The functions of the leaves appear to be a combination of those of the lungs and stomach of animals; they not only modify the food brought to them from the roots, so as to fit it for increasing the size of the parent plant, but they also absorb nourishment from the atmosphere. The sap, after elaboration in these organs, differs in every plant, though, as far as experiments have been tried, it appears to be nearly the same in all vegetables when it first arrives to them. The power of a leaf to generate sap is in proportion to its area of surface, exposure to the light, and congenial situation.

Leaves throw off a very considerable quantity of water. Dr. Hales found that a Cabbage emitted daily nearly half its weight of moisture, a Sunflower, three feet high, perspired 1 lb. 14 oz., and Spearmint exhales 1 1/4 times its weight in the same period. But of all the plants the diurnal perspiration of which has been ascertained, the Cornelian Cherry (Cornus mascula) transpires the most; the exhalation amounting to nearly twice the weight of the plant in twenty-four hours. This aqueous expiration takes place chiefly during the day, is much promoted by heat and checked by rain, or a reduction of temperature.

Evergreens transpire less moisture than deciduous plants; which would lead to the expectation that they are more capable of living in dry situations, which, in general, is really the case.

As the season of growth advances the transpiring power of leaves decreases. Under similar circumstances Sennebier found the transpiration much greater in May than in September.

The transpiration of plants decreases with that of the temperature to which they are exposed, as well as with the period of their growth. This explains why the gardener finds that his plants do not require so much water in cold weather, nor during the time that elapses between the fall of their blossom and the ripening of their seed. During this period they do not transpire more than one-half so much as during the period preceding and attending upon their blooming.

The transpiration takes place from the upper surfaces of the leaves; and, if these surfaces are coated with varnish, the leaves gradually decay and fall, and the growth of the plant ceases until fresh leaves are produced. Hence arises the benefit which plants derive in rooms, greenhouses, and other confined enclosures, from keeping those surfaces cleansed with the sponge and syringe. Some plants are particularly sensitive to injury from any check to their transpiration, among which are the Tea-scented Roses. The advantage derived by plants from having their leaves cleansed was exemplified by the following experiment: -

Two Orange-trees, weighing respectively 18 ozs. and 20 ozs., were allowed to vegetate without their leaves being cleansed for a whole twelvemonth; and two others, weighing 19 ozs. and 20 1/2 ozs. each, had their leaves sponged with tepid water once a week: the two first increased in weight less than half an ounce each; whilst of the two latter, one had increased two, and the other nearly three ounces. In all other respects they had been treated similarly.

It must be remembered, however, in using the sponge and the syringe, that the under side of the leaves is an absorbing surface, benefited by being kept clean, and by the application of moisture. The Kidney Bean, Sunflower, Cabbage, and Spinach, absorb moisture equally by their under and upper surfaces; the Cockscomb, Purple-leaved Amaranth, Heliotrope, Lilac, and Balm, absorb most freely by their upper surfaces; and the Vine, Pear, Cherry, Apricot, Walnut, Mulberry, and Hose, absorb most by their under surfaces. - The Science of Gardening.