GUMS 191 



Chondrus crispus) and of agar * is accounted for by the fact 

 that these substances have their carbohydrate residues com- 

 bined with calcium sulphate in the form of an ethereal sul- 

 phate represented by the formula — • 



o— SO,— Ov 



^o— so,— O^ 



in which R represents the polysaccharide residue. Substances 

 of this type have been shown to be colloidal electrolytes which 

 exert measurable osmotic pressures ; their solutions contain 

 calcium ions, but the sulphate complex is masked and is only 

 set free after hydrolysis : — 



yosop-. .on 



R<^ pCa + HjO ^ R\ + CaSO, + H2SO4 



^OSO^O-^ ^OH 



It will be seen that this hydrolysis involves the liberation of 

 a molecule of sulphuric acid, a fact which accounts for the 

 charring of this material which frequently occurs, owing to 

 spontaneous hydrolysis, when the material is heated in a 

 steam oven and even, sometimes, in the cold. Mucilages of 

 this type have been shown to occur in a number of marine 

 algae f both red and brow^n. 



Function. 



Mucilage, when it is a definitely secreted product or of a 

 definite and constant occurrence in a plant, may perform 

 several functions, but how far these are primary functions 

 cannot yet be stated. 



When it occurs in tubers, as in the OrchidaccEe, mucilage 

 is generally looked upon as a reserve food-material ; it may 

 serve as a check against too rapid transpiration, especially 

 when produced in connection with developing organs, such as 

 vegetative buds, young leaves, in the epidermis of mature 

 leaves, the sporangia of Cryptogams, etc. ; in the case of 

 aquatic plants, such as Algae, the hibernaculae of Myrio- 

 phyllum, etc., its presence may prevent a too rapid diffusion ; 



* Neuberg and Ohle : " Biochem. Zeit.," 1921, 125, 311. 

 t Haas and Russell-Wells : " Biochem. Journ.," 1923, 17, 696 ; also 

 Harwood ; " J. Chem. Soc," 1923, 123, 2254. 



