52 MAN AN ADAPTIVE MECHANISM 



which chances to alight upon the surface of the leaf, 

 or even to brush lightly against one of three tiny, hair- 

 like filaments which project upright from the surface of 

 each lobe, are about one twentieth of an inch in length, 

 and form the receptor mechanism by which the plant 

 is warned of the presence of its live prey. (Fig. 5.) 



The moment a small insect touches one of these 

 exquisitely sensitive filaments, the two lobes come 

 together quickly, the marginal spikes interlocking, 

 first at their tips, then down their entire lengths, while 

 the edges curve inward to form a shallow miniature 

 stomach, which begins at once to secrete digestive 

 fluid if the object caught contains animal matter. If 

 that object proves to be not of animal nature, 

 but a bit of wood, of cord, of paper, or of other non- 

 nitrogenous substance, the lobes will reopen within 

 twenty-four hours, and it may be seen that no digestive 

 fluid has been secreted. On the other hand, if a 

 piece of gelatine, of cooked or raw meat or of albumin 

 be substituted for the live insect, provided it weighs 

 about the same as the usual insect prey, the lobes will 

 close quickly, the digestive glands will become active 

 and the process of digestion and assimilation will pro- 

 ceed in the normal manner. In this case, as when 

 real insects are caught, the leaves remain closed for 

 many days. If the filaments be struck forcibly by 

 a needle or other hard object, no reaction will take 

 place. On the other hand, if the filaments be touched 

 lightly, so that the touch of an insect's foot is fairly 

 well imitated, the lobes will close. Any touch either 

 lighter or heavier than that of an insect invariably 

 fails to elicit a response. 



