THE CELL- WALL. 



?,5 



Fig. 39. — Epidermis of the central vein of the leaf of 

 the lioUy ; A transverse section ; B superficial appear- 

 ance. 



assumes the blue colour, as well as the cuticularised substance, is formed of several 

 layers in the sense indicated under (d) ; each is composed of a system of layers. In the 

 latter moreover the radial lamellar structure (striation) is more evident, as is shown in 

 Fig. 39, A, a, b ; these radial lines are not, 

 as was formerly thought, pores, but are the 

 appearance presented transversely by the la- 

 mellae ; they are to be seen (in Fig. 39, B, s, 

 a front view of the cuticle) as striae, which, 

 following the veins of the leaf lengthways, 

 pass over the septa of the cells (q). 



An example of strongly ligni/ied cell-walls, 

 divided into three shells, occurs in the dark- 

 brown-walled sclerenchyma-cells which com- 

 pose the firm bands between the vascular 

 bundles in the stem of Pteris aquilina (Fig. 

 40). The very thick wall between two cells 

 contains an intermediate, hard, deep-brown 

 lamella {a) ; this is followed on each side 

 by a light - brown, more horny shell (Jj) ; 

 and this encloses a third, likewise light-brown 

 shell. By boiling in nitric acid with potas- 

 sium chlorate the first {a) is dissolved, and 

 the cells arc thereby isolated (cf. Fig. 30) ; the 

 two other shells of the cell-wall (/>> and r) 

 remain unchanged by this maceration, except 

 that they lose their colour ; and hence the 

 shell c is shown to be composed of some 

 more and some less watery layers (Fig. 30, 

 C, c). The three shells also show a different 

 behaviour on treatment with concentrated sul- 

 phuric acid : a becomes a dark reddish brown, 

 and does not swell, or only slightly ; b swells 

 in the radial direction and becomes thicker ; 

 while c swells in the radial, tangential, and lon- 

 gitudinal directions (cf. Fig. 40, C, c, and Z), 

 c) ; in transverse sections c breaks away from 

 b, and curves spirally (C) ; in longitudinal sec- 

 tions it is bent in a wavy manner (D). 



In true wood-cells, e. g. in Pinus syl-vestris 

 (Fig. 26, A), three shells are likewise gene- 

 rally to be distinguished : a central one (Fig. 26, 

 A, w), next a thicker one (2), and an inner (/) ; 

 the two first turn yellow on treatment with 

 solution of iodine or iodine and sulphuric acid, 

 the innermost blue with the latter reagent ; 2 

 and / are dissolved by concentrated sulphuric 

 acid, and the central lamella m remains ; 

 here also the possibility of isolating the cell 

 depends on the circumstance that the central 

 lamella m may be dissolved by boiling in nitric 

 acid with potassimn chlorate ; and thus the iso- 

 lated cells consist only of the two inner shells. 



In many wood-cells (the 'Libriform Fibres' of Sanio) the inner thickening-layers form a 

 shell of cartilaginous and gelatinous consistence (as in the wood of many Papilionaceae). 



D 2 



Fig. \o.—Pteris aquilina; structure of the brown-walled 

 sclerenchyma in the stem (X55o)- ^^ a fresh thin transverse 

 section ; B the longitudinal wall between two cells, fresh 

 (a curved pit-channel at the lower end) ; C transverse sec- 

 tion in concentrated sulphuric acid; Z) longitudinal section 

 of the wall in sulphuric acid ; a tbe central lamella of the 

 wall; b second shell; c third, inner shell of the cell-wall; 

 p pore-channels ; / cavity of the cell. 



