The astringent and acid taste of unripe fruits is due to the tannin and the acids. Oxygen is necessary to ripen fruits and the slow natural maturing of the fruit on the tree enables the oxygen to enter into combination with these substances, lessening their reaction and altering the starch into glucose or levulose. Fruits ripened artificially lack this chemical action of sun and oxygen, hence the decreased palatability and digestibility of fruits so ripened. If underripe fruits are freely eaten they ferment in the alimentary tract and this fermentation causes the colic, vomiting, and diarrhea so often experienced. Overripe fruit, from the decomposition products which have already begun to form and which are further released in the stomach or bowels, may produce the same results. Care should therefore be exercised to select thoroughly ripe fruits which have not begun to decay. In order to reach their destination in fair condition, outwardly, many fruits are picked before they are ripe. Bananas are commonly picked green, because they decay so quickly that if they were picked ripe they would spoil before reaching the northern markets. One test of a naturally ripened apple is to cut it with a steel knife—if the blade turns black, or if the cut surface of the apple turns brown in a few minutes, it should not be eaten, for it indicates an excess of tannin. It is this tannin which gives the small boy excruciating pains from his green apples. It will be recalled that the tannin from the bark of trees toughens the skin of animals and forms leather. The effect on the membrane of the stomach and intestines, from the tannin in food, is not so pronounced, because of the activity and resistance of living matter. The juice of lemons and oranges is most valuable in seasickness and scurvy, and is of benefit in nausea. A slice of lemon will often clear a coated tongue and give a refreshing sense of cleanliness to the mouth, especially in feverish conditions. Sour lemonade is one of the best drinks in summer because of its thirst-allaying qualities. Table III shows that _bananas_ contain nearly twenty-three per cent. of carbohydrates, which, in an immature state, are largely starches. The natural ripening process changes the starch to sugar, thus making them more easily digested. The starch globules, when not matured on the tree, are not easily broken and are thus difficult of digestion. Bananas should not be given to children under two years of age because before this age the ptyalin and pancreatin are not sufficiently developed to digest the starch. Many of the inhabitants of the tropics use bananas almost to the exclusion of other food and appear well nourished. They obtain them from the tree when the fruit has thoroughly ripened, the starch having been transformed by Nature into an easily digested product. The reason many find they cannot digest bananas, as purchased in our markets, is due to the fact that the fruit is immature and unripe.