72 V. CELL-WALL, AND CELL CONTENTS. 



order to remove the air and chlorophyll, they are examined in 

 glycerine. The sections remain unchanged in glycerine or 

 glycerine-jelly. The pieces of plant can be removed from the 

 acetate of copper into alcohol for preservation, and examined later, 

 when desired, by the aid of acetate of iron. Iron-blue and iron- 

 green tannins are clearly distinguishable by this method. If it 

 is desired at the same time to fix the cell-contents, alcoholic 

 solution of acetate of copper can be used instead. 



Sclerenchyma Fibres of Vinca. If a strong stem of Vinca 

 major (the Periwinkle), cut off close above the ground, is broken, 

 we see numerous small fibres project from the edge of the broken 

 surface. We seize a tuft of such fibres with the forceps, draw 

 them out, and place them in a drop of water on an object-slide. 

 Under the microscope they appear as long, strongly thickened 

 sclerenchyma fibres, tapering at both ends. The cavity is 

 reduced to a narrow canal, which is entirely obliterated at both 

 ends of the fibre, and sometimes at intermediate places. In less- 

 thickened fibres the wall appears striate in one direction only ; 

 in more strongly-thickened fibres there are two opposite oblique 

 systems of striae, of which one belongs to the outer, the other to 

 the inner system of layers of which the wall is composed. These 

 are due to the spiral thickening of the wall. With chlorzinc 

 iodine solution the fibres quickly take on a violet colour, shading 

 into brown. Specially instructive, however, is the relation with 

 cuproxide ammonia, which reagent has the power of dissolving 

 pure cellulose. The action must be observed direct. On the 

 addition of the cuproxide ammonia solution the walls of the fibres 

 swell strongly ; at the first moment of the action the striation 

 becomes more distinct, but quickly disappears. The outer 

 complexes of layers are soon completely dissolved, while the 

 inner one resists longer, and therefore the observer can see 

 it completely isolated. At the beginning of the swelling a 

 still finer stratification appears in the stratification which 

 was previously visible ; each layer is therefore composed of 

 numerous exceedingly thin lamellae. This fine stratification 

 is stamped especially distinctly upon the inner more resistent 

 complex. 



Thickened Walls in Ornithogalum. We now divide in halves, 

 with the pocket-knife, a seed (not too old) of Ornithogalum, say 

 0. umbellatum (the Star of Bethlehem), clamp the half in the 

 hand-vice, damp the cut surface with water, and make with the 



