a8 



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 diflerent 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, and 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. 29. — Transverse section of a cell from 

 the root-tuber of the Dahlia (X 800); / the 

 cell-cavity; A' canals which penetrate the 

 stratified cell-wall ; sp a crack by which 

 an inner group of layers has become sepa- 

 rated. 



FIG. 28.— Sclerenchymatous cells ol Pteris aquilina', A half of a cell isolated and rendered colourless by Schulze's 

 maceration ; B a piece more strongly magnified (X 55o) ; 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. Pinus syl'vestris), 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- 



