Ill Other Insectivorous Plants 39 



rain. The triangular area between the bases of the hairs 

 is shghtly sensitive, and if the general surface of the leaf 

 be wounded closure may occur. Inorganic or non-nitro- 

 genous bodies placed on the leaves without touching the 

 hairs do not excite any movement, but nitrogenous sub- 

 stances, if in the least degree damp, cause after several 

 hours the lobes to close slowly. Leaves which have made 

 a mistake and have closed over useless bodies, reopen after 

 a few hours, or at most a day's rest, and are again ready for 

 action. According to Macfarlane, mechanical stimulus of 

 the fly-trap requires two touches, unless the stimulus be 

 very powerful, and the touches must be separated by an 

 interval greater than one-third of a second. If less than 

 one-third of a second be allowed as interval, no contraction 

 follows, and a third touch is then necessar}''. 



As to the movement of the fly-trap, Darwin detected a 

 measurable contraction or alteration of form, and showed 

 that the movement follows a stimulus passing through the 

 cellular tissue from the sensitive hairs. There are really 

 two kinds of movement — one rapid, which follows the irri- 

 tation of the sensitive hairs ; the other slow, excited chemi- 

 cally, as when the leaves gradually tighten their hold on a 

 fly and bring the glands on both sides into contact with it. 

 Burdon Sanderson shows that the electrical conditions 

 associated with the rest and activity of the leaf are closely 

 like those observed in our muscles. Not only is there 

 a normal electric current in the leaf, but at the moment 

 of closure there is what animal physiologists know as a 

 "negative variation," due to the conversion of electro- 

 motive force into mechanical work. These facts lead us 

 to believe with Burdon Sanderson that " the property by 

 virtue of which the excitable structures of the leaf respond 

 to stimulation is of the same nature as that possessed by 

 the similarly endowed structures of animals." 



