CARBOHYDRATES AND FATS 



469 



development that the largest quantities of non- nitrogenous food- materials 

 are required, and hence, unless special adaptations come into play (Sect. 80), 

 non-nitrogenous substances preponderate in the seed : thus in cereals, up to 

 80 per cent, of starch may be present, in fatty seeds 70 per cent, of the dry 

 weight may consist of oil, while a fresh beet-root may contain as much as 

 1 6 per cent, of cane-sugar. As far as is known, carbohydrates and fats 

 also form the main reserve food-materials in fungi, and carbohydrates 

 may be formed by the latter when fed with other carbon-compounds 

 (Sects. 66 and 80). Carbohydrates and fats do not function merely as 

 food-material, for cellulose is employed in cell-wall formation, and it is 

 questionable whether carbohydrates and fats may not form essential 

 constituents of the protoplasm, independently of the carbohydrates which 

 may be associated with the molecules of certain proteids (cf. Sect. n). 



The terms ' carbohydrate ' and ' fat ' are simply general names for large 

 groups of substances. Carbohydrates especially are extremely polymeric 

 (disaccharides, polysaccharides), and readily serve for the formation of such 

 substances as glucosides with widely different physical and chemical proper- 

 ties. Carbohydrate compounds of this kind are probably formed by every 

 plant, though there is perhaps no special combination which is essential to all. 

 Carbohydrates and fats may mutually replace one another to an extremely 

 wide extent, and hence it arises that different organs of the same plant may 

 contain different reserve-materials, and that not infrequently various carbo- 

 hydrates and fats may be present in the same cell. 



The most widely distributed plastic carbohydrates 1 are starch, dex- 

 trose (glucose, grape-sugar), laevulose (fructose, fruit-sugar), and cane-sugar 

 (saccharose). Starch is, however, absent from fungi, and of the carbo- 

 hydrates with a more limited range, inulin is found abundantly in Com- 

 positae ; glycogen and trehalose in many fungi. Mannite is less rare, and 

 often occurs in fungi, as do many of the different forms of reserve- 

 cellulose found in seeds. Many other carbohydrates frequently occur in 

 small amount, or in abundance, in special plants, but these are less im- 

 portant, and for the most part only those with special optical properties 

 have been detected. 



As far as is known, hexoses, as well as the di- and poly-saccharides, 

 seem to function as the chief plastic carbohydrates, whereas the pentoses 

 which appear to be universally present 2 seem for the most part not liable 



1 A complete account of the chemistry aftd distribution of the known carbohydrates is given by 

 Tollens, Handb. d. Kohlenhydrate, Bd. I, 1888; Bd. II, 1895 ; Lippmann, Chemie d. Zuckerarten, 

 1895. Our knowledge of their chemical constitution is mainly due to E. Fischer. 



2 Cf. Tollens, 1895, 1. c., pp. 198, 240; Journ. f. Landw., 1896, Bd. XXIV, p. 171, and Beibl. z. 

 Biol. Centralbl., 1896, Bd. vi, p. 331 ; Goetze u. Pfeiffer, Versuchsst., 1896, Bd. XLVII, p. 58. See 

 also Sect. 54. Trioses, Nonnoses, &c. do not seem to be present in plants, but may in certain cases 

 serve as food for fungi, yeast, &c. 



