754 Bancroft.—On the Xylem Elements of the Ptevidophyta. 
It would therefore appear that there must be some connexion between 
the bars, holding them in position, and a high magnification proved this to 
be the case. 
Gwynne-Vaughan considered that where he saw the connecting middle 
substance between the bars in longitudinal section, he was looking down at 
the corner of an element where he recognized that the primary parts were 
retained. This being the case, it is probable that his general reliance upon 
transverse sections was the cause of his overlooking the presence of a pit¬ 
closing membrane. 
In the case of Pteris Aquilina , in which Gwynne-Vaughan recognizes 
the presence of the primary wall at least between the thickened areas, he 
says ‘ these persistent parts ... do not become lignified, but remain pectic in 
character and stain readily with ruthenium red. When a tracheal wall 
treated with this reagent is regarded in surface view, the pectic middle 
substance will appear as a red area shining through the lignified and 
unstained secondary layers and outlining the inner limits of the pit cavities ’ 
(cf. 2 , PI. XXVIII, Fig. 10). A surface view of a tracheide of Lycopodium 
selago , stained with ruthenium red and methylene blue, showed in certain 
foci the appearance mentioned by Gwynne-Vaughan, except that the lignified 
layers, unstained in his sections, were here stained blue. At a lower focus, 
however, the red of the pit-membrane itself was readily seen. It is sug¬ 
gested that this effect was produced by the separation and raising of the 
lignified layers away from the membrane at the pits (as in a bordered pit), 
so that they occupied a different focus from that of the membrane, this 
being, in the actual pit-opening, at too low a focus to be seen. In the areas 
immediately surrounding the pit-space the primary red-stained pectic layers 
did not shine through, as here they were not backed and covered closely by 
the lightly blue-stained parts. 
A simple illustration will make this last suggestion more clear. Sup¬ 
pose a piece of red paper to represent the middle primary substance stained 
with ruthenium red, and two pieces of thin blue paper placed closely, one 
behind and one in front of it, to represent the layers of secondary thickening 
stained with methylene blue. If the light is allowed to pass through these 
three layers, when in contact with one another, the red will be seen through 
the blue. But if the three layers are separated, as is the case in a pit-area, 
the red will not shine through the blue so readily. 
In his description of the growth and formation of the xylem elements 
in Osmunda cinnamomea (as typical of the Osmundaceae and of multi- 
seriately pitted forms in general), Gwynne-Vaughan remarks that the 
pectose primary wall is present in immature elements, and that, as maturity 
is attained and lignification of the secondary layers becomes fairly strong, it 
gradually disintegrates, except at the angles and (in this particular type) 
between the series of pits. This is clearly shown in his Fig. 8. Eventually, 
