30 PHYSWLOG Y. 



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. The 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. 



64. 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 horizontal position by one 

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

 removed. 



65. Transverse tissue tension. — To illustrate this one may 

 take a willow shoot T,-^cm in diameter and saw off sections 

 about 2cm. long. Cut through the bark on one side and peel it 

 off in a single strip. Now attempt to replace it. The bark will 

 not quite cover the wood again, since the ends will not meet. It 

 must then have been held in transverse tension by the woody 

 part of the shoot. 



