Cuap. X. RE-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 suffice 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. 
