THE PHYSICAL FUNCTIONS OF LEAVES. 367 



Numerous experiments were made on the difference in evajjoratioii 

 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 hun- 

 dred 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 chest- 

 nut-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 epi- 

 dermis, designed especially to moderate the transpiration. In order to 

 see the remarkable retentive powder exercised by this epidennis, one 

 can expose for a few hours to the sun two cactus-leaves of the same su- 

 perficies, one of which has been deprived of its epidermis. The evap- 

 oration 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 spe- 

 cimen 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 con- 

 taining 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 eva2D- 

 oration 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 sur- 

 face 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- 



