More recently Escombe[753] has chemically analysed the cell-wall of _Cetraria islandica_: after the elimination of fat, oil, colouring matter and bitter constituents he found that there remained the compound lichenin, an anhydride of galactose with the formula C₆H₁₀O₅, which, as stated above, consists of two substances lichenin and isolichenin[754]; the latter is soluble in cold water and gives a blue reaction with iodine, lichenin is only soluble in hot water and is not coloured blue. Both are derivatives of galactose, a sugar found in a great number of organic tissues and substances, among others in gums. Lichenin has also been obtained by Lacour[755] from _Lecanora esculenta_, an edible desert lichen supposed to be the manna of the Israelites. Wisselingh[756] tested the hymenium of thirteen different lichens for lichenin. He found it in the walls of the ascus of all those he examined except _Graphis_. Everniin, a constituent of _Evernia prunastri_, was isolated and described by Stüde[757]. It is soluble in water and, though considered by Czapek[758] to be identical with lichenin, it differs, according to Ulander[759], in being dextro-rotatory to polarized light; lichenin on the contrary is optically inactive. Escombe[753] also obtained a substance from _Evernia_ which he considered to be comparable with chitosan. Usnein which has been extracted[756] from _Usnea barbata_ may also be identical with lichenin, but that has not yet been established. Ulander[759] examined chemically the cell-walls of a fairly large number of lichens. _Cetraria islandica_, _C. aculeata_ and _Usnea barbata_, designated as the “Cetraria group,” contained soluble mucilage-forming substances similar to lichenin. A second “Cladonia group” which included _Cl. rangiferina_ with the variety _alpestris_, _Stereocaulon paschale_ and _Peltigera aphthosa_ yielded almost none. After the soluble carbohydrates were removed by hot water, the insoluble substances were hydrolysed and the “Cetraria group” was found to contain abundant d-glucose with small quantities of d-mannose and d-galactose; the “Cladonia group,” abundant d-mannose and d-galactose with but little d-glucose. Hydrolysis was easier and quicker with the former group than with the latter. Besides these, which rank as hexosans, Ulander found small quantities of pentosans and methyl pentosans. All these substances which are such important constituents of the hyphal membranes of lichens are classed by Ulander as hemicelluloses of the same nature as mannan, galactan and dextran, or as substances between hemicellulose and the glucoses represented by lichenin, everniin, etc. They are doubtless reserve stores of food material, and they are chiefly located in the cell-walls of the medullary hyphae which are often so thick as almost to obliterate the lumen of the cells. Ulander made no test for chitin in his researches.