THE DETERMINATION OF SOLUBLE CARBOHYDRATES

It is generally agreed that soluble carbohydrates are the most available, as well as the original, source of metabolic energy for plants. Hence, studies of the formation, translocation, condensation, hydrolysis and assimilation of these compounds have commanded, and will undoubtedly continue to command, a major place in plant physiological analysis. Separation between soluble and insoluble carbohydrates is here made by classing as insoluble those compounds which require hydrolysis before they can be quantitatively extracted from plant tissues by water or alcohol. In the strictest sense the extraction of dextrins and pentosans is probably accomplished by suspension rather than by true solution, but for analytical purposes they may be termed "soluble" compounds. In the interest of completeness we have included the glucosides under this heading, although it is hardly probable that they should be classed with the carbohydrates from the standpoint of physiological function. A brief treatment of the occurrence, solubility, reducing power and other properties of the carbohydrates here considered is given by HAAs and HILL (4). All but pectins and the gums are readily soluble in water. These and the polysaccharides (dextrin, inulin and mannan) are insoluble in hot 80 per cent. alcohol. It is well known that all naturally occurring free sugars with the exception of sucrose and raffinose seduce Fehling's solution. The saponins also have this property. Other glucosides, the soluble polysaccharides, pectins and gums acquire reducing power by hydrolysis, as do the non-reducing sugars. Furthermore, such reducing disaccharides as maltose and lactose acquire an increase of this property by hydrolysis. The latter carbohydrate is added to tissue in the use of commercial taka-diastase, which may contain up to 75 per cent. of the sugar. The presence of more than two or three per cent. of pentose sugars, or of appreciable quantities of galactose or mannose, is generally considered to be an indication of poor technique in preservation or separation, since these sugars are transition compounds normally present in polymerized forms. DAVIS and DAISH (2) are authority for the statement that the presence of maltose or isomaltose is due to starch hydrolysis in preservation and they object on this ground to preservation by drying. Apparently germinating seedlings may be an exception to this rule, but it is general enough in its application to be of value.