582 



SENSORY SYSTEM 



B 



be executed, until the adhesive disc of the pollen-apparatus is detached 

 from the rostellum ; and this separation is a response elicited by the 

 contact of a solid body with the sensitive antenna. Under natural 

 conditions, stimulation of the antenna is effected by an insect-visitor, 

 ami the ejected pollen-apparatus becomes firmly attached to the back 

 of the animal." 5 ' 



In the species of Catasetum examined by the author (C. Darwin- 

 ianum and C macrocarjnim), each epidermal cell in the apical region of 

 the antenna bears a small tactile papilla, situated near the middle 



of the outer wall, or, more rarely, 

 towards one end. The membrane of 

 the papilla is thinner than the rest 

 of the outer wall, although the differ- 

 ence is not so marked as in the case 

 of Portulaca or Opuntia. According 

 to Von Guttenberg, tactile papillae 

 also occur in several other species of 



,, > Catasetum. 



J 11 |(__ Comparatively large, thin-walled 



tactile papillae are found on the 

 staminal filaments of many Cyna- 

 koideae, either as the only sensory 

 organs (Alfredia cemua) or in associa- 

 tion with tactile hairs (Centaurca, etc.; see below, p. 587). The most 

 distinctive feature of these papillae consists in the fact that they are 

 formed by pairs of sensory cells ; the adjoining ends of two epidermal 

 cells grow out together to form a common papilla, which is accordingly 

 divided into two halves by a thin vertical partition (Fig. 235). 



The tactile papillae of the stamens of Berberis vulgaris are constructed 

 on a somewhat different plan to those which have been described so far. 

 In this plant, sensitiveness is restricted to the adaxial side of the 

 filament, and even there is absent at the base, and also just below 

 the insertion of the anther. When any part of the sensitive region is 

 touched, the filament curves suddenly towards the stigma. The 

 sensitive adaxial surface is covered by a relatively large-celled and 

 remarkably papillose sensory epithelium. The sensory cells, which, 

 as usual, contain abundant protoplasm, are arranged in longitudinal 

 rows ; their length and breadth are approximately equal. The cells 

 usually alternate in adjacent rows, so that their cross-sectional outline 

 is normally hexagonal. Each papilla, which is about as high as it is 

 wide, arises by the protrusion of the entire outer wall of a sensory 

 cell : it has an obtuse apex, and is fairly thick-walled and provided 

 with a stout, smooth cuticle. The papillae are almost always relatively 



Fio. 235. 



A. Tactile papilla from a staminal fila- 

 ment of Centaurea Cyanus. B. Tactile pa- 

 pilla from a filament of Echenais earlinoides. 



