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In "Origin of Species", Darwin says (I have added bold for emphasis):

Certain plants excrete sweet juice, apparently for the sake of eliminating something injurious from the sap: this is effected, for instance, by glands at the base of the stipules in some Leguminosae, and at the backs of the leaves of the common laurel. This juice, though small in quantity, is greedily sought by insects; but their visits do not in any way benefit the plant. Now, let us suppose that the juice or nectar was excreted from the inside of the flowers of a certain number of plants of any species. Insects in seeking the nectar would get dusted with pollen, and would often transport it from one flower to another. The flowers of two distinct individuals of the same species would thus get crossed; and the act of crossing, as can be fully proved, gives rise to vigorous seedlings, which consequently would have the best chance of flourishing and surviving.

My first question is, has anyone done any experiments or chemical analysis or any other kind of science to show that nectar does in fact contain something "injurious" to the sap (or to the plant in general)?

More importantly, has anyone tried to "prove" Darwin's theory of the evolution of nectar in flowers?

I understand that Darwin seems to provide some hints by mentioning the

Leguminosae, and at the backs of the leaves of the common laurel.

as examples of a plant removing toxic substances from somewhere other than a flower, but I'm wondering if someone has "proved" that nectar in flowers started as a toxic substance the plant needed to remove.

Additionally, I'd be interested in how plants that haven't developed nectar/flowers deal with this waste (other than the common laurel and Leguminosae). For example, does the plant just store the waste in a vacuole until it dies (please pardon my bad biology if "vacuole" isn't the correct organelle -- I'm not a biologist)?

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