The Life of the Plant. 203 
or chloroform, usually temporarily paralyse it; and con- 
stant repetition of the irritation is followed by a diminution 
for a time of its sensitiveness. It is probable that the tissue 
of the lower side of the pulvinus, or mass of succulent tissue 
found at the articulation, contracts after irritation such as 
that produced by simple contact, its cells losing a large 
portion of their cell-sap, and thus becoming less turgid, 
and giving space for the expansion of the tissue of the 
upper side. A depression is thus caused of the leaf 
which is attached to the articulation ; and this is not coun- 
teracted by the vascular bundles in the pulvintis in con- | 
sequence of their flexibility. A fresh flow of sap into the 
part of the tissue which was previously emptied then 
causes a fresh elevation of the leaf. 
Venus’s fly-trap, Dionza muscipula (Fig. 372), a bog- 
plant of N. Carolina, manifests a similar irritability. When 
the upper side of the leaf, through which passes a strong 
mid-rib, is irritated, the two halves of the leaf close together, 
and remain closed until the irritation has ceased. If, there- 
fore, a small insect, such as a fly, settles upon the leaf, it 
becomes shut in, and is retained between the halves 'of the 
leaf until it is dead, and no longer exercises any irritation 
upon it. Itis believed by some botanists that the object 
of the capture of the insect is the nutrition of the plant. 
|The sensitive part in this case appears to consist of three 
small bristles on each half of the upper surface of the blade. ] 
Closely connected with these movements are those be- 
longing to fendrifs and tendril-like organs. In their early 
state, before they have coiled up backwards or have clung 
to a support, these organs are sensitive to simple and slight 
contact or to very light rubbing, the side on which they are 
touched becoming, after some minutes, concavely curved. 
The bent tendril subsequently straightens itself, and is then - 
again irritable. Bent tendrils are sensitive only on the con- 
cave side. | 
The stamen of many Composite, such as Centaurea, 
