"During the ripening much of the water disappears, while the cellulose or lignine and the dextrine are converted into sugar. Berard is of opinion that the changes in fruits are caused by the action of the oxygen of the air. Freney found that fruits, covered with varnish, did not ripen. As the process of ripening becomes perfected, the acids combine with alkalies, and thus the acidity of the fruit diminishes, while its sweetness increases. The formation of sugar is by some attributed to the action of organic acids on the vegetable constituents, gum, dextrine, and starch; others think that the cellulose and lignine are similarly changed, by the action of acids. The formation of sugar is said to be prevented by watering the tree with alkaline solutions. * * * In seasons, when there is little sun, but a great abundance of moisture, succulent fruits become watery and lose their flavor. The same thing frequently takes place in young trees with abundance of sap, and in cases where a large supply of water has been given artificially." Travelers, who have eaten the magnificent specimens of fruits produced by irrigation, in California, tell us that they are deficient in flavor, and the same thing is sometimes observed as a result of an unusually wet season. "It is not easy in all cases to determine the exact time when the fruit is ripe. In dry fruits, the period immediately before dehiscence,[21] is considered as that of maturation; but in pulpy fruits, there is much uncertainty. It is usual to say that edible fruits are ripe when their ingredients are in such a state of combination as to give the most agreeable flavor. After such are ripe, in the ordinary sense, so as to be capable of being used for food, they undergo further changes by the oxidation of their tissues, even after being separated from the plant. In some cases these changes improve the quality of the fruit, as in the case of the medlar, the austerity of which is thus still further diminished. In the pear, this process renders it soft, but still fit for food, while in the apple it causes a decay which acts injuriously on its qualities. By this process of oxidation, the whole fruit is ultimately reduced to a putrescent mass, which probably acts beneficially in promoting the germination of the seeds when the fruit drops on the ground. "The periods of time required for ripening the fruit, varies in different plants. Most fruits ripen within a year from the expansion of the flower, some come to maturity within a few days, others require months. Certain plants, as some Coniferæ, require more than a year, and in the Metrosideros the fruit remains attached to the branch for several years. The following is a general statement of the usual time required for the maturation of fruits:--