Review . 
1 20 
development all sporophytes are originally radial, a condition which 
has probably a very close relation to their production in the 
archegonium ” ; consequently he objects to a theory which would 
make the dorsiventral condition primitive, and the radial derived, 
as has been done in the form of this view recently maintained by 
the authors quoted. Finally he remarks that “ there is no known 
case of dichotomy in the sporophyte, where one branch develops 
as axis and the other as leaf.” 
It cannot be denied that the first two objections have 
considerable force. No one, certainly, would set out to explain the 
origin of the leaves of the microphyllous Pteridophytes, if these 
were the only forms known to us, as derived from branches of a 
dichotomous thallus of which the sister branches represented in 
each case the continuation of the axis beyond the insertion of the 
leaf. Nor can it be denied that the radial organisation of the young 
Pteridophyte is a strong, though not perhaps a conclusive, piece of 
evidence for regarding this type of organisation as primitive in the 
group. 
The third objection, that we never in fact get a case of 
“ dichotomy ” in which one branch becomes a leaf and the other an 
axis, will not however bear analysis. For what is the test of such 
a “ dichotomy ” ? Certainly not an equal symmetrical division of 
an apical cell or multicellular meristem, for Dictyota is practically 
the only known case in which such an equal division occurs. In 
practice, branching is said to be dichotomous when an elongated 
organ produces two branches of the same morphological nature 
and of approximately equal strength, neither of which appears to 
be a direct continuation of the original axis. Such branching may 
arise, so far as we know, in two different ways. In one case, found 
among Liverworts, the original apical cell or cell-group stops 
growing, and two new growing points are initiated right and left 
of it. In the other a new apical cell is initiated in a segment 
or the product of a segment of the original apical cell (which 
continues to divide), but the branch thus laterally produced grows 
as vigorously as the continuation of the original axis and a 
“dichotomy” is the result. Now there is no essential histogenetic 
difference between this process and the origin of a fern-leaf close 
to the apex of the stem-axis. It is therefore quite illegitimate to 
argue that a theory of the origin of the leaf and stem of a fern 
from originally equivalent “ dichotomous ” branches is excluded 
because there is no dichotomy as between the leaf and stem of a 
fern, and it is meaningless to say that the theory “ appears to 
break down on the test of fact.” If the histogenetic criterion is to 
be applied, the plain result is that no such thing as strict dichotomy 
exists among plants at all except in Dictyota and some filamentous 
Thallophytes. The problem we are really concerned with is the 
question whether the precursors of stem and leaf are derived from 
branches, originally equivalent in nature, of a forking thallus. 
While it is certain that we have no direct evidence of such an 
origin, it may be fairly contended that it is perfectly practicable, 
and not inconsistent with anatomical evidence derived from the 
more primitive ferns. 
If we use the word “dichotomous” as it is commonly applied, 
abandoning the histogenetic criterion, it becomes clear that the 
distinction between “ monopodial ” and “ dichotomous ” branching 
