SECT. xiii. IRRITABILITY OF LAS ELL UM. 401 



assumes when ripe by the process of gradual un- 

 twisting. This little plant belongs to a genus dis- 

 tinguished by having a movable and deciduous anther, 

 and is one of the British orchids that have allied forms 

 among the exotic genera. All Dr. Lindley's vast tribes 

 of Epidendreae, and the still more numerous and 

 splendid Yandeae, have not a single British represen- 

 tative. 



The structure of the exotic orchids is often very com- 

 plicated, and they possess many properties unknown in 

 the British genera. Their labellum, or lower lip, is so 

 varied, and sometimes so singular, that it baffles de- 

 scription ; besides, it often possesses peculiar motions, 

 sometimes from structure, at other times from irrita- 

 tion. In the Bolbophyllum Rhizophorse, the labellum is 

 attached to the column by a very narrow thin strap, 

 elastic as india rubber, which oscillates in a singular 

 manner ; while that of the B. barbigerum has a beard 

 of fine hairs irT almost constant agitation. The irri- 

 tability of the labellum in many of the allied forms of 

 the orchids is one of their remarkable properties ; the 

 slightest touch sets them into motion. The Australian 

 genus Caleana possesses it in the highest degree ; for 

 when an insect settles on its labellum, it suddenly shuts 

 up against the column, and encloses it, as it were, in a 

 box. 



Some of the exotic orchids have a pseudo-dioecious 

 character. Thus some species of Catasetum have two 

 long horns, or antennae, attached to the rostellum, which 

 stand over the labellum, and the pedicels of the anthers 

 are fastened down in a curved position ; so, when an insect 

 alights upon the labellum and touches the antennae, the 

 excitement is conveyed by them to the rostellum, the 

 attached edge of the disc of the anthers is ruptured, and 

 they straighten themselves with such force, that not only 

 do they drag the balls of pollen and anther cells from 



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