since meteorological conditions seem to determine its production. Unfortunately the observations are too superficial to be conclusive.
On the French Alps a sweet exudation is found on the small branches of young larches (Larix europaea), in June and July, in small, whitish, irregular, granular masses, which have the chemical composition of the sap of the trees, both containing the sugar mannite. It is known as Brianson manna, and is used for medicinal purposes. In Sicily the manna ash (Fraxinus Ornus) is extensively cultivated for manna or mannite sugar. In July and August incisions 2 inches long are made in the bark. On exposure to the air the sap hardens into flakes, which are secured by scraping. Its chief constituent is manna sugar. Maple sugar, the evaporated sap of the rock maple, is obtained in New England by a somewhat similar process. There are a number of other plants which are reported to yield sugars, but little definite information in regard to them is available. But not all of the sweet white substances sold as manna are plant exudations. Australian manna, obtained from the manna gum tree (Eucalyptus viminalis), is excreted by plant-lice. It has been observed falling in minute drops, until the twigs on the ground were covered with a snow-white incrustation a quarter of an inch in thickness.
The following distinctions should be carefully observed: Nectar is the sweet secretion of nectaries, either floral or extra-floral. Honey-dew is the sweet excretion of insects living on plants. The sweet exudations of plants, whether natural or artificial, should not be confused either with nectar or honey-dew. Their composition is the same or nearly the same as the plant sap. They should be known as sugars or mannas. The term plant honey-dew should never be used.
The exudation of nearly pure water by the leaves of many plants is clearly very closely allied to the exudation of sweet liquids. The Fuchsia, Indian corn, jewelweed, cabbage, primrose, grapevine, potato, elm, plane-tree, the aroids, and other species of plants often excrete drops of water from the tips and marginal teeth of the leaves. These drops may be observed on lawn grass, the ends of corn leaves, and the margins of jewelweed leaves in the morning, when they are likely to be mistaken for dew. The exudation of drops of water may easily be shown experimentally by placing a young cabbage plant grown in a flower-pot under a bell jar. In a few hours drops of water will appear on the apices or margins of the leaves, gradually increase in size, finally fall off, and new drops form. A surplus of water in the plant thus escapes through the leaf pores. A great amount of water may thus be exuded, and in a single night a leaf may excrete half of its weight in water. A vigorous leaf of Calocasia has been observed to eject water at the rate of 195 minute drops per minute, so that there seemed to be an almost continuous jet of water. The liquid is pure water except for a trace of salts (one-tenth of one per cent.). Certain tropical flowers bear glandular hairs which secrete great quantities of water containing a little sugar, thus exhibiting an intermediate stage between the exudation of pure water and saccharine liquids.