28 
MORPHOLOGY OF THE CELL. 
Every system of stratification or striation consists of layers of visible thickness and 
of different refractive powers, so that a more strongly refractive layer or stria always 
alternates with a less strongly refractive one. This difference of refraction results 
from a different distribution of water and of solid particles in the cell-wall; the less 
strongly refractive layers contain more water and less cellulose, and are therefore 
less dense ; the more strongly refractive and denser layers contain less water and more 
cellulose. Hence stratification and striation of the cell-wall disappear both when water 
is completely eliminated, an-d also when it absorbs much water; because, in the first 
case, the more watery layers are reduced to the condition of the less watery ones, 
in the latter case the less watery become similar to the others. On the other hand 
stratification and striation become most conspicuous when, from the particular propor- 
tion of water in the cell-wall, the difference between the dense and the watery layers 
is greatest. In many cases this may be brought about by addition of acids or alkalies 
FIG. 28.— Sclerenchymatous cells 6i Pterts aquilina; A half of a cell isolated and rendered colourless by Schulze's 
maceration ; B a piece more strongly magnified (X 55°) ! the fissure-like pits cross ; i.e. the fissure is twisted ; at/ side- 
view of a fissure, appearing here as a simple canal. C transverse section of the same. 
which occasion a moderate swelling. But if the dense layers are very dense, and the 
others very watery, as is the case with some wood-cells [e. g. Pinns sylvestris), the stria- 
tion becomes more evident through desiccation, because this brings out the dense layers 
and effaces the less dense ones. 
The systems of striation and stratification of a cell-wall intersect one another, like 
the cleavage-planes of a crystal splitting in three directions. But since the striation and 
stratification are produced by layers of a measurable thickness, composed of alternately 
denser and less dense substance, the cell-wall appears to be composed of parallelopipedal 
pieces, distinguished from one another by the proportion of water contained in them. 
If we for a moment disregard the stratification, and assume that we have two inter- 
secting systems of striation ; then, where two dense striae intersect, the densest or least 
watery places are always to be found ; where two watery ones intersect, the least dense 
or most watery, and where places of greater and less density intersect, areolae of inter- 
