The Water Balance in Plants 91 



them, and this reduces the water loss. Under these conditions, 

 the cells of the plant quickly become turgid and the leaves recover 

 their firmness. The leaves of many plants hke lettuce, pumpkin, 

 and ragweed, that have little or no woody tissue in them, 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 presence of a water bal- 

 ance may be clearly seen by cutting the stem of a potted plant in 

 two and then connecting the two pieces by a T-tube filled with 

 water. To the side arm of the T-tube connect a U-shaped tube 

 containing mercury and water. Then allow the U-tube to dip 

 into a beaker of water. If transpiration is more rapid than ab- 

 sorption, the mercury will be drawn toward the plant. If ab- 

 sorption is more rapid than water loss, the mercury will move 

 up the outer end of the tube. Figure 50 shows a sketch of the 

 experiment. Adding water to the soil, placing it in full sunlight 

 or in darkness, or moving it from a high to a low temperature will 

 soon change the position of the mercury in the tube. 



The water balance can be further illustrated by using the ap- 

 paratus shown in Figure 49. This consists of a porous cup at 

 the top, connected by a glass tube, with two bulbs, to a porous 

 cup filled with a solution of sugar. The middle of the tube is 

 further connected with a U-tube containing mercury. The 

 whole interior is filled with water. When the lower cup is placed 

 in a beaker of water, absorption begins and evaporation takes 

 place from the upper cup. By placing the apparatus in different 

 conditions, the changes in internal water balance will be shown 

 by movement of the mercury in the tube. 



Transplanting and the watef balance. When the skillful 

 gardener transplants a tree, he cuts off a number of branches to 

 reduce the number of leaves, in order that the plant may not dry 

 out before new water-absorbing roots are developed. Before 

 lettuce, tomato, and cabbages are Hfted for transplanting, the 



