142 CONVERSION OF ACIDS INTO SUGAR. 



III. This formation of acid proceeds for a certain time, the fruit be- 

 coming sourer and sourer ; the acidity then begins to diminish, sugar is 

 formed, and the fruit ripens. The acid rarely disappears entirely, even 

 from the sweetest fruits, until they begin to decay ; a considerable por- 

 tion of it, however, must be converted into grape sugar, as the fruit ap- 

 proaches to maturity. This conversion may take place in either of two 

 ways. 



1°. By the direct evolution of the excess of oxygen. Thus 



3 of Tartaric Acid = 0^2 Hg Oj^ 



6 of Water . . . = H. O^ 



Grape Sugar. 



Sum . . . == Ci2 Hi2 O2, = C12 H12 0,3 + 90. 



Or grape sugar may be formed from 3 of tartaric acid and 6 of the water 

 of the sap, by the evolution, at the same time, of 9 of oxygen. Citric 

 and malic acids, in the same proportion, would form grape sugar by the 

 evolution of 6 of oxygen only. 



Do fruits, when they have reached their sourest state, begin thus to 

 give oflf an excess of oxygen ? I know of no experiments which as yet 

 decide the point. 



2°. By the absorption of oxygen and the evolution of carbonic acid. 

 Thus in the case of tartaric acid, 

 1 of Tartaric Acid = C4 Hg O5 

 1 of Oxygen . . . = O. 



 . Xth of Grape Carbonic 



Sugai". Acid. 



Sum . . . = C4 H2 Oe = C2 H2 O3 + 2 CO2 

 Where one of oxygen is absorbed and two of carbonic acid given off. 

 Or in the case of the malic and citric acids, 



1 of Malic Acid = C4 H2 O4 



2 of Oxygen . . = O2 



'■ Xth of Grape Carbonic 



Sugar. Acid. 



Sum . . = C4 H, O2 = Co H2 O2 + 2 CO, 

 Where 2 of oxygen are absorbed and 2 of carbonic acid given on. 



We know from the experiments of Berard that, when unripe fruits 

 are plucked, they do not ripen if excluded from the access of oxygen 

 gas — but that in the air they rif en, absorbing oxygen at the same time, 

 and giving off carbonic acid. This second method (2°) therefore ex- 

 hibits the more probable theory of the ripening of fruits after they are 

 plucked; and if — as they become coloured — fruits imitate the petals of 

 the flower in absorbing oxygen from the air and giving off carbonic acid, 

 it will also represent the changes which lake place when they are per- 

 mitted to ripen on the tree. 



During the ripening of the fruit, it has been stated that the woody or 

 cellular fibre it contains gradually diminishes, and is converted into su- 

 gar. This is familiarly noticed in some species of hard or winter pears. 

 In sour fruit, the cellular fibre seldom exceeds 2^ per cent, of their 

 whole weight ; — in ripe fruits, however, it is still less, and as the con- 

 stitution of this substance is so analogous to that of grape sugar, there is 

 no difficulty in understanding that it may be readily converted into the 

 latter, though the immediate agency by which the transformation is 

 effected is as yet unknown to us. 



