10 ELECTRO-PHYSIOLOGY CHAP. 



Iii order to explain these electromotive effects in the 

 " resting " leaf, Munk formulated a " molecular theory," in which 

 the parenchyma cells function as cylindrical molecules, giving such 

 an electrical reaction that " the positive electricity is directed 

 from the middle of the cell towards either pole, the latter being 

 therefore positive towards the centre." Dioncea (in common with 

 a few other plants) has the power of making visible movements 

 under special conditions, consisting in this case of a rapid 

 closing of the two lobes, which enables it to capture the in- 

 sects that have alighted on it. This kind of excitatory move- 

 ment is distinguished by its rapid energetic character from the 

 slow " movements of absorption " which commonly succeed the 

 first, and only appear alone when absorbable substances (meat, 

 albumen, etc.) are applied cautiously, and without touching the sen- 

 sitive hairs, to the inner surface of the leaf. The excited leaf of 

 Dioncea closes with extreme rapidity (in a minute at the longest), 

 while the reopening may extend over many hours (24-3G). If 

 the leaf has closed on an absorbable substance it may not reopen 

 for several days. 



The whole inner (upper) surface of the leaf is sensitive, yet, as 

 was said above, it is chiefly the six hairs (three sessile on each lobe) 

 which are so strikingly excitable that they were long held to be 

 the only sensitive part. Amputation of the petiole, or division 

 of the joint between leaf and stalk, does not act as a stimulus 

 unless the incision reaches the lower border of the mid-rib of 

 the leaf, where the vertical rows of excitable parenchyma cells 

 are situated beneath the epidermis of the upper surface. Am- 

 putation of the marginal bristles is equally ineffective. On the 

 other hand, incision of the lobe at any point induces closure. 



Slight pressure has no effect upon the upper surface of the 

 lamina, while stronger pressure, as well as stroking with a pointed 

 needle, discharges an excitatory movement. The lower surface 

 is totally insensible to all these stimuli. Only the upper layer of 

 the parenchyma of the lobe and mid-rib is therefore excitable 

 and conductive, agreeing with the fact that the hairs (which are 

 not intrinsically excitable) are sessile upon the cones of excitable 

 parenchyma cells that break through the epidermis, and act upon 

 these something after the nianin-r of a lever (cf. taste-hairs of 

 rrrlaiii animals). Kadi hair can be shorn away from apex t<> base 

 with sharp scissors without evoking any excitatory movement, 



