518 Gwynne- Vaughan. — On the Real Nature of 
It appears from our observations that the above statement is thoroughly 
misleading, and that a return must be made to the views held by the earlier 
anatomists, 1 who believed that the xylem elements of the Ferns in general 
were true vessels and not tracheides at all. 
The xylem of the Osmundaceae will first be described, and in particular 
that of Osmunda cinnamomea , in which plant the point in question was the 
most thoroughly investigated. To begin with, it appears that the Osmun- 
daceae as a whole must be added to the list of Ferns whose tracheae con- 
stantly possess more than one vertical series of pits on each side of their 
walls. Almost all the sides or facets have two or more series of pits 
(PI. XXVIII, Fig. i) ; indeed, the typical scalariform marking with a single 
series of pits is only to be found on the smallest elements at the external 
periphery of the xylem ring and on those of the decurrent protoxylem 
strands of the leaf-trace. In the fossil representatives of the order the 
multiseriate nature of the pitting is even more pronounced, walls being 
often found with as many as four or five regular series. In the largest 
elements the pits are so numerous, and so irregularly arranged, that in 
surface-view the walls present the same reticulate or porose appearance as 
do the xylem elements of Botryopteris (Fig. 2). 
To turn now to the actual nature of the pits themselves ; it soon 
became clear that there is no pit-membrane whatever separating the cavities 
of two contiguous tracheae from one another. The pits are, in fact, actual 
perforations permitting free lateral passage through the side walls of the 
tracheae. This is not all, for the repeatedly unsuccessful attempts to 
demonstrate a pit-membrane gradually brought to light the surprising fact 
that there was no ‘ middle substance * in the other parts of the wall ; not 
even between the bars of thickening that separate the pits. For instance, 
in a wall bearing a single series of pits the bars of thickening stretch across 
quite freely from one corner to another without coming into contact in the 
median plane of the wall (Fig. 3). The bars are in opposite pairs, one 
belonging to each of the two contiguous tracheae, and there is an empty 
space between them. It follows, therefore, that there is a free passage both 
up and down in the middle of the common wall from one pit cavity to the 
other throughout the whole series of pits. When there is more than one 
series of pits on the same side of the wall, matters are not quite so simple, 
for the parts of the wall separating the several series of pits vertically 
are solid right through (Fig. 4). At these points the bars belonging to the 
two tracheae are firmly cemented together by an intervening ‘ middle 
substance.’ In this case, therefore, although there is the same free passage 
up and down in the middle of the wall for each series of pits, there is 
no direct lateral passage from one vertical series to the other. 
1 Dippel, Das Mikroskop (new ed.), p. 280 (1st ed. 1869); Sachs, Textbook of Botany 
(English ed.), pp. 25-26 ; Gustav A. Weiss, Anatomie der Pflanzen, p. 271, 1878. 
