4 8 



BOTANY. 



Exercise 22. 



87. Transverse tissue tension. — To show this take a willow shoot 3-5«rc 

 in diameter and saw off sections about 2cm long. Cut through the bark on 

 one side, and peel off the bark in one piece carefully. Now attempt to re- 

 place it. What has happened ? Describe the tension. 



Demonstration 16. 



88. Importance of tissue tension. — To demonstrate the efficiency of this 

 tension in giving support, let us take a long petiole of caladium or of rhubarb. 



Hold it by one 

 end in a hori- 

 zontal position. 

 It is firm and 

 rigid, and does 

 not droop, or 

 but little. Re- 

 move all of the 

 outer portion of 

 the tissues, as 

 described 

 above, leaving 

 only the central 

 portion. Now 

 attempt to hold 

 it in a horizon- 

 tal position by 

 one end. It is 

 flabby and 

 Fig. 40. S' B \ droops down- 



Caladium leaf petiole rigid from longitudi- 

 nal tensions. 



ward because the longitudinal ten- 

 sion is removed. (See figs. 40, 41.) 

 Synopsis. — When plants are re- 

 moved from the soil, or plant parts 

 are removed from the shoot, they 

 soon become flabby and limp. 



Fig. 41. 



When these partly wilted plants Same leaf, longitudinal tension partly removed 

 are placed with the stems in water, h ? the loss of two °««ide strips, 



they may become rigid again by the absorption of water and the restora- 

 tion of the rigidity of the cells. 



