650 BOTANY AND PHARMACOGNOSY. 



in Sicily. Manna is obtained by making transverse or oblique 

 incisions in the bark, the extidation flowing down the side of the 

 tree where it hardens, or it is collected in special receptacles. 

 Several commercial varieties are recognized : large flake manna, 

 consisting of light-colored pieces 10 to 20 cm. long; and small 

 FLAKE manna, which occurs in smaller light yellowish-brown 

 pieces. The former is preferred. 



Description. — In irregular, 3-sided, more or less elongated 

 pieces, one side being smooth and concave ; externally yellowish- 

 white ; friable, somewhat waxy ; internally whitish, porous and 

 crystalline; odor suggestive of maple sugar; taste sweet, slightly 

 bitter and acrid. 



Constituents. — The principal constituent is mannitol (80 

 to 90 per cent.), which crystallizes in colorless needles that are 

 soluble in water and sparingly soluble in alcohol ; on sublimation 

 it yields a sweet, syrupy liquid, mannitan ; the solutions of 

 mannitol do not ferment nor is it decomposed with dilute acids. 

 Manna also contains a green, fluorescent glucosidal principle 

 fraxin (resembling sesculin), which occurs in bitter, colorless 

 prisms that are soluble in water and alcohol ; dextrose, as high as 

 16 per cent. ; mucilage ; resin, and 1.3 to 4 per cent, of ash. 



Allied Products. — A number of other species of Fraxinus 

 indigenous to Europe also yield manna. The term " manna '' is 

 applied to a number of exudations obtained from different sources 

 and of varying composition, none of which, however, appear to 

 be supplied as a substitute for true manna, although they might 

 be employed in a similar manner. 



Manna of inferior quality, known as " sorts," is obtained 

 from incisions lower down on the stem, and consists of brownish- 

 yellow, more or less agglutinated tears, which are sticky and but 

 slightly crystalline. 



The leaves of a number of species of Eucalyptus (Fam. Myr- 

 tacese) secrete a manna-like carbohydrate, as E. Gunnii and E. 

 resinifera. (See in this connection Coniferee, p. 81 ; Legumin- 

 osse, p. 292; Myrtacese, p. 346.) 



TRAGACANTHA.— -TRAGACANTH.— A gummy exuda- 

 tion from the stem of Astragalus gummifcr and other species of 

 Astragalus (Fam. Leguminosse), shrubs (Fig. 152) indigenous 



