588 



SENSORY SYSTEM 



arise side by side from adjacent epidermal cells. In C. jacea and 

 C. C // nus the hairs have thin walls, and hence are flexible throughout 

 their length. Where, on the other hand, the hairs are more or 

 less thick-walled (as in 0. montatia, Fig 239), they are usually 

 provided with a " hinge," in the shape of an unthickened basal zone. The 

 hinge-area never completely encircles the hair, and hence is only visible 

 in transverse sections through the filament ; it can accordingly only 

 facilitate lateral displacement of the hairs. It can readily be shown 

 by experiment that these hairs actually represent sense-organs. For if 

 one rubs the hairs of a filament, which has been dissected out, with a 



needle or bristle, without bending or even 

 touching the filament itself, the charac- 

 teristic contraction at once takes place. 

 This experiment can be most satisfactorily 

 performed upon Ccntaurca orientalis. In 

 nature, when an insect visitor alights upon 

 a capitulum of Centaur ea, and inserts its 

 proboscis into the corolla-tube of a floret 

 in order to reach the nectar concealed 

 therein, it cannot fail to rub against 

 several of the tactile hairs, and thus bring 

 about stimulation of the filaments."' 4, " 29a 



The author has shown that the sensitive 

 foliage-leaves of Biophytum scnsitivum and 

 Mimosa pudica are likewise provided with 

 In these cases, the motor-tissues respond to 

 direct stimulation in the shape of shock or vibration ; they can also be 

 indirectly affected through the agency of the tactile hairs, which are 

 sensitive to a very light touch. From the ecological point of view, the 

 movement probably represents a method of protection against crawling 

 insects ; for such undesirable visitors will sooner or later brush against 

 one of the tactile hairs, and the sudden movement that ensues will 

 jerk them off the leaf, or perhaps cause them to fly away affrighted." 91 ' 

 The tactile hairs of Biophytum are located on the rhachis and on the 

 pinnae. On the rhachis they are arranged in transverse series ; a 

 row, consisting of from four to seven hairs, extends between each 

 pair of secondary pulvini, while, in addition, three or four hairs occur 

 in the angles between the secondary pulvini and the rhachis. 

 Similar hairs are found on the upper sides of the pinnae, six to 

 twelve being situated on the mid-rib, and five to eighteen on the 

 lateral veins, and in the meshes of the vascular network. TJie lower 

 surface of the leaves, on the other hand, is entirely devoid of such 

 appendages. If the sensitiveness of the leaves represents a form 



Fig. 239. 



Tactile hair from a staminal fila- 

 ment of Centaurea monla.ua ; the 

 section being taken across the fila- 

 ment, only one of the two cells of 

 which the hair is composed can bo 

 seen. 



tactile hairs or bristles. 



