586 SENSORY SYSTEM 



their protoplasts are not more sensitive than those of the ordinary 

 epidermal cells. 



C. TACTILE HAIRS AND BRISTLES. 



Trichume-structures, whether unicellular or multicellular, are par- 

 ticularly well adapted for the perception of mechanical stimuli. The 

 superficial position of these structures renders them peculiarly accessible 

 to the influence of such stimuli : they can also be readily utilised as 

 levers, the function of which is to ensure that the stimulus is strictly 

 localised, or, in other words, that all the available energy derived from 

 the shock is concentrated at one point, so as to produce the greatest 

 possible deformation of the sensitive protoplasm. 



The simplest types of tactile hair or bristle merely serve to 

 transmit the stimuli of shock or contact to the sensitive motor-tissue ; 

 these perform a purely mechanical function. Structures of this kind 

 which are not necessarily trichomes in every instance are termed 

 stimulators by the author, in contradistinction to sense-organs in 

 the strict sense. The bristles which are found on the main pulvinus 

 of the leaf of Mimosa Spegazzini may serve as a type of such 

 stimulators. The bristles in question are composed of thick-walled, 

 lignified mechanical elements ; distally each terminates in a single 

 cell, whereas the lower extremity is provided with a sharply conical 

 and particularly thick-walled process, which is deeply embedded in 

 the sensitive motor- tissue that composes the lower half of the pulvinus. 

 Figuratively speaking, this thick- walled process constitutes a " thorn 

 in the flesh " of the sensitive tissues, since even a slight displacement 

 of the stiff bristle must result in a very pronounced deformation of 

 the cells adjoining the embedded thick-walled process. The action of 

 the process is probably accentuated by the fairly numerous large 

 crystals of calcium oxalate that occur in the sensitive cells. 291 



In the case of Mimosa pudica, also, the majority of the bristles 

 which occur on the under side of the main pulvinus serve only as 

 stimulators. 



Stimulators of a different type occur on the stamens of 

 Sparmannia africana. These consist of peculiar reversed hooks, 

 representing transverse folds of the epidermis, and are situated near 

 the distal ends of the stamens and staminodes, on their inner side. 

 The smooth inner surfaces of the filaments are quite insensitive to 

 contact. When the outer side of a filament, on the other hand, is 

 rubbed with a needle or bristle, the characteristic movement of the 

 stamen at once follows, the filament bending outwards i.e. towards 

 the corolla more particularly near its base. If the needle is drawn 

 upwards that is, contrary to the direction in which the barbs point, 



