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X. On the Functions and Structure of the Rostellum of Listera ovata. 
By J. D. Hooker, M.D., F.R.S. &;c. 
Received June 15, — Read June 15, 1854. 
My attention having been directed by the Rev. Professor Henslow, to a remarkable 
irritability of the rostellum in a common English orchideous plant, the Listera ovata, 
I have taken the earliest opportunity of following up his observations on the living 
plant, and of dissecting the organ in question ; an account of the anatomy and 
functions of which I have now the honour of laying before the Royal Society. 
The rostellum of Listera ovata is a broad, blunt, tongue-shaped organ, terminating 
the column, and projecting forwards, below the anther, and above the stigma. 
Soon after the flower opens, if the rostellum be touched or irritated, two white 
viscid masses are instantaneously protruded, — one from each side of the apex ; these 
coalesce and form a considerable gland-like body, which attaches itself at once to the 
bases of the pollen-masses, and usually continues firmly to adhere to the apex of the 
rostellum also. 
Such are the effects of artificial irritation; and a careful inspection of a great 
number of flowers proves, that in a state of nature this glandular secretion secures 
the impregnation of the plant, by freeing the pollen-masses from the anther-case, and 
retaining them on the rostellum, where they break up, and the granules falling over 
the edges of the latter become applied to the stigmatic surface. 
The form and position of the labellum seems also to be of considerable importance 
in this operation. This organ is attached to the base of the column by a rather 
narrow claw, beyond which it is bent upwards, so that its anterior surface is brought 
immediately in front of the rostellum. Soon after the flower opens, a viscid fluid 
is secreted along the mesial line of the labellum, which retains the pollen-masses 
when these have accidentally become detached from the anther-case previous to the 
emission of the viscous masses from the rostellum. In some flowers I also found 
that the viscous masses had been projected with such force from the apex of the 
rostellum, that they had carried the pollen-masses with themselves beyond that 
organ, and had alighted on the viscid surface of the labellum, where they were re- 
tained by their bases, whilst their apices were brought into contact with the stigma. 
The structure of the rostellum in Listera ovata is extremely curious, and, as far as 
I am aware, quite unique in the Order, though it is highly probable that a similar 
organization will be found in allied species and genera. From its earliest appearance 
in the young bud (as a simple transversely elongated cellular ridge) to its fully-formed 
condition, it undergoes no morphological change of any consequence; its development 
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