CHAP. XIII. RE-EXPANSION. 319 



leaves thus treated re-expanded, one to a partial ex- 

 tent in 24 hrs., a second to the same extent in 48 

 hrs., and the third, which had been previously in- 

 jured, not until the sixth day. These leaves after 

 their re-expansion closed quickly when the filaments 

 on the other lobe were irritated. These were then cut 

 off one of the leaves, so that none were left. This 

 mutilated leaf, notwithstanding the loss of all its fila- 

 ments, re-expanded in two days in the usual manner. 

 When the filaments have been excited by immersion 

 in a solution of sugar, the lobes do not expand so soon 

 as when the filaments have been merely touched ; and 

 this, I presume, is due to their having been strongly 

 affected through exosmose, so that they continue for 

 some time to transmit a motor impulse to the upper 

 surface of the leaf. 



The following facts make me believe that the 

 several layers of cells forming the lower surface of the 

 leaf are always in a state of tension ; and that it is 

 owing to this mechanical state, aided probably by 

 fresh fluid being attracted into the cells, that the lobes 

 Segin to separate or expand as soon as the contraction 

 of the upper surface diminishes. A leaf was cut off 

 and suddenly plunged perpendicularly into boiling 

 water : I expected that the lobes would have closed, 

 but instead of doing so, they diverged a little. I then 

 took another fine leaf, with the lobes standing at an 

 angle of nearly 80 to each other ; and on immersing 

 it as before, the angle suddenly increased to 90. A 

 third leaf svas torpid from having recently re-expanded 

 after having caught a fly, so that repeated touches of 

 the filaments caused not the least movement ; never- 

 theless, when similarly immersed, the lobes separated a 

 little. As these leaves were inserted perpendicularly 

 into the boiling water, both surfaces and the filaments 



