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 (5, d) remains between their margms; 

 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 

 f f in section, at e in front view. Where the wall of a vessel bounds a parenchymatous 

 cell (£), the scalariform thickening takes place only on one side of the vessel {g), and 

 is absent from the other side {p). 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. 



^P i^pi ^iiMgm 





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 /3 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 ; Fa piece of the wall of a vessel, 

 where it adjoins a parenchymatous 

 wood-cell ; a 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). 



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, A and 5, 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 



