18 



EXPERIMENTAL PLANT PHYSIOLOGY. 



FIG. 14. 



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 

 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. 



Transverse tissue-ten- 

 sions. (Detmer.) 



Cut a ring of bark from a young twig of 

 Willow or Poplar, and after a few minutes 

 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.) 



