572 SENSORY SYSTEM 



moreover, the perceptive faculty is often associated with certain stages 

 in the development of the cells. In such cases, there are no histological 

 features, apart from the ultra-microscopic irritable structure, to indicate 

 the perceptive function of the cells. 



This diffuse type of sensitiveness, in which the perceptive capacity 

 may be distributed through an entire organ or over extensive portions 

 thereof, was long thought to constitute an important distinction 

 between plants and animals ; while it does actually occur more fre- 

 quently in plants than among animals, it can by no means be regarded 

 as a universal characteristic of vegetable organisms. 



At a higher level of specialisation, the faculty of perception is 

 localised to the extent of being assigned to a definite tissue, instead of 

 being diffused through an entire organ or distributed among a number 

 of tissues ; w T hile the sensitive tissue is still primarily concerned with 

 other functions, its anatomical structure does exhibit a certain amount 

 of adaptation in connection with the subsidiary function of perception, 

 some of the features of its component cells being designed to facilitate 

 the perception of particular stimuli. 



On account of its exposed position, the epidermis is particularly liable 

 to experience a variety of external stimuli : as a matter of fact, epidermal 

 cells do frequently exhibit histological features indicating a perceptive 

 activity which is supplementary to their principal function of protection. 

 Thus, in tendrils, the epidermis is often sensitive to contact, while in 

 leaves it is frequently responsible for the perception of photic stimuli. 

 When it is desired to discuss or emphasise the perceptive faculty of the 

 epidermis, or the anatomical features correlated with perception, it is 

 quite justifiable to apply the term sensory epithelium to the layer in 

 question. This term is most appropriate, where perception of stimuli 

 takes precedence over the protective function and, accordingly, finds 

 permanent expression in the histological structure of the cells con- 

 cerned. The sensory epithelium of the stamens of Bcrberis provides 

 an excellent illustration of this grade of specialisation. 



The highest stage of adaptation is characterised by the fact that the 

 perceptive faculty is strictly confined to definite cells, cell-groups or cell- 

 organs, which have perception as their sole, or at any rate as their 

 principal function. Here the anatomical structure of the sensory organ 

 is primarily arranged with reference to its perceptive function. In 

 zoological nomenclature, organs concerned with the perception of 

 external stimuli have always been known as sense-organs, even among 

 the lower animals, and in other cases in which it is doubtful if the 

 organs in question are responsible for sensation in the psychological 

 sense. It is therefore not only permissible, but necessary in the 

 interests of consistency, to apply the term sense-organ to the analogous 



