Temporal variation in pollinarium size after its removal in species of Bulbophyllum: a different mechanism preventing self-pollination in Orchidaceae

In Bulbophyllum involutum and B. ipanemense (Orchidaceae), two closely rela ted species, shortly after removal the pollinarium has a diameter of approx imately twice that of the entrance of the stigmatic cavity, requiring a mea n time of 105 to 135 minutes to shrink in width and allow pollination. Beca use the pollinators of these species remain for some minutes in the same fl ower after removing the pollinia this mechanism, previously unknown in Orch idaceae, may be very important in preventing self-pollination. This mechani sm does not occur in B. weddellii, and the pollinator does not remain in th e flower after removing the pollinia. The smaller diameter of the stigmatic cavity in B. involutum reduces by 50% the chances of interspecific pollina tion with B. weddellii, and interspecific crossing is strictly unidirection al. This is important in maintaining isolation between these sympatric spec ies, which share the same pollinators and have synchronized flowering.