Bancroft.—On the Xylem Elements of the Pteridophyta . 749 
vermilion or Indian ink. Longitudinal sections of the injected stems 
showed that the granules of colouring matter rose to different heights 
throughout the xylem, and that they were crowded together in the ends of 
the pointed elements. It was also noted, both in transverse and longitudinal 
section, that one element might show granules, while its neighbours were 
quite free from them (cf. Text-fig. 2 (a) and ip)). 
Text-fig. 2. ( a ) Diagrammatic transverse section of the xylem elements of Marattia laxa , 
illustrating the presence or absence, at the same level, of injected matter adhering to the inner 
surface of the cell-wall. ( b ) Diagrammatic longitudinal section of M. laxa, showing the accumula¬ 
tion of granules in the pointed ends of the elements. 
If the xylem had been a communicating system, as Gwynne-Vaughan 
states, the granules of cinnabar or Indian ink would have been drawn to the 
same height throughout, not being impeded by either end or side walls. 
Again, if the xylem had consisted of true vessels, there would have been no 
great amount of accumulation of colouring matter in the pointed ends of the 
elements, for the perforations would have allowed of their passage. But the 
fact of the presence of injected matter in some, and of its absence in others, 
of neighbouring elements at the same level, points to the presence of a mem¬ 
brane closing the pits of the lateral walls. Hence it would appear that the 
xylem of Pteridophyta cannot be a communicating system of the type 
described by Gwynne-Vaughan ; neither can it consist of vessels (at least 
in the majority of known cases), for the accumulation of granules at the 
pointed ends of elements shows that the end walls must have acted as 
filters, not permitting the passage of suspended matter. This points to the 
presence of a pit-closing membrane here, as well as in the truly lateral por¬ 
tions of the walls ; so that the above-described physical tests strongly 
3 u 
