558 Bower.—On Medullation in the Pteridophyta. 
be prepared to demonstrate the extrastelar origin of the pith in each 
phyletically distinct series of Medullated Plants if his statement is to hold. 
The plain fact is that a statement has been made ‘ for all cases ’ before 
‘all cases’ have been adequately investigated, and it is not put forward as 
a working hypothesis, but as a rigid law. Whether Professor Jeffrey’s 
generalization be ultimately substantiated or disproved, his handling of 
the origin of pith cannot be quoted as a model of scientific method. 
I do not propose to enter here upon any critical examination of the 
facts for or against the stelar or cortical origin of the pith in all the several 
phyla of Vascular Plants which show medullation. This will be left to the 
professed anatomists. But two extreme examples may be taken from the 
Pteridophyta, which illustrate, to my mind satisfactorily, that the origin of 
the pith has not been uniform. The one case provides an example of extra¬ 
stelar pith, the other of pith of intrastelar origin. The first example is 
taken from the Filicales. No one who has followed the evidence based 
upon the endodermal barriers would now deny that the tissue filling the 
cavity in rhizomatous solenostelic Ferns is of extra-endodermal, that is of 
cortical, origin, certainly in the greater part, and probably in most cases 
altogether. The suggestion of this came first from Van Tieghem in 1890, 
and the idea was taken up and amplified by Professor Jeffrey in 1897. But 
as Mr. Tansley pertinently remarks in his ‘ Lectures on the Evolution of the 
Filicinean Vascular System’ (p. 132), ‘Professor Jeffrey has nowhere dis¬ 
cussed in detail the origin of the siphonostele (solenostele) from the proto- 
stele, though he himself stated the view of the primitiveness of protostely.’ 
The most exact demonstrations of the steps of the progression in distinct 
phyla of Ferns, from the protostele to the siphonostele, and of the modifica¬ 
tions which the latter undergoes, are due to Professor Gwynne-Vaughan, 
Mr. Tansley, and Mr. Boodle, while by far the best comprehensive account 
of the process and its results is that given by Mr. Tansley. 1 It may be 
taken that, speaking phyletically, the cortical origin of the pith in such 
cases has been demonstrated by these writers. It is, however, to be clearly 
understood, as pointed out by Mr. Tansley, 2 that such siphonostely is 
characteristic of Ferns belonging to the middle grades of evolution, though 
it also occurs in several of the lower and a few of the higher types. Strati- 
graphically it has repeatedly been found in the fossils of the secondary and 
more recent rocks, and persists in many present forms. It remains yet to 
be shown that this stelar structure figured largely, or even at all, in the 
really primitive types of vascular construction. 
The second example, where the pith appears to have been clearly intra¬ 
stelar in its origin, is in the Lepidodendreae. This is indicated by a com¬ 
parison of early protostelic types such as Lepidodendron esnostense and 
L. rhodumnense from the Culm, with the condition seen in L . vasculare 
1 1. c., 1908. * 1. c., p. 56. 
