MOVEMENTS OF WATER IN THE PLANT. 



31 



EXPERIMENT 35. 



NEGATIVE PRESSURE. 



Fig. 26. 



In September bore a small hole 6 cm. 

 in depth in the trunk of a small Birch, 

 and fit into the opening a glass tube a 

 meter in length which has been bent 

 once at right angles. Make the fitting 

 " air-tight " by means of wax. Place 

 the lower end of the tube in a dish of 

 mercury. In a day or two the mercury 

 will rise in the tube to a varying height. 

 The rapid transpiration from the leaves 

 has withdrawn so much water from the 

 trunk of the tree that a partial vacuum 

 is formed. (Fig. 26.) This may also 

 be demonstrated as follows (Fig. 27) • 



Negative pressure in Birch 

 stem. (After Oels.) 



Negative pressure in shoot of Lonicera. (Detmer.) 



