This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
Manna is strictly a generic term applied to the saccharine exudations from a number of different plants belonging to various natural orders, but when not otherwise specified is understood to mean the saccharine exudation from the stem of the manna ash, Fraxinus Ornus, Linne (N.O. Oleaceoe), a small tree widely distributed over southern Europe and cultivated especially in Sicily for the production of manna.
The trees are considered fit for yielding manna when they are about ten years old. Every day a transverse or oblique incision is made through the bark on one side of the stem; the saccharine liquid that exudes flows down the stem in favourable seasons and dries, but in rainy weather it drops from the trunk and is caught upon cactus leaves (more strictly branches), placed beneath it, yielding an inferior quality. In the following year the tree is cut upon the opposite side, and in the succeeding year again on the first side. The stem is then exhausted, the tree is cut down, and from the stool two or more shoots are allowed to grow, which in ten years are again ready for tapping. In good seasons about 500 grammes of manna are obtained from each stem.
The finest qualities of manna, known in commerce as ' flake manna,' are in pieces about 10 or 15 cm. long and 2 or 3 cm. wide, which are more or less conspicuously three-sided, one of the sides (that which has been next the stem) being concave and smooth. It is yellowish white in colour and very brittle, even friable, exhibiting when broken an indistinctly crystalline structure. It has a slight agreeable odour and a sweet taste.
Inferior qualities of manna are of a darker, brownish yellow colour, and composed of broken flakes agglutinated into a more or less sticky, gummy mass.
Manna consists principally of mannite (mannitol), C6H8(OH)6, of which it may contain from 40 to 60 per cent. Mannite is a well-defined, crystalline substance belonging to the class of alcohols (a hexahydric alcohol), and is widely diffused throughout the vegetable kingdom. It is associated in manna with two sugars, viz. manninotriose (6 to 16 per cent.) and manneotetrose (12 to 16 per cent.). Each molecule of manninotriose yields by hydrolysis two molecules of galactose and one of dextrose; each molecule of manneotetrose yields two of galactose and one each of dextrose and levulose. Dextrose, mucilage, inorganic substances, a minute quantity of a fluorescent substance, fraxin, and about 10 per cent. of moisture also occur in manna.
Manna is used medicinally as a gentle laxative.
Australian manna obtained from Myoporum platycarpum, Robert Brown (N.O. Poly-galece), in brownish masses containing about 89 per cent. of mannite.
Alhagi manna from Alhagi Maurorum, Tournefort (N.O. Leguminosoe), Persia, in brownish, brittle, crystalline or amorphous masses; consists chiefly of sucrose; contains no mannite.
Tabashir or Bamboo manna secreted in the surface of the stem of Bambusa stricta, Roxburgh (N.O. Gramineoe), almost entirely sucrose (not to be confounded with the tabashir which occurs in the interior of the stem and is mainly silica).
Oak manna, secreted in the leaves and acorn cups of Quercus Vallonea, Klotschy (N.O. Cupuliferoe), consists of sucrose and dextrose; contains no mannite.
Tamarisk manna, from Tamarix gallica, var.
mannifera, Ehrenberg (N.O. Tamariscineoe), contains sucrose, 55 per cent.; levulose, 25 per cent.; no mannite.
Briancon manna secreted by the leaves of Larix europoea, de Candolle (N.O.
Coniferoe) contains the sugar melezitose.
Fig. 244. - Portion of the stem of a Manna tree, showing the incisions and adhering manna. Reduced. (Vogl).