THE CELL- WALL. 



35 



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 

 Papiiionaceae. 



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, 

 md also in the endosperm of Ceratonia Siliqua (Fig. 39) ; cc are the outer layers of the 



/5 « 



Fig. 37.— Epidermis of the central vein of the leaf of 

 the holly ; A transverse section ; B superficial appearance 

 (front view). 



I 



Fig. 38.— Structure of the sclerenchyma in the stem 

 of Pteris aqttilina (X 550). A 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 ; b second shell ; c third or innermost shell of the 

 cell-wall ; p pore-canals ; / cavity of the cell 



wall of the cells <?, 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 develop^'d in the spores of Pilularia (Fig. 33, p. 32) and 

 Marsilea. In the sporocarp of these plants arc certain masses of parenchyma, the 

 cell-walls 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., Rhizocarpeae). 

 A similar transformation into mucilage of inner layers of cell-wall, while an outer, 

 thin, and cuticularised shell resists, occurs with linseed and quince-seed. The inner 

 thickening-masses of the epidermis of the seed, transformed into mucilage, absorb the 

 surrounding water, swell up violentlyj and, bursting the cuticle which is incapable 



D 2 



