Chap. X. EE-EXPANSION OF THE TENTACLES. 261 



• are moderately rigid and elastic ; if bent by a needle, 

 the upper end yields more easily than the basal and 

 thicker part, which alone is capable of becoming in- 

 flected. The rigidity of this basal part seems due to 

 the tension of the outer surface balancing a state of 

 active and persistent contraction of the cells of the 

 inner surface. I believe that this is the case, because, 

 when a leaf is dipped into boiling water, the tentacles 

 suddenly become reflexed, and this apparently indi- 

 cates that the tension of the outer surface is mecha- 

 nical, whilst that of the inner surface is vital, and is 

 instantly destroyed by the boiling water. We can 

 thus also understand why the tentacles as they grow 

 old and feeble slowly become much reflexed. If a 

 leaf with its tentacles closely inflected is dipped into 

 boiling water, these rise up a little, but by no means 

 fully re-expand. This may be owing to the heat 

 quickly destroying the tension and elasticity of the 

 cells of the convex surface ; but I can hardly believe 

 that their tension, at any one time, would suflice to 

 carry back the tentacles to their original position, 

 often through an angle of above 180°. It is more 

 probable that fluid, which we know travels along the 

 tentacles during the act of inflection, is slowly re- 

 attracted into the cells of the convex surface, their 

 tension being thus gradually and continually in- 

 creased. 



A recapitulation of the chief facts and discussions 

 in this chapter will be given at the close of the next 

 chapter. 



