THE CELL- WALL, 



25 



the cell-cavity of the vessel a narrow fissure {c) opens bet^^een the expanded thickening- 

 masses {b) into a M^ider cavity, which is bounded on the sides by the narrow part of the 

 thickening-masses {a), on the outside of the primary cell-wall. These processes can 

 only be seen in sections of extraordinary tenuity ; but these are easily obtained if good- 

 sized pieces of the parts to be studied are allowed to lie for months in plenty of 

 absolute alcohol, taken out before the preparation is made, and the alcohol allowed to 

 evaporate ; in this manner pieces of sufficient hardness are obtained to cut extremely 

 well with a very sharp knife. 



Fig. ■2j,.—Pinus sylvestris ; radial longitudinal sec- 

 tion through the wood of a rapidly growing branch ; 

 cb cambial wood-cells ; a — e older wood-cells ; 1 1" t" 

 bordered pits of the wood-cells, enlarging with age ; 

 St large pits where cells of the medullary rays touch 

 the wood-cells (X SSo). 





Fig. 24.— Pimes sylvestris; A transverse section of mature 

 wood-cells (X 800) ; m middle lamella of the common partition; i 

 inner layer, clothing the cavity ; z intermediate layer of the cell- 

 wall ; t a mature pit cut through the middle ; 't' the same, but at a 

 thicker part of the section, the part of the cavity of the pit lying be- 

 neath is seen in perspective ; t" a pit cut through beneath its inner 

 opening ; B transverse section through the cambium (X 800) ; c 

 cambium ; h wood-cells still young ; between them two very young 

 wood-cells with pits it beginning to form ; (C — F diagrammatic). 



In the walls of scalariform vessels, which are developed with peculiar beauty in 

 the higher Cryptogams, the bordered pits are fissure-like ; they often stretch across 

 the partition-wall of two adjoining cells, but are very narrow in the longitudinal 

 direction. In Fig. 27, A^ is shown the lower half of a vessel of this kind with fissure- 

 like pits, separated by thickening-masses like rungs of a ladder ; the larger white spaces 



wood of Robinia pseudacacia, &c, [See Joum. of Bot. 1872, pp. 321-323, t. 126; and Reess, Bot. 

 Zeitg. 1868, pp. i-ii, t. I ; also infra, Book iii. Sect. 16.] 



