160 PLANT ANATOMY. 
iodine. A solution of chloride of zinc with iodine colors cellu- 
lose membranes violet. 
The cellulose of fungi, and suberified and lignified membranes 
are not colored by the above-described treatment; in the case of 
the latter two, however, this may be brought about if they are 
previously boiled with nitric acid (specific gravity 1.185) with 
the occasional addition of a few crystals of potassium chlorate 
(‘‘ Schultze’s maceration”). The cellulose of fungi, however, 
even after this treatment, is not colored by iodine. 
Concentrated sulphuric acid alone (specific gravity 1.84) 
dissolves cellulose with complete chemical change. This is not 
the case when cellulose is dissolved in ammoniacal oxide of cop- 
per (see Micro-chemical Reagents). In contradistinction to the 
lignified membrane, pure cellulose possesses an exceedingly 
slight inclination to take up aniline colors (page 161). 
Pure cellulose may be prepared by the successive treatment of 
tissues consisting of this substance (cotton, the pith of the elder 
and of Aralia papyrifera) by means of caustic potassa, acids, 
water, alcohol, and ether, or by the precipitation of its solution 
in ammoniacal oxide of copper by means of water. 
One of the most widely distributed modifications of cellulose 
is formed through the deposition of lignin (xylogen’). A 
membrane thus altered is termed lignified.” 
Lignification appears at very early stages in the so-called wood- 
cells. The wood-cells which, in dicotyledons, are separated 
toward the interior by the growth in thickness, already possess 
lignified membranes long before they become thick. The bast- 
cells and many stone-cells (sclereids) are also often lignified. A 
1 EvAor wood, and yervaa produce. 
*Compare in this connection Stackmann, ‘‘ Studien iiber die Zusam- 
mensetzung des Holzes,” Inaugural Dissertation, Dorpat, 1878, and the 
previously mentioned dissertation of Schuppe.—M. Niggl, ‘‘ Ueber die 
Verholzung der Pflanzenmenbranen ” (an historical survey). Jahres- 
bericht der Pollichia, Kaiserslautern, 1881.—Ebermaier, ‘‘ Physiologische 
Chemie der Pflanzen,” 1882. In this work (p. 175) will also be found 
statements relating to the amount of lignin contained in some woods. 
Thus (according to Schulze) oak-wood contains 54.12 per cent, and fir- 
wood 41.99 per cent of lignin. 
