26 
MORPHOLOGY OF THE CELL. 
are the angles of contiguous cells. The formation of such a scalariform thickening 
begins by the growth, on the thin wall which separates two vessels (C, /), of transverse 
ridges {v), which unite on either side with the thickening which always lies at the 
angle of a cell-wall. C shows this in front view, D in vertical section. When com- 
pletely developed, the thin cell-wall (/) is absorbed {c, c, in B), the thickening-ridges 
have overarched, so that only a narrow fissure {B, d) remains between their margins ; 
still further inwards the ridge again becomes narrower. The interior cavities of two 
adjoining vessels are thus united by a number of long narrow fissures {B, s) ; the frame- 
work of the ladder is formed of peculiarly-shaped rungs, which may be seen in B at 
c c m section, at e in front view. Where the wall of a vessel bounds a parenchymatous 
cell {E), the scalariform thickening takes place only on one side of the vessel {g), and 
is absent from the other side (/). In this case also the thin original wall remains, 
closing externally the space of the fissure-like pit. 
The variety in the formation of pits is by no means exhausted by these examples ; 
but all the processes cannot be described here; we can only indicate a few. 
In the formation of vessels in the Dahlia (Fig. 25) the pit occupies at first a large 
round space, while the edges of the overarching thickening enclose a fissure. By 
a modification of this process of growth the fissure may attain a length much greater 
than the diameter of the pit, which then appears, on a front view, as a roundish opening 
crossed by a fissure (Fig. 26, P). It also sometimes happens that the pit-fissure changes 
its direction as the thickening increases ; in this case, on a front view, two fissures 
appear as if they cross one another (Fig. 28, ^ and B, st). But in order to be certain 
that this takes place within the layers of the wall of a single cell, the cells must be 
isolated by maceration. Similar appearances are also often seen if the partition-wall 
of two cells is observed from the front. If the fissure inclines to the left in the one 
Fig. 25.— Wall of a vessel with bordered pits from the 
tuberous root of the Dahlia ; A front view of the wall of a 
vessel from without ; B transverse section of the same (hori- 
zontal, at right angles to the paper) ; C longitudinal section 
(vertical, at right angles to the paper) ; q septum ; a the 
original thickening-ridges ; b the overarching part of the 
thickening-masses ; c the fissure through which the cavity of 
the pit communicates with the cell-cavity. At a and ß the 
corresponding front view is appended in order to make the 
transverse and longitudinal sections more clear (x 800). 
Fig. 26.— From the root-tuber of 
the Dahlia ; P parenchymatous wood- 
cell ; Va piece of the wall of a vessel, 
where it adjoins a parenchymatous 
wood-cell ; rt b the thickening-masses 
of the wall of the vessel cut through 
at right angles ; c fissure of the pit ; 
d simple pits in the parenchymatous 
wood-cell (x 800). 
