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. 16.— From the transverse section of a leaf oi Camellia Japonica ; /> parenchymatous cells with chlorophyll- 

 granules and drops of oil ; i^ a very thin fibro-vascular bundle ; vv z. large, branched, thick-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 alteration 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-wall-layer 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 flexible like horn, as the cell-walls of many Algse, 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 



