FORMATION OF THE COMMON WALL OF CELLS. 



71 



The middle lamella is generally thin in lignified tissues, but strongly refractive, 

 and formed of dense substance not capable of swelling; when the rest of the 

 substance of the cell- wall has been dissolved in concentrated sulphuric acid, it 

 remains (in fine transverse sections) as a delicate net-work; if, on the other 

 hand, the cells are isolated by boiling in potash or nitric acid, solution of this 

 middle lamella which resists sulphuric acid takes place, while in this case the rest 

 of the cell-wall is preserved (as in all wood-cells and very many bast-cells). In 

 other cases, as has already been mentioned in sect. 4, the middle layers of the 

 partition-wall of adjoining cells are, on the contrary, converted into mucilage; 

 the layer of cell-wall immediately surrounding each cell-cavity is dense, and 

 appears like the entire cell-wall imbedded in a mucilaginous, swelling, weakly 

 refractive matrix (the so-called intercellular substance) ; this occurs very com- 

 monly in many Fucacese and in the endosperm of Ceraionia Siliqua (Fig. 41, 

 p. 36). On a fine transverse section through the cambium tissue of a branch 

 of Pinus sylvestris, the two phenomena here described may be seen simulta- 

 neously; the wood-cells show the thin dense middle lamella, the young bast-cells 

 appear deposited in a soft mucilaginous substance, which is especially thick 

 between the radial rows of cells, and is interspersed with fine strongly refractive 

 granules; but both forms of tissue arise out of the same young tissue (the cam- 

 bium), the walls of which are simple thin lamellae, between which the cell-cavities 

 themselves appear as so many compartments. Objects of this kind are well 

 adapted to prove the correctness of the supposition that in general the formation of 

 denser or softer middle lamellae depends only on a diflferentiation of the substance of 

 the partition-walls during their thickening, a view which explains in a perfectly simple 

 manner all the phenomena belonging to it, and agrees altogether with growth by 

 intussusception. 



The thin entirely homogeneous lamella of cellulose which bounds the young 

 cells never allows a separation into two lamellae to be recognised ; the bounds of the 

 two cells are never marked by a fissure dividing the partition-wall. Nevertheless such 

 a splitting of the still very thin 

 lamella often takes place later 

 locally, when the surface grows 

 more quickly, as in the forma- 

 tion of the intercellular space of 

 the large-celled succulent tissue 

 (parenchyma) of vascular plants, 

 in the formation of stomata, &c. 

 Fig. 58 shows some fully grown 

 parenchyma-cells from the stem 

 of Zea Mais in transverse section ; 

 the cells were at first bounded 

 by perfectly flat walls, which met 

 nearly at right angles. As the 

 growth increased, a tendency towards the rounding off of the polyhedral forms 

 arose ; the unequal growth clearly leads to tensions which are compensated by the 

 fact that on the line where one wall meets the other, the cohesion is destroyed in 



Fig. 58.— Transverse section through the succulent parenchyma of the stem 

 of Zea Mais; gw common partition-wall of each pair of cells; z intercellular 

 space caused by their splitting {X5S0). 



