This thesis explores the dynamic patterns in the secretion, reabsorption and concentration of nectar, and their relation with microclimate, flower visitors and the innervation of the nectaries. Case studies are presented, comprising Impatiens glandulifera, Brassica napus, Fritillaria imperialis and Borago officinalis. Nectar secretion rate and nectar solute concentration are affected by the environment, and probably by the genetic composition of the plants. Significant differences in nectar secretion rate and nectar concentration are found between plants, between times of the day and between days, but not among flowers on the same plant. Correlation matrices and correlograms help us to disentangle multiple interactions. In all the species studied, the environmental factor most likely to affect nectar secretion rate seems to be the temperature of the air. Other factors also contribute to explain the variation in nectar secretion rate, among them the stand age (probably acting through the relative sink strength of the flowers), and the number of flowers and fruits per module. The supply of assimilates to the nectary is explored by experimental defoliations and deprivation of light. In both cases an immediate response is elicited, but the degree of response varies between species. Brassica napus, for example, is much less sensitive to light deprivation or total defoliation than Fritillaria imperialis. Nectar solute concentration seems to depend mainly on the relative humidity of the air. However, some evidence suggests that plant water status might affect nectar concentration. These results, obtained from field experiments, were confirmed in controlled conditions in growth chambers. Other minor factors probably play a role, but their effects are obscured by complex physiological interactions. We conclude that in a given plant the nectar secretion rate will depend mainly on its physiological age, and on variations in air temperature, while the nectar solute concentration at a given moment will be mainly the product of the short-term plant-microclimate interaction. In this context, constraints on the evolution of nectar presentation systems are considered.