Abstract. 1. This paper describes field observations on diel changes in the nectars of Crataegus, Tilia and Echium in relation to microclimate and insect visits.2. Nectar concentration is highly correlated with ambient relative humidity, but the concentrative properties of the nectars differ from those of pure sugar solutions in ways that could be accounted for by microclimate or chemical effects.3. The microclimate inside flowers may influence the rate of equilibration of nectar with the relative humidity of the air, or the equilibrium concentration itself.4. The vapour pressure relationships of nectars may be influenced by the presence of components with low r/P values (that is, solutes in which solution show a relatively large lowering of vapour pressure, P, for a small change in refraction, r). Such components might be released by micro‐organisms in the nectar.5. The quantity of sugar per flower depends on the relative rates of secretion and reabsorption, among other things. The flowers we studied showed evidence for morning and evening peaks of secretion, and in Crataegus substantial quantities of sugar were ‘sequestered’ around midday.6. The pattern of visits by bumblebees to the flowers of Tilia and Echium can be related to the changing concentration of sugar in the nectar; caloric reward was probably not limiting for bumblebees visiting Tilia.

The concentration of sugar in the nectars of unprotected flowers of several species was measured and did not reach the high values that would be in equilibrium with the daytime humidities recorded outside the corolla, although the sugar concentration was highly correlated with ambient relative humidity. This paper examines features that maintain low nectar sugar concentrations at low ambient humidities. Postsecretory changes in concentration are influenced to a small extent by nectar composition but depend largely on physico-chemical and microclimatic effects. Factors contributing to the maintenance of steep gradients in water activity between the nectar and the ambient air include corolla morphology, sugar concentration gradients and waterproofing lipid monolayers on the nectar surface. This paper considers the relative importance of such features in relation to the pollination syndrome. A simple technique is described for the measurement of intrafloral relative humidity.

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Luthraea clandcstina 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 inteiart in this defence. The nectar, which is abundant and contains 3538% 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 ofhairs. 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 pungext ammoniacal fluid, apparently tolerated by bumblebees, serves as a deterrent to other animals, and to ants in particular.

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