474 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



Dextrose, Lacvulose, Cane-sugar. These are the commonest forms of sugar, 

 and function both as translocatory and as reserve material (cf. Sects. 16, 54, 109). 

 All three forms frequently occur together, but usually one preponderates. Even 

 the beet-root contains traces of glucose along with its cane-sugar, and similarly 

 small quantities of cane-sugar have been detected in many plants where its presence 

 was formerly overlooked '. Of the hexoses, sometimes dextrose, at other times 

 laevulose is more abundant, but in many cases only the total amount of reducing 

 sugar has been determined, and in such cases it remains uncertain whether other 

 glucoses (galactose) or reducing polysaccharidcs (maltose, lactose) may not also be 

 present. Saccharose may possibly also be frequently accompanied by other non- 

 reducing sugars ". 



Cane-sugar occurs in great abundance in the beet-root and sugar-cane, in the 

 sap of the sugar-maple, in certain fruits and inflorescences (banana, pine-apple\ 

 in the rhizomes and roots of Rubia tinctoria and of certain I^abiatae, Umbelli- 

 ferae, &c., and small amounts may even be present in ripe seeds along with 

 other sugars. The sap of the sugar-cane may contain as much as 20 per cent, 

 of sugar, and that of wild beet-roots 6-8 per cent., and under cultivation it may 

 rise to as high as 16 per cent. Dextrose and laevulose are usually employed for 

 translocation, but frequently for storage also. Both are very common in pulpy fruits, 

 and they are stored up in the bulbs of Allium cepa and of Ornithogalum arabicum, 

 as well as in the subterranean parts of many species of Primula and Globularia *. 

 The amount accumulated may often during life reach 5-10 per cent., and in 

 grapes even 25 per cent., of the total weight. In such cases the osmotic pressure 



closely allied to starch, and soluble starch occurs dissolved in the cell-sap of the epidermal cells of 

 certain plants (Safonaria officinalis, &c.)- Cf. Dufour, Kech. sur 1'amidon soluble, Bull. d. 1. Soc. 

 Yand. d. Sci. Nat., T. XXII. Similarly, the cell-sap in the epidermal cells of Arum italicum turns 

 violet when treated with iodine, the colour disappearing on heating and returning on cooling. 

 The substance giving this reaction escapes from the cells as soon as they are killed, and the watery 

 extract yields on evaporation a transparent, slightly gummy residue, which turns violet or blue with 

 a watery solution of iodine, but reddish-brown when alcoholic iodine is added, turning blue in the 

 presence of water. After prolonged boiling a more reddish reaction is given, and also after partial 

 digestion with diastase or ptyalin, while ultimately the colour reaction disappears, a reducing sugar 

 being formed. This ' soluble starch ' has a very much feebler osmotic value than cane-sugar or 

 dextrose, and its molecule is presumably large and complex. Its peculiar distribution points 

 rather to its possessing some biological function (hindrance to transpiration, protection, &c.) than 

 to its having any special value in nutritive metabolism. It may occur in small quantity in the 

 cell-sap of the guard-cells of the stomata, though it seems always to be more abundant in the 

 surrounding epidermal cells, and it may be still present in almost undiminished abundance after 

 a prolonged sojourn in darkness (ten days), although no starch is then present in the mesophyll. 

 The soluble starch soon escapes from the epidermal cells when placed in 50 per cent, alcohol, 

 and the same also occurs in absolute alcohol, though more slowly.] 



1 Cf. Tollens, 1. c., 1888, p. 104; 1895, p. 155 ; E. Schulze, Zeitschr. f. physiol. Chemie, 1895, 

 Bd. XX, p. 511. [C. Hoffmeister, (Jber d. mikrochem. Nachweis v. Rohrzucker in pflanzlichen 

 Geweben, Jahrb. f. wiss. Bot.. 1898, Bd. XXXI, p. 688.] 



J Raffinose in the wheat embryo, according to Frankfurt, Versuchsst., 1896, Bd. XLVII, p. 469. 

 Ct also Schulze, 1895, 1. c., p. 534. Here and in Tollens' Handbook an account of Stachyose, 

 Melezitose, &c. is given. 



3 Cf. Tollens, I.e., 1888, pp. 32,83; 1895, pp. 76, 126; G. Kraus, Bot. Zeitung, 1876, p. 604. 



