The work on Arabidopsis here may help our understanding of how Passiflora species manage to maintain their individual characteristics in the wild, with the stigma only allowing species-specific bonding. In captivity hybridization is easy if the species are closely related enough..so why does this seem to occur so rarely despite closely related species growing wild in the same areas?

Dr. John MacDougal comments, ''As do all plants, Passiflora vines have the ability to distinguish their own pollen from pollen of other individual plants of the same species and from other species. This poorly understood ability to recognize 'self' from 'non self' prevents hybridization. In a sort of plant 'immune system'*, 'recognition' proteins interact between the stigma and style of the receiving plant and the pollen and pollen tubes of the pollen-donor plant. Additionally, recent studies have shown the importance of the very first step in pollination, the first embrace. Pollen grains must first attach and bond to the female receptive surface, the stigma."

*Dr. Jen Sheen, Molecular Biology Department at Massachusetts General Hospital (MGH) comments that ''plants have an effective and sophisticated immune system''  See February 28 2002 issue of Nature.

Pollen adhesion hints at a small part of the story of how many species manage to remain individual & distinct, responding only to their own pollen. At the same time however many Passiflora species do not self pollinate, so the stigma cells may be sensitised to respond preferentially (an immune response?) to pollen from different clones of their own species. Perhaps there is a lack of response to their own pollen. Now that we are beginning to understand the mechanism, perhaps we may one day be able to induce selfing in non-selfing rare species e.g. by treating the flower and its own pollen in such a way as to go from initiating the binding process right through to the sperm within the pollen grain making its way down the tube and fertilizing eggs within the ovary.

Which pollen the flowers respond to is also complicated by the movements of the styles & stigma from the time the flower first opens to when it shuts, moving in & then out of range of the pollinators as they collect the pollen from the anthers. The stigma may also only be responsive at certain times. Perhaps when they are returning the the up position at the end of the day it is too late for successful pollination.