On the Xylem Elements of the Pteridophyta. 
BY 
NELLIE BANCROFT, B.Sc., F.L.S. 
Research Scholar in the University College of Nottingham. 
With Plate LVI and three Figures in the Text. 
A. Historical. 
HE question regarding the nature of the xylem elements of Pterido- 
1 phyta—whether they are vessels of ordinary type or of a special 
type, or whether they are tracheides—is one which has given rise to much 
discussion. The differences of opinion on this point are due to the difficulties 
attending the detection of the primary portions of the cell-wall in much 
altered tissues ; for upon the recognition of the presence of a pit-membrane, 
itself a part of the middle lamella of the primary cell-wall, depends the 
solution of the whole question. Dr. Fr. Halft ( 3 ) has recently given an 
excellent resume of the views of the earlier investigators of this subject, 
pointing out that it was Sanio ( 11 ) who, in 1873, caused general opinion to. 
turn in favour of the presence of the pit-closing membrane, at least in 
Phanerogams. 
With regard to the Vascular Cryptogams, it was at first maintained 
that their xylem elements were true vessels, the presence of a pit-membrane 
being denied. De Bary (1), however, after the appearance of Sanio’s work 
on the Scots Pine was led to investigate other forms, including Vascular 
Cryptogams. He, too, noted the pit-membrane, mentioning especially 
Pteris Aquilina , where it occurs in the side walls of the elements ; and 
in 1891 Strasburger ( 16 ) demonstrated that the Vascular Cryptogams in 
general possess pointed tracheides, dovetailing into one another and present¬ 
ing no distinction between their end and side walls. At present only two 
exceptions to this general rule are known—the root of Nephr odium Filix - 
mas and Pteris Aquilina having true vessels, since the end walls are 
perforated (Russow, 10 ). 
In 1908 Prof. Gwynne-Vaughan (2) returned to the former theory 
of the vascular nature of the xylem elements of Ferns. His conclusions 
are based upon investigations suggested by the appearance of transverse 
sections of certain fossil forms referred to the Osmundaceae (Kidston and 
[Annals of Botany, Vol. XXV. No. XCIX. July, 19x1.J 
