Exchange and Movements of Gases and Liquids. 55 



pressure of a long knife-blade. Note the position and form of 

 the two portions a few minutes later. What parts of the stem 

 were most highly turgid? What causes the curvature of the 

 separated portions? (Fig. 32.) 



Divide the hollow flower stalk of a dandelion (Taraxacum) 

 into several longitudinal strips, and note the action of these 

 strips. Place all the above material in a shallow dish contain- 

 ing a 5 per cent, solution of common salt, for half an hour, 

 and compare the positions assumed when plasmolyzed with 

 those assumed immediately after 

 their isolation. 



Cut a ring of bark from any rapidly 

 growing woody stem, and allow it to 

 lie in a moist chamber for a few min- 

 utes. Now replace in its original 

 position. It will be found that it 

 does not completely encircle the 

 stem, and must have been in a 

 stretched condition when taken from 



the Stem. (Fig. 33.) 



50. Wilting.-If the portions 

 of the stem which were laid in a salt Detmer. 

 solution are examined, they will be 



found quite limp and flaccid. A similar condition prevails 

 when for any reason turgidity is lost. The loss of turgidity by 

 the cells of a stem is followed by a drooping position of leaves, 

 and by a bending of stems under their own weight. Leaves are 

 constantly losing water, and, if not supplied, the turgidity is soon 

 lost and wilting follows. Wilting may be induced by immersion 

 of a shoot in a salt solution, which plasmolyzes the cells but 

 induces a different condition from that following an insufficient 

 supply of water. 



Cut off a young shoot of sunflower (Helianthus) or Coleus, 

 and force the base of the excised stem through a perforation 



Fig< 33 ._ Diagram of a cross . 



section of a young stem from 



