58 



PHYSIOLOGY. 



shorter now. In other words there is also a tension in the tissue 

 of the central portion of the petiole, the direction of which is 

 opposite to that of the superficial tissue. I'he parts of the petiole 

 now are not rigid, and they easily bend. These two longitudi- 

 nal tissue tensions acting in opposition to each other therefore 

 give rigidity to the succulent shoot. It is only when the indi- 

 vidual cells of such shoots or petioles are turgid that these tissue 

 tensions in succulent shoots manifest themselves or are promi- 

 nent. 



117. To demonstrate the efficiency of tliis tension in giving support, let us 

 take a long petiole of caiadium or of rliubarb. Hold it by one end in a hori- 

 zontal position. It is firm and rigitl, 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 horizontal position by one 

 end. It is flabby and droops downward because the longitudinal tension is 

 removed. 



118. Longitudinal tension in dandelion stems. — Take long 



and fresh dandelion stems. Split 

 them. Note that they coil. The 

 longitudinal tension is very great. 

 Place some of these strips in 

 fresh water. They coil up into 

 close curls because by the ab- 

 sorption of water bv the cells the 

 turgescence of the individual cells 

 's increased, and this increases 

 the tension in the stem. Now 

 place them in salt water (a 5 per 

 cent solution). Why do they 

 uncoil ? 



119. To imitate the coiling 



of a tendril. — Cut out a narrow 



strip from a long dandelion stem. 



(,, Fasten to a piece of soft wood, 



with the ends close together, as 



shown in fig. (12. Now place it in fresh water and watch it coil. 



Part of it coils one wav and jiart another way, Just as a ten- 



^W^^ 



Fig. 62. 

 Strip from dandeliMn stem mad' 

 imitate a r'lant tt-ndril. 



