, 
which is in any case excluded from the comparison, since 
the vascular system in Pteridophytes is confined to the 
sporophyte. And at the least it is extremely unlikely that 
the Pteridophytes have been derived from a Bryophytic 
ancestor with a sporophyte showing anything approaching 
the specialization of the moss-sporogonium, in which con- 
ducting tissues also occur. But it must not for this reason 
be supposed that the Bryophytes are of no interest in con- 
sidering the problem of the evolution of the vascular system 
in Pteridophytes. We see among the former group, plants 
in the very act, so to speak, of developing a conducting 
system in response to vital needs, and others in the most 
various stages of its evolution in complexity. The conditions 
under which this evolutionary development occurred must 
have been practically identical with those to which the 
primitive Pteridophytic sporophyte was subjected — gradually 
