THE BIOLOGY OF ALKALINE NECTAR IN THE PURPLE TOOTHWORT (LATHRAEA-CLANDESTINA) - GROUND-LEVEL DEFENSES

Lathraea clandestina flowers emerge directly from the soil and have the difficult problem of providing sufficient nectar rewards for suitably specialized legitimate visitors (some bees) whilst defending these resources from nectar thieves such as ants and birds. Flower morphology and nectar chemistry interact in this defence. The nectar, which is abundant and contains 35–38% w/w sugar, has a pungent unpleasant taste and an abnormally high pH; our tests indicate the presence of dissolved ammonia. This is produced within the nectar, after primary secretion of an initially slightly acidic fluid, probably by enzymic degradation of amino acids. It is retained within the nectar by the architecture of the nectary chamber in the corolla base, with apical narrowing and a barrier of hairs. In addition a water jacket formed by the calyx, around the corolla base, serves to keep the nectar chamber cool and reduces vapourization of the ammonia. Reasons for this peculiar basic nectar are discussed; we believe the punge ammoniacal fluid, apparently tolerated by bumblebees, serves as a deterrent to other animals, and to ants in particular. Copyright © 1992, Wiley Blackwell. All rights reserved