CELLS AND CELL PRODUCTS 211 



colouring substance which could be dissolved out from the lichenin and 

 which he considered to be true starch. Berg 1 also demonstrated the com- 

 pound nature of lichenin: he isolated two isomerous substances with the 

 formula C 6 H 10 O 5 . The name " isolichenin " was given to the second blue- 

 colouring substance by Beilstein 2 in 1881. 



More recently Escombe 3 has chemically analysed the cell-wall of Cetraria 

 islandica: after the elimination of fat, oil, colouring matter and bitter consti- 

 tuents he found that there remained the compound lichenin, an anhydride of 

 galactose with the formula C 6 H 10 O 5 , which, as stated above, consists of two 

 substances lichenin and isolichenin 4 ; 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 6 from Lecanora esculenta, an 

 edible desert lichen supposed to be the manna of the Israelites. Wisselingh 6 

 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 Stude 7 . 

 It is soluble in water and, though considered by Czapek 8 to be identical 

 with lichenin, it differs, according to Ulander 9 , in being dextro-rotatory to 

 polarized light; lichenin on the contrary is optically inactive. Escombe 3 

 also obtained a substance from Evernia which he considered to be comparable 

 with chitosan. Usnein which has been extracted 6 from Usnea barbata 

 may also be identical with lichenin, but that has not yet been established. 

 Ulander 9 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 dex- 

 tran, or as substances between hemicellulose and the glucoses represented 



* Wiesner 1900. 

 9 Ulander 1905. 



1 Berg 1873. 2 Beilstein ex Errera 1882, p. 16 (note). 3 Escombe 1896. 

 5 Lacour 1880. 6 Wisselingh 1898. 7 Stude 1864. 8 Czapek 1905, I. p. 515. 



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