557 
Bower.—On Medullation in the Pteridophyta . 
(iii) It may originate sometimes in one way, sometimes in another: 
indeed, it may be contemplated as possible that the pith may be partly 
intrastelar and partly extrastelar in origin in the same individual organism. 
This is the view which appears to me to be in accordance with the known 
facts. It commends itself as being in harmony with general morphological 
experience, and especially in that it provides for possible cases of parallel 
development, which on the experience of external morphology may be 
expected to occur. 
As the extreme intrastelar view drops out of the discussion, the 
question which concerns us here will be whether the pith of Vascular 
Plants is constantly extrastelar or whether it originates in various ways. 
In discussing this alternative, it must first be remarked that Professor 
Jeffrey appears entirely to misapprehend what will naturally be expected 
of him in support of his assertion of a constantly extrastelar origin. For 
he says 1 that, ‘ although it is not possible to prove in all cases that the 
pith may be derived from outside the stele, it is fortunately only necessary 
to demonstrate this in a few instances, in order to invalidate the position of 
those who claim that the pith is differentiated from the stele itself.’ This 
may be a satisfactory reply to such anatomists as are said to take the 
extreme intrastelar view, but it is no answer to those who are prepared to 
see a varied origin of the pith. To show that their position is erroneous 
and that Professor Jeffrey is right ‘in all cases’, the phyletic story of the 
pith must be known ‘ in all cases ’, and a single discrepancy destroys his 
generalization. He has made his statement in rigid and comprehensive 
terms, and the onus probandi lies with him. Nor will the establishment of 
the phyletic history of the pith in any one line of descent have any direct 
bearing upon that in any other, except as an interesting analogy, unless it 
can be proved that the origin of the pith antedated the phyletic segregation 
of those stocks from a common ancestry. As regards the Pteridophyta, it is 
not yet known for certain that any two of the recognized phyla, viz. Fili- 
cales, Equisetales, Sphenophyllales, or Lycopodiales, had an origin from 
a common ancestor, much less do we know whether that ancestor was 
pithed or not. But we do know that more than one of those phyla includes 
protostelic types, while comparison indicates that the vascular system in all 
of them is probably referable to protostely as the original condition. More¬ 
over, protostely is present in examples which on other grounds are held 
to be relatively primitive. Such considerations clearly indicate that the 
pith in each phylum, where present, has originated phyletically distinct 
from that of the rest. Consequently, each separate phylum in which pith 
appears will present its own problem of medullation, which must be solved 
independently of the others. There is no common rule which can be estab¬ 
lished by observations made ‘in a few instances’. Professor Jeffrey must 
1 k c., p. 403. 
pp 
