IRRITABILITY. 547 



a single touch is not sufficient, as appears from Darwin's ob- 

 servations. Thus, a tentacle of Drosera requires to be touched 

 several times before any movement occurs, and reacts better 

 under the continued contact of some small body ; the in- 

 curving of the margin of the leaf of Pinguicula is only in- 

 duced by placing small objects on it. These organs are 

 remarkable for their ready reaction to chemical stimuli. The 

 tentacles of Drosera, for instance, are caused to inflect much 

 more rapidly when the bodies placed on the leaves are such 

 that they contain some nitrogenous substance which can be 

 absorbed by the plant, such as pieces of raw meat, of hard- 

 boiled eggs, fragments of insects. For example, a small 

 piece of raw meat placed on the gland of a tentacle caused 

 inflexion in five or six minutes, whilst with a piece of cinder 

 the shortest time of inflexion was 3 hours and 40 minutes. 

 Drops of nitrogenous liquids, such as milk, solution of al- 

 bumen, of urea, infusion of raw meat, or of peas or cabbages, 

 cause rapid and well-marked inflexion ; this effect is pro- 

 duced also in a remarkable degree by dilute solution of salts 

 of ammonia applied either to the glands of the marginal 

 tentacles or to the disc of the leaf-blade. In some cases the 

 application of a nitrogenous substance caused an incurvation 

 of the margin of the blade as well as inflexion of the 

 tentacles. Similarly, the application of organic nitrogenous 

 substances, such as pieces of meat and fragments of insects, 

 or drops of nitrogenous liquids caused more rapid and more 

 marked incurvation of the margins of the leaf of Pinguicula 

 than when other substances were employed. And not only 

 do these nitrogenous substances give rise to more active 

 movements, but the new position induced by them is main- 

 tained for a much longer time than when other substances 

 are employed. Ammonia gas is one of the most powerful 

 chemical irritants; exposure to it induces movement in all 

 irritable motile organs. 



In many of these organs it can be clearly observed that 

 the stimulus is transmitted from one part to another. Thus 

 if the terminal pair of leaflets of a pinna of the leaf of the 

 Sensitive Plant be irritated, not only will they fold up, but 



352 



