122 
Review. 
system was always and exclusively dichotomous) we pass to a 
consideration of the “ third view ” (which is Professor Bower’s own), 
“ that the axis pre-existed, and the foliar appendages arose as out 
growths upon it.” 
It will of course be admitted by the author’s opponents that a 
thallus-axis of some sort pre-existed among remote algal ancestors. 
The point in dispute is whether an axis without foliar appendages 
existed in the immediate ancestors of the Pteridophytes. From 
such an axis Professor Bower believes that leaves arose by “enation,” 
i.e. by exogenous outgrowth from a previously vacant surface. We 
have to imagine an ancestral sporogonial axis without appendages. 
This, according to the author, became differentiated into a fertile 
upper region with the power of apical growth, and a vegetative 
base; the sporogenous cells were then relegated to a superficial 
position and segregated into separate pockets or sporangia ; and 
finally there was enation of the foliar appendages. It also had to 
originate a root-system, whose mode of origin is left doubtful. 
It may be freely admitted that this is a perfectly clear, clean- 
cut theory, which harmonises the Bryophytic sporogonium with the 
Pteridophytic sporophyte, and accords well with the cytological 
facts of chromosome reduction in the process of spore-formation 
throughout the Archegoniate series. It also accords with the 
general similarity of the Fern-prothallus and the Hepatic thallus, 
and of the archegonium in the two groups. Let us see what is the 
main evidence brought forward in its favour in The course of this 
eleventh chapter—“ The Theory of the Strobilus.” 
Professor Bower remarks that “ much of the difference of view 
as to foliar origin centres round the question whether originally 
the leaf was relatively large or small.” In other words if we can 
establish a presumption that the small-leaved strobilate Pteri¬ 
dophytes are relatively primitive and the Ferns derived we shall 
reach a firmer basis for the sporogonium and enation hypothesis. 
On this point he cites anatomical evidence. Adopting Jeffrey’s 
distinction of the cladosiphonic type in which the gaps in the stele 
are branch-gaps only, and the phyllosiphonic type, in which the 
stelar gaps are connected with both branches and leaves, Professor 
Bower states (p. 140) that “ in certain Ferns the progression may 
be traced from the cladosiphonic in the young plant to the phyllo¬ 
siphonic in the mature, thus suggesting a similar progression in 
descent, viz : that the large-leaved phyllosiphonic Ferns were 
derived from a smaller-leaved cladosiphonic stock.” The reviewer 
is unaware of any case in which the stele of a young fern shows 
branch-gaps and not leaf-gaps. The cladosiphonic and phyllo¬ 
siphonic types of structure distinguished by Jeffrey are charac¬ 
teristic respectively of the microphyllous and megaphyllous phyla of 
Pteridophytes, when the structure of the stem is siphonostelic. .To 
the protostelic types of any phylum it does not apply, because 
there cannot be leaf-gaps in protostelic forms. The ordinary pro¬ 
gression seen in the young fern-stem, is from the protostelic to 
the siphonostelic condition. But protostelic forms are in no sense 
cladosiphonic, and afford no presumption whatever of derivation 
from a microphyllous type. 
The leading argument used by Professor Bower in this chapter 
may perhaps be fairly stated as follows: that there is a presump¬ 
tion in favour of the origin of foliar appendages by enation because 
