580 



SENSORY SYSTEM 



of these marginal pockets to the tactile pits of Cucurbitaeeous 

 tendrils. 



B. TACTILE PAPILLAE. 286 



The term which forms the heading of this section may be applied 

 to any mechanical sense-organ that projects above the general level of 

 a sensory epithelium in the form of a more or less well-defined papilla. 

 As a rule, it is only a limited area near the middle of the outer wall 

 of the sensory cell that projects in this way ; generally, this protruding 

 part of the wall is specially thin. The tactile papillae on the tendrils 

 of Uccremocarpus scaler are cut off from the underlying epidermal 

 elements by cell-walls, so that each represents an independent cell. 



Fig. 234. 



A, Epidermal cell from a staminal filament of Portulacagrandiflora in L.S., showing 

 a tactile papilla. B. A similar preparation from a filament of Opuntta vulgaris. In 

 both cases the protoplasts have shrunk away from the walls, the sections having been 

 prepared from spirit-material. 



Where the papilla consists of the whole outer wall, it often remains 

 thin throughout ; it may, on the other hand, be more or less thick- 

 walled, except for an unthickened marginal strip. In the latter case, 

 only this marginal zone or rather the ectoplast adhering thereto 

 suffers the deformation upon which stimulation actually depends. So 

 far, tactile papillae have only been observed in connection with floral 

 organs and tendrils. 



The staminal filaments of Portulaca grandiflora are sensitive to con- 

 tact at every point, except near the base. When touched or rubbed with 

 a needle, they curve towards the stimulated side. Most of the narrow 

 elongated epidermal cells of the filament bear, near the middle, or more 

 rarely towards one end, a minute papilla, with a wall which is so thin 

 as to consist of little beyond the very delicate cuticle (Fig. 234 a). 



