180 DIFFERENT ABSORBING POWERS OF THE TWO SURFACES. 



length of time, by whichever surface they received their supply 

 of fluid. 



274. This arrangement is admirably adapted, for obtaining 

 the greatest supply of the moisture, contained in the lowest layer 

 of the atmosphere ; for when, by the cooling of the earth's sur- 

 face, which takes place on a clear night, this moisture falls in the 

 form of dew, it will manifestly be received on the upper sides of 

 the leaves of plants, which are but little raised above the ground ; 

 whilst, on the other hand, the leaves of more elevated trees would 

 not benefit by this deposition, being situated above its influence ; 

 and these would receive and iinbibe the vapour, as it afterwards 

 rises from the surface of the moistened earth. "We thus learn 

 that, if it be desired to revive drooping or sickly plants or trees 

 by the application of moisture, the mode in which it should be 

 distributed over them will depend upon their size ; if they bo 

 herbaceous plants, they should be watered from above ; and if 

 they be tall shrubs or trees, the water should be thrown up by a 

 syringe from below. 



275. The absorbing power of leaves has been shown by other 

 satisfactory experiments. Some plants of Mercurialis (Mercury) 

 were placed in water, some of them being immersed by their 

 roots, and the others touching it by a part of their leaves alone, 

 a small shoot of each being kept out, for the purpose of compa- 

 rison with the rest. After five or six weeks, the shoots of the 

 plants which were nourished by the leaves, differed little in vigour 

 from those which had been supplied by the roots. Experiments 

 upon single leaves, which nave already partially faded, are stili 

 more striking. Some leaves of Potamogeton natans (Pond-weed), 

 after being wiped dry, were weighed; and after remaining out 

 of the water for two hours, they were found to have lost from 

 3J to 5^ grains each. They were then put in water ; and after 

 the lapse of two hours more, were again wiped dry and weighed. 

 It was then found, that they had severally gained from 3 to 5 

 grains each ; and this increase (which was also evident from the 

 restoration of their natural freshness and plumpness) could only 

 have taken place by absorption through the cuticle, as the cut 

 nds of their footstalks were defended by soft cement. 



