164 PLANT ANATOMY. 
The relations of these bodies to each other, as also to cellulose, 
have, however, not yet been made clear. 
According to Giraud’ these substances may be grouped in the 
following manner: 
1. Ordinary varieties of gum: arabin, bassorin, cerasin; 
2. Pectose: gum tragacanth (adragantin); 
3. Plant mucilages in a more restricted sense: 
(a) insoluble in alkalies and dilute acids (the cellulose of 
quince-mucilage); 
(6) insoluble in alkalies, and forming with acids glucose 
and a variety of dextrin: flaxseed, mucilage of Irish 
moss; 
(c) soluble in hot, concentrated alkalies, and converted by 
acids into dextrin and glucose. 
Mucilaginous substances, in the broadest sense, are also dis- 
tinguished by their behavior to nitric acid; some afford with 
it mucic acid, C,H,(OH),(COOH),, while others do not. 
Furthermore, the aqueous solutions of many varieties of gum are 
precipitated by the normal acetate of lead, while others are only 
precipitated by the basic acetate.’ 
Gum arabic and cherry-tree gum (cerasin), as well as traga- 
canth, are formed by a retrograde metamorphosis of the cell- 
membrane,’ that is, by a pathological process; the former two by 
a conversion of the membranes of the peripheral layers of the 
“‘horn-bast prosenchyma ” * into gum, the latter through a meta- 
‘Compt. rend., 80, 477; compare also Husemann and Hilger, ‘“ Die 
Pfianzenstoffe,” I. (1882), 131. 
* Compare Kirchner and Tollens, Liebig’s Annalen, 175 (1874), 205. 
* In reference to this and the following statements compare: Mohl, 
Bot. Zeit., 1857, 33.—Frank, Jour. f. Pract, Chem., 95, 479; idem, Prings- 
heim’s Jahrb. fir wissenschaftl. Botanik., V., 25.—Wigand, ‘‘ Ueber die 
Desorganisation der Pflanzenzelle,” in Pringsheim’s Jahrb., III., 115.— 
Prillieux, ‘‘ La formation dela gomme.” Ann. des sc. nat., 6 Ser., Bot. 
B;..1 76. 
* With regard to horn-bast, compare Wigand, Flora 1877, p. 369, and 
“‘ Lehrbuch der Pharmacognosie,” 1879, pp. 9 and 38; further Flickiger, 
“ Pharmakognosie,” 349, The word “horn-bast” should be expunged 
pre the modern terminology. 
