Journal Number 104
August 2007

EDITORIAL

The Posterior Protruberance of the Pterostylis Petal
By Ian St George

Many Pterostylis flowers have petals with a triangular lobe a third of the way up the posterior
edge ( arrowed in the photographs of Pterostylis alobula below ). Some Australian species
have a trichome, a hairy area, in the same place [Jones DL, Clements MA.
A review of Pterostylis. Australian Orchid Research 2002; 4: Fig.1.7].

We understand roughly the pollination mechanism in Pterostylis, with the irritable labellum flinging the fungus gnat back, the gnat crawling down toward the light coming into the base of the "window flower", then up past the stigma into the cavity formed by the column wings, and on up that narrowing passage to brush pollinia from the anther before escaping, pollen-laden, to visit the next flower.

We surmise the gnat is attracted by pheromones but nobody has done the analysis of volatile compounds necessary to prove that, and we don't really understand the function of many of the curious structures that have evolved inside the flower.

The fimbriate labellar appendage may be a lever or it may be sexually attractive to the gnat - we just don't know. The "horns" on top of the column wings (and the upward/forward pointing anther cap tip) may act like the tines of a crayfish pot in reverse to prevent the gnat entering from above and effecting self pollination.

What about the triangular petal lobes then? I think they act as an additional guide to the gnat, blocking the possibility of an exit to the side of the column, and, with the triggered labellum, blocking the possibility of re-entry by that route.