The Water Relations of Leaves 



53 



l4 



soil has been increased and more water is taken into the plant. 

 The shower has also covered the leaves with a film of water 

 and made the air moist around them, and this reduces the 

 water loss. Under these conditions, the cells of the plant 

 quickly become turgid, — that is, become fully distended 

 with water, — and the leaves recover their firmness. The 

 leaves of many plants like lettuce, pumpkin, and ragweed 

 depend for their firmness almost entirely upon the turgidity 

 of the leaf cells. 



The balance between the rate of water supply 

 and the rate of water loss is the most important 

 water relation of the plant. 



The water balance illustrated. The in- 

 ternal water balance of the plant may be 

 crudely illustrated by a glass tube with an 

 inverted porous porcelain cup sealed to one 

 end and wdth a stopcock attached near the 

 other end. If the cup and tube are filled 

 with water and the open lower end of the 

 tube is placed in a dish of mercury, the 

 mercury will rise as the water evaporates 

 through the porous surface of the cup. If we 

 nearly close the bottom of the tube by means 

 of the stopcock, the rate at which the mer- 

 cury rises is diminished. This is because the 

 evaporation is decreased as the amount of 

 water suppUed to the cup is lessened. If we 

 open the stopcock, but cover the outside of f '^; ^f ;^,^^^''['''^"' 



^ ^ ^ ' toillustratcthedraw- 



the cup with a thin layer of some substance ing up of water in 

 like wax, which does not allow water to pass ^ tree through tran- 



^ spiration from the 



through it freely, the rate of evaporation will leaves. 



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