20 
MORPHOLOGY OF THE CELL. 
but in different proportions ; portions less and more watery, denser and less dense, 
alternate. Thus, in every cell-wall sufficiently thick, a system of concentric layers 
becomes visible, of which the outermost and innermost are always denser, while 
between them alternate more and less watery layers. The stratification is visible 
on the transverse and longitudinal sections of the cell-wall, the striation on the 
surface as well ; it is usually most evident there, but is in general less easily seen 
than the stratification; it depends on the presence of alternately more and less dense 
layers in the cell-wall, meeting its surface at an angle. Generally two such systems 
of layers may be recognised mutually intersecting one another. There are thus 
altogether three systems of layers present in a cell-wall, one concentric with the 
surface, and two vertical or oblique to it mutually intersecting, like the cleavage- 
planes of a crystal splitting in three directions (Nageli) ; and just as this cleavage 
is sometimes more evident in one direction, sometimes in another, so is it also with 
the stratification and striation. 
Independently of this internal structure, chemical changes arise in the cell-wall 
which never affect the whole mass uniformly, but usually mark out the thickened 
Fig. i6.— From the transverse section of a leaf oi Camellia japonica ; P parenchymatous cells with chlorophyll- 
granules and drops of oil ; Fa. very thin fihro-vascular bundle ; vv a. large, branched, tliick-walled cell, which 
intrudes its branches between the parenchymatous cells. 
cell-wall into concentric layers differing from one another chemically and phy- 
sically. These chemical changes, which are always attended by an akeration of 
physical properties, are very various, but can conveniently be reduced to three 
categories ; — Suberous or Cuticular change, Ligneous change, and Mucilaginous 
change. The first consists in the change of the outer layers of the cell-wall into 
an extensible very elastic substance, which water is unable, or nearly so, to penetrate 
or cause to swell, as the outer cell-wall4ayer of the epidermis (cuticle) and of 
pollen-grains and spores, and cork. Lignification increases the hardness of the 
cell-wall, diminishes its extensibility, and renders it more easily permeable to water 
without considerable swelling. The conversion into mucilage renders the cell-wall 
capable of absorbing great quantities of water, so as to increase its volume and 
give it a gelatinous consistence. In the dry state such cell-walls are hard, brittle, 
or fliexible like horn, as the cell-walls of many Algae, the so-called 'intercellular 
substance' of the endosperm of Ceratonia Siliqua, linseed, and quince-mucilage. 
Several of these changes may occur simultaneously in a cell-wall, so that, for 
