The droplets of water on the foliage of large Mediterranean spurge hint at an ecological adaptation to hot, dry environments. During episodes of intense heat and dry air, the amount of evapotranspiration (the combination of evaporation and plant transpiration) can be so high that plants are unable to draw enough water from the soil (or internally) to retain biochemical balance and turgor, so they begin to wilt and burn. If this environmental stress continues long enough, the plant will shrivel and die.
Plants growing in hot, dry environments, have evolved a number of strategies to reduce the likelihood of “death by thirst”. This Euphorbia illustrates one example. Its leaves are covered by a thick epicuticular wax that prevents the loss of water from the leaf surface via evaporation; the wax forms an impenetrable barrier that water cannot pass through and be lost. The waxy layer works from both directions, though; the hydrophobic lipids of the wax repel the water molecules and prevent adhesion, while water's cohesive properties cause the beading.
Photography resource link: for study and inspiration, the flora and fauna photography of Sean and Jennifer McCormick of Alberta, Canada.