Numerous experiments were made on the difference in evaporation during the day and during the night. Those carried out with leaves of the grapevine gave the following hourly averages per square metre of foliage: in sunshine, thirty-five grammes; in shade, eleven; during the night, 0.5. The trellis on which the vine was trained was one metre high and thirty-eight metres long, and presented a surface of one hundred and thirty-eight square metres of foliage. In sunny weather this would lose by evaporation, in the course of twenty-four hours, forty-eight kilogrammes of water, and nearly half of that amount during cloudy weather. To give an idea of the enormous amount of aqueous vapor dissipated by plants in the sunshine, calculation showed that an acre of beets could lose in the course of twenty-four hours between 8,000 and 9,000 kilogrammes. Another experiment made with a chestnut-tree thirty-five years old showed that it lost over sixty litres of water in the course of twenty-four hours. The structure of the leaf, however, containing seventy to eighty per cent, of water, and possessing a thickness frequently of but one-tenth of a millimetre, would suggest the question why the evaporation is not much more rapid. The answer to this is found in the peculiar structure of the tissue forming the epidermis, designed especially to moderate the transpiration. In order to see the remarkable retentive power exercised by this epidermis, one can expose for a few hours to the sun two cactus-leaves of the same superficies, one of which has been deprived of its epidermis. The evaporation in the latter case will be about fifteen times as rapid as in the other. It is the presence of a similar tissue forming the skin of fruits which prevents an otherwise rapid evaporation. For instance, an apple deprived of its skin loses fifty-five times as much water as a whole specimen in the same time. Losses by rapid evaporation lessen notably the physiological energy of leaves. Thus an oleander-leaf containing sixty per cent, of water, when introduced into an atmosphere containing carbonic acid, decomposed sixteen centimetres of this gas; one containing thirty-six per cent, decomposed eleven centimetres; and one containing but twenty-nine per cent, was without action.
A series of observations was made on the relative powers of evaporation on the upper and lower sides of leaves. They consisted in plucking two leaves of the same kind at the same moment, covering on the one the upper, oil the other the lower side with melted tallow, and then noticing the loss of weight by evaporation in a given time. The average of the results showed that the proportion between the amounts of water evaporated on the upper and lower side of a dozen varieties of leaves was 1: 4.3. In all cases the amount evaporated from the two exposed sides of two equal leaves was greater than from the entire surface of a similar leaf under the same circumstances.
A point of no small interest with regard to the physical function of leaves is that of their ability to replace the roots of a plant in serving as the agent of absorption. A variety of tests were undertaken to set-
