TACTILE HAIRS OF FLORAL ORGANS 



587 



B 



the movement of the stamen takes place much more promptly than it 

 does if the filament is rubbed from above downwards. The barbs 

 serve to convert a uniform friction into a series of jerks, and thus act 

 as mechanical stimulators. It is only necessary to substitute an 

 insect visitor for the needle or bristle used in 

 the above experiment, in order to realise what 

 happens under natural conditions. 292 



Some account must now be given of typical 

 tactile hairs and bristles. We may first of all 

 deal with the unicellular tactile hairs of 

 Mormodcs, a genus of orchids in which the 

 pollen-apparatus is ejected much in the same 

 way as in Catasctum. Here the twisted column 

 is enclosed within, and compressed by the 

 peculiarly formed labellum, which in M. Buc- 

 cinator is more or less trumpet-shaped. When 

 an insect alights upon the column, this pres- 

 sure is increased, and the tactile hairs, which 

 are present in large numbers upon the apical 

 part of the column, are bent and consequently 

 stimulated. The final result is the same as 

 in Catasctum ; the disc of the pollen-apparatus 

 separates from the column, the tension in the 

 stalk is relieved, and the whole pollen-appa- 

 ratus is violently shot forward. In the 

 distal region of the column, all the epidermal 

 cells are prolonged outwards to form short 

 tactile hairs. Each hair is from four to six 

 times longer than its breadth, and has a 

 blunt apex, and a more or less well-defined 

 thin- walled basal zone (Fig. 238), which 

 closely resembles the hinge of the tactile 

 papilla of Berb&ris, and, like that structure, serves to produce a very 

 intense local deformation of the ectoplast. 293 



In many Cynakoideae the irritable filaments of the stamens bear 

 two-celled tactile hairs, which have been most thoroughly studied 

 by the author in the genus Centaurca. The hairs in question, which 

 are usually developed on all sides of the filaments, tend to be longest 

 and most abundant towards the middle of the filament ; near its 

 base, on the other hand, they are altogether absent. In Ccntaurea 

 Cyanus they form a continuous ring or collar around the middle 

 of the filament, and on its outer side become united into a ciliated 

 scale. Each tactile hair is composed of two elongated processes which 



Fig. 23S. 



Tactile hair from the upper 

 part of the column of Mormodet 

 Buccinator (plasmolysed). B. Ba- 

 sal portion of another hair, show, 

 ing the hinge-area very distinctly. 



