On the Sensitive Labellum of Masdevallia 
muscosa, Rchb. f. 
BY 
F. W. OLIVER, B.A., B.Sc., F.L.S. 
With Plate XII, 
HE irritability of the labellum in certain orchids would 
-a- appear to have been discovered by Robert Brown about 
1830 — in the first instance for Pterostylis, and to have been 
communicated by him to Lindley. In his Vegetable King- 
dom 1 Lindley quotes the following orchids as possessing 
irritable labellums : — Pterostylis, Meg actinium, and C ale an a 
nigrita , in each case the contractility residing in the narrowed 
hinge or neck. 
It is however a mistake to include Caleana in this category; 
for Fitzgerald 2 , in a most careful and intelligent account of the 
mechanism of this orchid, conclusively shows that the motility 
of its labellum arises solely from the great flexibility of its 
narrowed hinge. In this flower, by inversion, the labellum 
stands above the column. An insect alighting on the under 
(i.e. side towards column) surface of the labellum drags this 
down with it and becomes shut into the cup-like column, the 
labellum forming a close-fitting lid. It is no case of irritability. 
Megaclinium falcatum was the subject of one of Ch. 
Morren’s classical memoirs 3 . The movement here is entirely 
spontaneous (as in Desmodiuni gyrans), and consists of a slow 
oscillation in a vertical plane. Mechanical stimulation has 
no effect. In Pterostylis the movement of the labellum is 
1 Ed. 1853, p. 179. 
2 R. D. Fitzgerald, Australian Orchids, vol. i. Sydney, 1882. 
3 Ch. Morren, Recherch.es sur le mouvement et l’anat. du labellum du Mega- 
clinium falcatum , in Mem. Acad. Roy. d. Sci. et Belles-lettres de Bruxelles, 1841. 
[Annals of Botany, Vol. I. Nos. Ill and IV. February 1888.] 
