574 SENSORY SYSTEM 



living substance. The protoplasmic stimulation itself is quite beyond 

 the reach of direct observation. 



It is not essential that every component cell of a sense-organ 

 should be endowed with the perceptive faculty. Structures, for 

 example, which serve to transmit the external stimulus to the seat of 

 perception need not themselves be sensitive. The conical distal por- 

 tion of the tactile bristle of Dionaea muscipula, which, from the 

 mechanical point of view, represents the active part of the whole 

 organ [but is itself insensitive to contact], illustrates this point very 

 clearly. If we wish to apply a distinctive name to the actual perceptive 

 elements, we may term them the sensory cells. In its narrow sense 

 this term only refers to those elements of sense-organs, in which the 

 actual protoplasmic perception takes place. It may, however, con- 

 veniently be extended, so as to include all cells which are shown by 

 their anatomical structure to be concerned with the perception of 

 stimuli. 



It has already been explained that, when a stimulus acts upon a 

 sense-organ, it starts a series of processes, which usually terminates 

 with the execution of an adaptive paratonic movement. Sense-organs 

 are very frequently situated in the immediate vicinity of the associated 

 motor-tissues or organs. Thus, the sensory epithelium of the Berberis- 

 stamen immediately overlies the motor-tissue of the filament; some of the 

 tactile hairs and bristles of Mimosa pudica and Biophytum sensitivum, 

 at any rate, are located upon the pulvini ; finally, it is the rule for 

 the statocysts or geotropic sensory cells of aerial organs to be 

 situated in the internodes or pulvini that execute the geotropic 

 curvature. In other instances, the sense-organs are further removed 

 from the motor-tissues. One of the best illustrations of this separation 

 in space of the perceptive and motor regions is furnished by ordinary 

 roots, where the statolith-apparatus is largely or entirely confined to 

 the extreme tip, whereas geotropic curvature takes place in the sub- 

 apical growing zone. 



A transmission of stimuli from the sense-organ or sensory cell to the 

 motor-tissue must take place in all cases. 



II. TACT HE SENSE-ORGANS.-* 1 



A great many plants respond to shock, friction, contact and other 

 stimuli of a " mechanical " nature, by executing movements which 

 entail some ecological advantage. Instances are furnished by the 

 foliage-leaves of Mimosa pudica (and other species of Mimosa) and 

 Biophytum sensitivum, by certain insect-catching organs such as the 

 leaves of Dionaea muscipula and Aldrovandia vesiculosa or the tentacles 



