36 



MORPHOLOGY OF THE CELL. 



When the outermost layers of cells which are combined into tissues become gelatinous 

 or mucilaginous, the boundary-line readily disappears ; and the appearance may then be 

 presented as if the cells, enclosed by the inner shell, which is not mucilaginous, were im- 

 bedded in a homogeneous jelly as a ground-work ; and this latter especially gave rise in 

 time past to the theory of ' Intercellular Substance,' to which we shall recur. This be- 

 haviour occurs in the tissue of some Fucaceae, and also in the endosperm of Ceratonia 

 Siliqua (Fig. 41) ; cc are the outer layers of the wall of the cells a, which have become 

 entirely converted into mucilage and rendered indistinguishable, their inner layer appear- 

 ing as a strongly refractive shell. In the dry 'state the mucilaginous mass is almost horny; 

 it swells up strongly in water with solution of potash ; with iodine and sulphuric acid it 

 does not become coloured, but the sharply defined layer turns blue. In free-lying cells 

 numerous layers of cell-wall may also form a mucilaginous shell, which is most beautifully 

 developed in the spores of Pilularia (Fig. 35) and IVIarsilea. In the spore-fruit (sporan- 

 gium) of these plants are 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 absorb so much water that they increase in bulk several hundred-fold, and 



burst the pericarp (Book II. Rhizocarpeae). 

 On a similar transformation into mucilage 

 of inner layers of cell-wall, while an outer, 

 thin, and cuticularised shell retains its power 

 of resistance, depends also the formation of 

 the mucilage of linseed and quince-seed. The 

 inner thickening-masses of the epidermis of 

 the seed, transformed into mucilage, absorb 

 the surrounding water with great force, swell 

 violently, and, bursting the cuticle which 

 is incapable of swelling, appear, in the pre- 

 sence of a small quantity of water, as a 

 hyaline layer enveloping the seed ; and, with 

 more copious addition of water, become 

 more and more diluted into thin mucilage. 

 A similar process occurs in some other seeds, 

 such as those of Teesdalia nudicaulis and Plantago Psyllium, in the seed-hairs of Ruellia, 

 and the pericarp of Salvia. Gum-tragacanth consists of the cells of the pith and medul- 

 lary rays of Astragalus creticus, A. Tragacantha, and other species, transformed into 

 mucilage. When the walls of these cells become mucilaginous, and swell up on copious 

 addition of water, they force themselves through slits in the stem as viscid masses, and 

 dry up on the outside into a horny mass capable of swelling. Vegetable mucilage can, 

 however, arise in other ways l. 



(f) Incombustible Deposits occur in every cell-wall. The presence of lime and silica can 

 be directly proved ; but it can scarcely be doubted that potash, soda, magnesia, iron, 

 sulphuric acid, &c., also occur in small quantities. The deposit of lime-salts and silica 

 increases with age. The deposit may take place in two ways ; usually only extremely 

 small particles of incombustible substance are deposited regularly between the mole- 

 cules of the organic substance of the cell-wall ; and this may be recognised by the ash 

 remaining behind after ignition in the organised form of the cell- wall (as a skeleton) ; 

 but lim.e salts may also be contained in the cell-wall in the form of numerous very 

 small crystals ; they then lie imbedded in the substance of the cell-wall itself, sometimes 

 in the form of particular growths which project into the cell-cavity and are termed 

 Cystoliths (cf. ^ect. 10). 



Fig. 41.— Section of the endosperm of Ceratonia 

 SiliqJia. 



^ Compare further, Frank: Ueber die anatomische Bedeutung und die Entstehung der veget. 

 Schleime. (Jahrb. fiir wissen. Bot. V. 1866.) 



