848 CHEMICAL CONSTITUTION OF FRUITS. 
salts with an acid reaction. The nature of these acids and salts 
varies in different fruits ; thus the Grape contains tartaric acid 
chiefly and acid tartrate of potassium ; the Apple, malic acid ; and 
the Lemon, citric acid. As the pericarp ripens, saccharine 
matter is formed, and the quantity of free acids diminishes, 
partly from their conversion into other matters, and partly from 
their combination with alkalies. In order that these changes 
may be properly effected, it is necessary that the fruit be ex- 
posed to the sun and air, for if grown in the dark it will continue 
acid ; and it will be much less sweet even when developed in 
diffused daylight, than when freely exposed to the sun. As 
fruits ripen they evolve carbon dioxide, as already noticed, give 
off watery fluids, and a sensible elevation of temperature may 
be noted. 
The origin of the sugar of fruits, and even its nature, is not 
satisfactorily determined. According to most observers, ripe 
fruits contain grape sugar (glucose), but M. Buignet states that the 
sugar which is primarily formed in acid fruits is sucrose or cane 
sugar, and that during the process of ripening this sugar is gradu- 
ally changed into fruit sugar (glucose and levulose), but very often 
there remains in the ripe fruit a mixture of these two sugars. 
The origin of the sugar is variously attributed to the transforma- 
tion of the acids, cellulose, lignin, starch, dextrin, gum, and other 
matters of a like nature. According to M. Buignet’s investiga- 
tions, the cause of the change of the primarily formed cane sugar 
into fruit sugar is not the acids of the fruits, but appears to 
depend upon the influence of a nitrogenous body playing the 
part of a glucosic ferment, analogous to that which M. Berthelot 
has extracted from yeast. M. Buignet adds, that ‘the abun- 
dance in which starch is found distributed through the Vegetable 
Kingdom leads to the supposition that it is the true source of 
the saccharine matter in fruits. Its presence cannot, however, 
be detected in green fruits, either by the microscope or by 
iodine, excepting in green bananas, which contain a notable 
quantity of starch.’ M. Buignet also notices that green fruits 
contain an astringent principle resembling tannin, which is 
capable of being converted into a sugar identical with the sugar 
from starch (maltose), under the influence of dilute acids and a 
proper temperature. The proportion of this tannin diminishes in 
fruits in the same ratio that the proportion of sugar increases. 
The pericarp of some fruits has developed in it during the 
process of ripening, fixed and essential oils, as well as other 
substances of an aromatic nature. According to Frémy, the 
inner walls of the cells of succulent fruits in an unripe state 
consist of a substance called pectose, which is insoluble in water, 
alcohol, or ether. This body has not been isolated, but is con- 
verted in ripe fruits by the agency of acids into pectine, which is 
soluble in water. Pectine is afterwards transformed into pectosic 
and then into pectic acid through the agency of a peculiar ferment 
