THE CELL-WALL. 



27 



original breadth, where it is raised on the thin cell-wall ; but the free rim which grows 

 further inwards, expands, and becomes arched over the thin parts of the cell-wall. But 

 in this case the overarchings do not grow uniformly, but in such a manner that their 

 rims form at least a fissure (c, in A and B). Here also, when two similar cells adjoin, 

 the same process takes place on both sides of the partition-wall ; and here also lenticular 

 spaces are formed by the overarchings ; these are at first bisected by the original thin 

 lamella of the cell-wall, which afterwards disappears, and the two cell-cavities are placed 

 in communication at each bordered pit ; the chan- 

 nel or bordered pit which unites them is wide in 

 the middle, and opens right and left into each cell 

 by a narrow fissure (Fig, 27, B, C). If, on the other 

 hand, a vessel of this kind adjoins a parenchyma-cell 

 which remains always full of sap and closed, the 

 thickening and overarching of the pit occurs only 

 on the side of the vessel (Fig. 28, V); the thin parts 

 of the cell- wall are retained 1, and the bordered pits 

 remain closed ; from the cell-cavity of the vessel 

 a narrow fissure {c) proceeds between the expanded 

 thickening -masses [b) to a wider cavity, which is 

 bounded on the sides by the narrow part of the 

 thickening-masses {a), on the outside by the primary 

 cell-wall. These processes can only be seen in sec- 

 tions of extraordinary tenuity ; but these are easily 

 obtained if larger pieces of the parts to be observed 

 are allowed to lie for months in plenty of absolute 

 alcohol, then taken out before the preparation is 

 made, and the alcohol allowed to evaporate : in this 

 manner pieces of some hardness and toughness are 

 obtained, which may be cut extremely well and 

 smoothly if the knife is very sharp. 



In the walls of vessels thickened like ladders or 

 steps, which are developed with peculiar beauty in 

 the higher Cryptogams, the bordered pits are fis- 

 sure-like ; they are often as broad as the partition- 

 wall of two adjoining cells, but very narrow in the 

 direction of the longitudinal axis of the cell. In 

 Fig. 29, A, is shown the lower half of a vessel of 

 this kind with the fissure-like pits, between which 

 the thickening-masses of the wall lie like rungs of 

 a ladder ; the larger clear spaces are the angles of 

 the contiguous cells. The formation of such a 

 scalariform thickening begins by the growth, on the 

 originally very thin wall which separates two vessels 

 (C, /), of transverse ridges of thickening (y), which pass over, right and left, into that 

 thickening which always lies on the angle of a cell- wall. C shows this horizontally, 

 D in vertical section. When completely developed, the thin lamella (/) has disap- 



Flf,. 29. — Pteris aquilina, vessel from the un- 

 derground stem thickened in a scalariform man- 

 ner ; .'/ a half-vessel, isolated by Sthulze's mace- 

 ration ; B — D obtained from pieces of the stem 

 liardened in absolute alcohol; B after a very 

 clean section, represented half as a diagram ; to 

 the right, front view of the wall of the vessels 

 from within ; c c vertical section of the same ; 

 C front-view of the young wall of a vessel ; 

 D its vertical section; E place where a vessel 

 adjoins a succulent cell, in section vertical to the 

 thickening-ridges of the vessel (X8oo). 



^ These thin pieces of cell- wall which close up bordered pits may, by rapid surface- growth, 

 form bag-like prominences, which grow through the pores of the pits into the vessels, spread them- 

 selves out there, become separated by septa, and thus form a thin-walled tissue, which not unfre- 

 quently fills up the whole of the cavity. These formations were long known under the name of 

 'Tiillen '; they are abundantly and easily seen, for instance, in old roots of Cucurbita, and in the wood 

 of Rohinia pseiidacacia, &c. [These cells contained in the ducts are, according to Mohl and Reess, 

 really hernioid protrusions from adjacent cells; see Journ. of Eot. 1872, pp. 321-323, t. 126; and 

 Reess, Bot. Zeitg. 1868, pp. i-ii, 1. 1.] 



