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Whether the sugar of fruits is formed within them, or introduced through the stem, and, if formed in the fruits, from what substance formed, are questions which have been investigated, but not wholly settled. It has been pretty generally held that starch in the unripe fruits is converted into sugar in the ripe fruits; the fruit acids inducing the change, as we know they have power to do. But starch is not found in the unripe stage of all fruits, and, in the cases where found, its quantity is sometimes too small to serve as the source of all the sugar of the ripened fruit. In the investigation of Hilger, above quoted, the immature fruit was at no time found by microscopic examination to contain starch. It appeared in the fruit-stalks in June; after August it almost wholly disappeared from the fruit-stalks, and was found only in the wood of the vines. Payen (Comptes Rendus, liii, 313) reported that he had demonstrated the presence of starch in unripe fruits and its conversion to sugar during ripening; but did not ascertain how much of the sugar of fruits is formed in this way.

It has been advanced that sugar is formed from malic and other acids, during ripening, either in the fruit or in the parts of the plant supplying juices to the fruit. Six molecules of malic acid and six molecules of tartaric acid, with nine molecules (eighteen atoms) of oxygen, would furnish the atoms for formation of four molecules of glucose, twelve molecules of water, and twenty-four molecules of carbonic anhydride. Mercadante (Gazetta Chimica Italiana, cxxv.; Journal of the Chemical Society, xxviii. [1875], 904) made a series of determinations of the malic acid and sugar in plums, commencing May 20th. The quantities of both acid and sugar increased in the fruit so long as it was green and emitting oxygen in the daylight; the branches which bore the fruit containing acid and pectous substances but no sugar. During the same time, the pectous and gummy substances in the green fruit had decreased from six per cent, of the pulp to three per cent, of the pulp. The investigator believed the sugar of the green fruit to have been chiefly formed, in the fruit, from the pectous and gummy substances, under contact of the acids. As soon as the fruit, losing green color, began to emit carbonic acid in the daylight, the acid in it began steadily to decrease as the sugar increased. The increase of sugar at expense of the acid in the pulp of plums is shown as follows: