386 CARBOHYDRATE METABOLISM 



are due to the sjmthesis of anthocyanins in the leaf cells of these species. 

 Autumnal development of anthocyanins is favored by periods of bright, clear, 

 dry weather, during which cool, but not freezing temperatures prevail. From 

 the preceding discussion of the factors favoring anthocyanin formation it is 

 evident that such meteorological conditions should be almost ideal for the 

 synthesis of these compounds. 



Tannins. — These are a rather heterogeneous group of complex compounds 

 of common occurrence in plants. Upon hydrolysis they produce certain 

 complex acids, nearly always a sugar (usually glucose) and sometimes other 

 substances. The proportion of sugar found in the hydrolytic products of the 

 tannins is relatively small, however. While the tannins may be regarded as 

 remotely related to the glycosides, their properties are distinctive as com- 

 pared with the glycoside type of compound, and they must be considered 

 as a separate class of substances. 



Tannins vary greatly in amount from one species to another. They are 

 sometimes present in the cell sap but are of more frequent occurrence in the 

 cell walls, often accumulating in very considerable amounts in dead tissues. 

 Tannins are found in the leaves of many species such as tea (15 per cent 

 of the dry weight) oaks, and many conifers. The woody tissues of many 

 species contain tannins. The bark of oaks, chestnut, hemlock, sumac and 

 other species is very rich in tannins. In some species of oak they may com- 

 pose as much as 40 per cent of the dry weight of the bark. Unripe fruits 

 of some species (persimmon, plum, etc.) contain relatively large quantities 

 of tannins. 



Carbohydrate Transformations in Plants. — Of the many carbohydrate 

 transformations which occur in plant cells, the conversions of glucose to 

 starch, and of starch back to glucose probably are of most frequent occur- 

 rence. Certain conditions within plant cells are known to favor the con- 

 densation of glucose to starch; others the digestion of starch to sugar: 



I. Temperature. — Low temperatures in general favor the starch to sugar 

 transformation in plant cells. This fact is clearly illustrated by the seasonal 

 behavior of the carbohydrates in the leaves of evergreen species and in woody 

 stems as already discussed in Chap. XI. 



Another interesting example of a shift in the starch-sugar equilibrium 

 with temperature occurs in potato tubers (Hopkins, 1924). If stored at 

 too low a temperature a gradual accumulation of sugars will occur in the 

 tubers at the expense of the starch present. This accounts for the sweet taste 

 which is sometimes found in potatoes purchased on the market. Contrary to 

 popular opinion this sweetening is not necessarily due to a freezing of the 

 tubers, since it has been found that temperature of the inception of sugar 



