48 



mechanism of pollination in certain australian 



Orchids. 



By R. S. Rogers, M.A., M.D. 



[Read June 12, 1913.] 



Plates VI. to IX. 



DiPODIUM PUNCTATUM, Br. PI. vi. 



The column (figs. 1 and 2) is about quarter of an inch 

 long and semicylindrical in shape. It is flattened in front, the 

 upper third of this surface being bevelled antero-posteriorly. 

 The lid-like anther rests on its summit, and is attached thereto 

 by a minute posterior hinge. The rostellum is on a level with 

 the base of the anther, and immediately below the rostellum is 

 the elliptical stigma. 



There is little or no evidence of winging. In the raceme 

 of flowers it occupies a horizontal position, but for convenience 

 of description it will be assumed (except where otherwise 

 stated) to be vertical. Anteriorly its central third is in close 

 contact with the middle lobe of the rigid sessile labellum ; its 

 upper portion forms with the latter a wedge - shaped space 

 (fig. 9), in which are the rostellum and the stigmatic cavity. 

 The margins of the anterior surface are accurately embraced 

 throughout their length by the lateral lobes of the labellum 

 (fig. 3). 



The rostelluTn can be seen in the immature flower as a 

 small, blunt, and almost horizontal projection immediately 

 above the stigma (fig. 5). It extends also horizontally back- 

 wards so as to form a sort of incomplete roof to the column. 

 An examination of the top of the rostellum at this stage will 

 reveal in relief the outlines of the future ''disc," and passing 

 horizontally backwards from the latter the outlines of the two 

 pointed caudicles or "stipites." These structures have as yet 

 no connection with the pollinia, and are developed from ros- 

 tellar tissue. The disc is somewhat oyster-shaped, and lies 

 upon the blunt point of the rostellum. At a later period well- 

 marked separation lines appear between this portion (disc and 

 appendages) of the pollinarium and the subjacent tissues, but 

 even in the comparatively advanced bud it may with care and 

 a little trouble be removed en masse. At this stage a rather 

 blunt triangular tongue of tissue may be seen projecting up- 

 wards and backwards from the rostellar roof so as to insert 

 itself into the triangular space between the anther loculi. 



