FORMATION OF THE COMMON WALL OF CELLS. 
73 
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 Pinns syl- 
vesh'is, 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 lamellce; the boundary-line between the 
two cells is never marked by a 
fissure dividing the partition-wall. 
Nevertheless such a splitting of 
the still very thin lamella often 
takes place when the surface- 
growth is more rapid, as in 
the formation 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 
maize in transverse section; the 
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 hne where one wall meets 
