11. J TRANSPIRATION. ABSORPTION 19 



evaporation of water from the leaves. Take six or eight 

 tolerably large, healthy leaves with petioles an inch or two 

 in length ; two tumblers filled to within an inch of the top 

 with water, two empty dry tumblers, and two pieces of 

 card, each large enough to cover the mouth of a tumbler. 

 In the middle of each card bore three or four small holes 

 just wide enough to allow the petiole of a leaf to pass 

 through. Let the petioles hang sufficiently deep to dip into 

 the water when the cards are put upon the tumblers con- 

 taining it. Having arranged matters thus, turn the empty 

 tumblers upside down, over each card, so as to cover the 

 blades of the leaves. Place one pair of tumblers in the 

 sunshine, the other in a shady place. In five or ten minutes 

 examine the inverted tumblers. That exposed to the sun 

 you will find already lined with dew on its cool side, while 

 that kept out of the sun is still nearly or quite clear. It is 

 manifest, therefore, that evaporation from the leaves must 

 be not only rapid, but considerable in amount when plants 

 are exposed to the sun or a dry atmosphere. 



This exhalatioix of vapour from the surface of plants is 

 termed transpiration. A correct understanding of the pro- 

 cess explains how it is that plants growing in parlours are 

 apt to become faded even when watered, because the taking 

 up of water (termed absorption) by the roots cannot keep 

 pace with the transpiration from the leaves, owing to the 

 rapid evaporation excited by the dryness of the air. Since 

 the specimen No. 3, experimented upon at the beginning of 

 the present lesson, faded, notwithstanding the immersion of 

 some of its leaves, it is clear that the root is the part which 

 performs the office of absorption principally. 



Every part of a plant or animal appropriated to a distinct 

 purpose or function is termed an organ. Hence the root 

 may be called the organ of absorption of the plant. 



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