324 Dr. G. Ogilvie on the Vascular 
as the woody fibres of the Lime and the Yew, which are marked 
in the interior with a spiral thread. 
Not that woody fibres of any kind are themselves to be 
regarded as elements in progress of conversion into vessels or 
ducts, or as in any sense in a transition state. Both fibres and 
vessels are equally ultimate forms, though it may be surmised 
that, according to the energy of the developmental process, the 
result is either simple woody fibre or some of the varieties of 
vascular tissue. The transformation, according to Schleiden, 
takes place progressively from within outwards in the fasciculi 
of Endogenous plants, but in Ferns simultaneously throughout 
the whole of that portion of the cambium-tissue which is to 
undergo conversion. 
A summary is here subjoined of the more prominent characters 
of the petiole and rhizome in the species which have been enu- 
merated, and a few others. It was my wish to have included 
all the well-defined species of our flora; but in a few cases I 
have not been successful, after every exertion, in obtaining rhi- 
zomes in a state fit for examination. The notice could not have 
been made even so complete as it is, but for the kind assistance 
of some friends, among whom I have especially to tender my 
thanks to Professors Balfour of Edinburgh and Dickie of Belfast, 
Mr. L. Squire of Falmouth, the Rev. W. Gregor of Macduff, and 
Mr. Beverley and Mr. Roy, Sen., of Aberdeen. All the descrip- 
tions have been verified by the examination of fresh specimens. 
Polypodium vulgare. 
The petiole has three or more fasciculi, derived from the 
netted cylinder of the creeping rhizome, which acquire sheaths 
of dark tissue, and finally run into one central bundle as they 
ascend towards the frond. 
The rhizome consists of pale tissue, invested by a brown cuti- 
cular layer. 
Polypodium Dryopteris. . 
The petiole has two fasciculi running the whole length of the 
frond, with sheaths of dark tissue. 
The rhizome as in the last species, though less tortuous and 
much more attenuated. 
Polypodium Phegopteris. 
The petiole has two fasciculi running into one above the base, 
and without dark sheaths. 
The rhizome as in the former species, except that the general 
tissue (as well as that of the lower part of the petiole) has its 
cells somewhat elongated, and of a decided brown tint. 
