CELLULOSE. 159 
The cell-wall is subject to chemical and physical changes, 
either through the deposition of woody matter, lignin’ or cork- 
fat (suberin),? as also through a retrograde metamorphosis of the 
cellulose into gum and mucilage. 
All young cell-membranes, and most of the walls of cells, with 
which we shall become acquainted under the designation of 
parenchyma, as also many appendages of seeds which are de- 
veloped as hairs (Cotton, Asclepias, Eriodendron, Salix), consist 
of purecellulose. ‘The phloém, Jeptom (sieve-tubes and cambi- 
form tissue), always remains unlignified. Membranes consisting 
of cellulose show, even by superficial microscopical observation, 
an entirely different capacity for the refraction of light from 
lignified and suberified membranes; they appear clearer, more 
strongly refractive, and jelly-like (collenchyma, the cell-mem- 
branes of Macis). Cellulose membranes are digestible. 
As already intimated on page 123, there are some exceptional 
cell-walls which, in contact with iodine-water, are colored in a 
similar manner to amylum. By the treatment of pure cellulose 
with mineral acids, this faculty may quite generally be imparted 
to it. The respective sections or objects (for instance, cotton) 
are moistened for an instant with sulphuric acid of the specific 
gravity 1.84, washed without delay with much water, and then 
powdered iodine strewn upon the moist preparation, or it is 
impregnated with iodine-water (see Micro-chemical Reagents). 
The reaction succeeds with still greater certainty with phos- 
phoric acid, which is first concentrated as much as possible on a 
water-bath. When hydriodic acid has been formed in an iodine 
solution which has been long preserved (see Micro-chemical 
Reagents), such a solution can effect the blue coloration of cellu- 
lose without the co-operation of other acids, The reaction admits 
of demonstration, without further preparation, with moistened 
parchment-paper, which is sprinkled over with finely-powdered 
certain vegetable fibres,” Ibid., 1882; Schuppe, ‘ Beitrige zur Chemie 
des Holzgewebes.” Inaugural Dissertation, Dorpat, 1982, in which the 
older literature is also to be found. 
1 Lignum, wood, 
2 Suber, cork. 
