752 Bancroft.-—On the Xylem Elements of the Pteridophyta . 
Halft finds true tracheides ; Gwynne-Vaughan finds true vessels, often 
of a peculiar type. 
Halft’s observations point to the retention of the primary wall between 
the transverse bars of lignified substance—those of Gwynne-Vaughan to its 
retention in elements of the type presented by Pteris Aquilina ; to its partial 
disappearance in multiseriately pitted types of element such as those of 
Osmunda cinnamomea ; and to its complete disappearance (except, of course, 
at the angles of the cells) in uniseriately pitted forms, as found in Nephro - 
dium Filix-mas. 
As mentioned in the Introduction, Gwynne-Vaughan was led to his 
conclusions by observations made in connexion with the fossil Osmundaceae. 
Transverse sections of Osmundites skidegatensis showed apparently clear 
splits between contiguous xylem elements (cf. 2 , PI. XXVIII, Fig. 2). In 
the present investigations it was found that sections must be examined 
under a high magnification (550-1500 diameters), and that they must be 
fairly thin, if the middle lamella and its continuation as pit-membrane were 
to be seen distinctly. For example, in a transverse section of Matonia pec- 
tinata stained lightly with ruthenium red and methylene blue, the middle 
lamella was at first overlooked, under a magnification of 325 diameters, 
though a higher power revealed its presence. But the ordinary fossil section 
is not prepared for examination under a very high power, and extremely 
minute histological points may easily be overlooked. Miss Kershaw men¬ 
tions and figures the occurrence of spaces between the xylem elements in 
Solenoslelopteris ( 4 , p. 686, PI. LVIII, P"ig. 5); and an examination of 
transverse sections of the fossil plants Botryopteris sp., Psaronius sp., Lepi - 
dodendron Harcourtii , Z. vasculare , Lepidophloios fuliginosus ) Sphenophyllum 
plurifoliatum , and others, proved the frequent occurrence of these appa¬ 
rently clear spaces. It seems, however, unlikely that this appearance is 
due to the fact that the lamella has disappeared (at least normally—it may 
be broken down by the attacks of Fungi (cf. Seward, 12 )). It is more 
probable that the means of preparation and examination are, as a rule, not 
perfect enough to demonstrate its presence, in the case of fossil specimens, 
especially since its occurrence has been definitely proved in allied types 
among recent plants, e.g. in members of the Ophioglossaceae, Osmundaceae, 
and Hymenophyllaceae (cf. Seward, 13 ; Kidston and Gwynne-Vaughan, 
5 and 6) ; the Marattiaceae ; the Lycopodiales; and the Psilotales. 
In the xylem of the Lepidodendreae, fine connexions are often present 
between adjacent bars of secondary lignified substance, as, for example, in 
the short elements in the central part of the protostele of Lepidodendron 
vasculare (Seward, 13 ). Professor Seward, in describing these delicate con¬ 
nexions, refers to Gwynne-Vaughan’s view and remarks that ‘ in the Lepido¬ 
dendron tracheae we seem to have a stage in which the intervening mem¬ 
brane is in process of absorption. It is, however, possible that the threads 
