THE CELL-WALL. 
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 potassium 
chlorate; and thus the walls of the isolated 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 or gelatinous consistence, as in the wood of many 
Papilionacese. 
When the outermost layers of the walls of cells which are combined into tissues 
become gelatinous or mucilaginous, their boundary-line disappears; and the cells, 
enclosed by the inner shell which is not mucilaginous, appear to be imbedded in a 
homogeneous jelly ; this appearance gave rise to the theory of ' Intercellular Sub- 
stance,' to which we shall recur. This behaviour occurs in the tissue of some Fucaceae, 
and also in the endosperm of Ceratonia SU'iqua (Fig. 39) ; cc are the outer layers of the 
Fig. 38.— Structure of the sclerencr.3 ma in the stem 
of Pteris agiiih'jta (X 550). .4 a fresh thin transverse 
section ; B the longitudinal wall between two cells, fresh 
(a curved pit-canal at the lower end) ; C transverse sec- 
tion in concentrated sulphuric acid ; D longitudinal sec- 
tion of the wall in sulphuric acid ; a the central lamella of 
the wall ; h second shell ; c third or innermost shell of the 
cell-wall ; p pore-canals ; / cavity of the cell. 
wall of the cells a, which have become entirely converted into mucilage and rendered 
indistinguishable, their innermost system of layers {b) appearing as a strongly refractive 
shell. In the dry state the mucilaginous mass is almost horny ; it swells up strongly 
in water with potash solution ; with iodine and sulphuric acid it does not become 
coloured, but the sharply defined inner shell b turns blue. 
In isolated cells numerous cell-wall-layers may also form a mucilaginous shell 
which is most beautifully developed in the spores of Pllularia (Fig. 33, p. 32) and 
Marsilea. In the sporocarp of these plants are certain masses of parenchyma, the 
cell-wails of which become mucilaginous on the inner side ; when dry the mucilaginous 
masses are firm and horny, but can absorb so much water that they increase in bulk 
several hundred-fold, and burst the wall of the sporocarp (Book II., Rhizocarpese). 
A similar transformation into mucilage of inner layers of cell-wall, while an outer, 
thin, and cuticularised shell resists, occurs with Unseed and quince-seed. The inner 
thickening-masses of the epidermis of the seed, transformed into mucilage, absorb the 
surrounding water, swell up violently, and, bursting the cuticle which is incapable 
D 2 
Fig. 37.— Epidermis of the central vein of the leaf of 
the holly; A transverse section; 7? superficial appearance 
(front view). 
