246 
Boodle and WorsdelL — On the 
wall of a vessel having at each end bordered pits, while in the 
centre were one to two small perforations. 
As evidence of the great amount of sliding-growth taking 
place between the ends of the vessels, macerated material 
showed a long, pointed piece of wall extending a long way 
past the locality of the perforation (Fig. 25). 
The fibrous tracheides form the great bulk of the wood, 
and, as already described as seen in transverse section, they 
vary considerably in appearance. The type which is perhaps 
of the most frequent occurrence has a cell-wall about equal in 
thickness to the width of the cavity, with conspicuous bordered 
pits at short intervals from one another, the cavity of the pit 
being fairly large. The outline of the pit is usually circular, 
and the opening is slit-like and oblique, so the super- 
posed openings on each side of the common wall often give 
the appearance of a cross (Fig. 26). In other parts of the 
stem, however, of the same or of another species, broad 
zones occur, consisting entirely of fibrous tracheides whose 
walls have become excessively thickened, so much so as to 
reduce the width of the cell-cavity to a minimum, while the 
wall itself is, in many cases, split rather widely apart at the 
middle lamella, so that at first this gives the false appearance 
of a cell-cavity. In these elements no pits of any kind are to 
be seen ; but we may presume that they are of the same 
nature and origin as the above-mentioned fibrous tracheides, 
and that during elongation of the element, and the process of 
thickening of the cell-wall, the rudiments of the bordered 
pits, which most likely had begun to be formed, soon become 
entirely suppressed and obliterated. This is the more pro- 
bable, as in other parts of the stem elements were found with 
a wall of considerable thickness, but in which bordered pits 
occurred, though of very small size, and at distant intervals 
from each other ; they were in process of becoming suppressed 
like those in the more highly differentiated elements 1 (Fig. 27). 
The fibrous tracheides have bordered pits on all their walls ; 
1 Solcreder, loc. cit. 
