262 
DR. J. D. HOOKER ON THE FUNCTIONS AND STRUCTURE 
The relation of these glands to those which congenitally connect the pollen- 
masses of so many genera of Orchidece (either directly, or indirectly by means of the 
caudicula) is on the other hand very evident, and the structural passage between 
them may be traced through those genera in which each pollen-mass has a separate 
gland developed at its base, in a fovea of the rostellum. Another modification of 
structure in the rostellum occurs in certain genera allied to Listera , in which that 
organ is described as becoming divided into two arms, through the absorption or 
falling away of the intervening substance. 
In all these cases the special function of the rostellum appears to be the same, 
and they afford beautiful instances of a great variety of modifications of structure in 
one organ, being all adapted to the performance of one very simple function, namely 
the retention of the pollen-masses. With regard to the degree of complexity presented 
by the several modifications alluded to, it would appear, that, taking Listera ovata as 
an extreme case, a very simple form of pollen here accompanies a very complex 
organization of rostellum, whereas in some Vandeae which have eight pollinia attached 
to a strap-shaped caudicula, and that again fixed congenitally by an externally 
developed viscid gland to the rostellum, the latter is a mere cellular protuberance. 
Between these remarkably contrasted forms there are very numerous and evidently 
intermediate grades and modifications of structure, besides a great many which 
have not hitherto been reduced to any theoretical law of comparative development. 
Though I have given here the results of a long and very careful examination of 
Listera , I consider the subject as far from exhausted. Professor Henslow has had 
the kindness to overlook my dissections and drawings, which will I trust be found 
tolerably faithful, but there are many minute points in this species which yet require 
to be carefully investigated, whilst an examination of its allies would doubtless throw 
much light upon the structure and functions of the many curious forms of the column 
in Ophrydece. 
