222 A TEXT-BOOK OF BOTANY. 



(epidermis and hypodermis), as these in many cases may be 

 mistaken, as in the Violaceae, for distinct cells, although only the 

 inner walls of the epidermal cells are gelatinized. 2. Mucilage 

 cavities arise from the simultaneous gelatinization in the walls 

 of a group of cells. These are found in the cells of the pith, 

 cortex, and petioles in a number of plants of the Malvaceae, 

 Sterculiaceae, Simarubaceae, etc. 3. Mucilage canals are large 

 cavities formed either (A) as a result of the enlargement of the 

 intercellular spaces between the cells, the primary lamellae being 

 modified to mucilage; or (B) are formed by the disintegration or 

 breaking down of a number of cells, the walls of which become 

 gelatinized. In the former case they are spoken of as " schizog- 

 enous canals," and in the latter as " lysigenous canals." The 

 latter are the more common form and occur in the pith and 

 primary cortex of a number of plants belonging to the Guttiferae, 

 Malvaceae, Sterculiaceae, Oleaceae, Rhamnaceae, Vitaceae, Legumi- 

 nosae, Rosaceae, Cactaceae, Piperaceae, Moraceae, and Urticaceae. 

 4. Glandular hairs (Drusensotten) . In this form (Fig. 118) 

 they are found in the lobes of the leaves and calyces of Viola 

 tricolor, Coffca arabica, and of Primus aviitm. 



CHEMICAL CLASSIFICATION OF MUCILAGE. Mucilages may 

 be distinguished, according to their behavior with special reagents, 

 as cellulose-mucilages or pectose-mucilages. The former are 

 colored blue by chlor-zinc-iodide, and are soluble in ammoniacal 

 solution of cupric oxide. To this class belong the mucilages of 

 the tuber of salep and the seeds of cydonium. The pectose- 

 mucilages are distinguished by the fact that they are dissolved 

 on being heated with solutions containing from 35 to 65 per cent, 

 cane sugar. They are also stained intensely with solutions of 

 saffranin, methylene blue, or ruthenium red. 



Mucilage is formed in large quantities in certain trees, and the 

 exudation which is collected forms the so-called gums of com- 

 merce. As these are largely used for a variety of technical pur- 

 poses, their chemical properties have been studied, so that four 

 distinct classes of gums are recognized. 



i. Gums containing arabin or arabic acid. In this group are 

 included gum arabic, obtained from Acacia Senegal and other 

 species of Acacia ; Feronia gum, obtained from Fcronia clcphan- 



