18 EXPERIMENTAL PLANT PHYSIOLOGY. 



from the centre of a young branch of Elder (Sambucus) or stem 

 of Rhubarb. Divide the slice in halves and note the outward 

 curvature of the two parts. Describe the FlG T 



tensions which existed. Prepare another 

 slice and separate the parenchyma (pith) 

 from the wood and epidermis. The pa- 

 renchyma expands and the other portions 

 contract. (Fig. 13.) 



EXPERIMENT 23. 



TRANSVERSE TISSUE-TENSIONS. 



Cut a ring of bark from a young twig of Transverse tTssue-ten- 

 Willow or Poplar, and after a few minutes sions. (Detmer.) 

 replace in its original position. It now does not extend entirely 

 around the twig. When in that position it must have been in a 

 stretched condition. (Fig. 14.) 



