FORMATION OF THE COMMON WALL OF CELLS. 



n 



In lignified tissues the middle lamella is generally thin but strongly refractive, 

 and is 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, this middle lamella is 

 dissolved, while 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 cellulose immediately surrounding each 

 cell-cavity is dense, and the whole appears as if the cell-wall were imbedded in 

 a mucilaginous refractive matrix (the so-called 'intercellular substance'); this occurs 

 in many Fucacese and in the endosperm of Ceratonia Siliqua (Fig. 39, p. 36). On 

 a fine transverse section through the cambial tissue of a branch of Pinus syl- 

 vesiris, the two phenomena here described may be seen side by side; 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 (crystals); but 

 both forms arise out of the same young tissue (the cambium), 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 correct- 

 ness of the supposition that in general the formation of denser or softer middle 

 lamellae depends only on a differentiation 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 altogether accords with growth by intus- 

 susception. 



The thin perfectly homogeneous lamella of cellulose which bounds the young 

 cells never exhibits a separation into two lamellae; the boundary-line between the 

 two cells is never marked by a 

 fissure dividing the partition-wall. 

 Nevertheless such a spHtting of 

 the still very thin lamella often 

 takes place when the surface- 

 growth is more rapid, as in 

 the formadon of the intercellu- 

 lar space, in the large-celled 

 succulent tissue (parenchyma) of 

 vascular plants, in the formation 

 of stomata, &c. Fig. 58 shows 

 some fully grown parenchyma- 

 tous cells from the stem of the fig. 58.— Transverse section through the succulent parenchyma of the 



stem of the maize ; ^r^" common partition-wall of each pair of cells : si inter- 

 maiZe m transverse section; the ceUular space caused by the spatting ofthewal)(X 530). 



cells were at first bounded by , 



perfectly flat walls, which met nearly at right angles; as the size increased, a 

 tendency arose towards a rounding off of the polyhedral forms, the unequal 

 growth clearly leading to tensions which are only neutralised by the destruction 

 of the cohesion in the substance of the cell-wall on the line where one wall meets 



