> I have no clue why maples would produce nectar.  
> Because they are either "mostly" or "partly" insect pollinated.

The idea that every feature on a plant or animal has a definite function is simply not true. Species do not simply do this "because of" that. 

> Some highly self-pollinating plants that do not require pollinators, like Arabidopsis, have maintained functional nectaries and proven useful for studying nectary development and function]. Similarly, wind- pollinated plants with relictual nectaries do exist. It is unclear if these nectaries enhance outcrossing and genetic fitness, or if not enough time has passed for functional nectaries to be lost. 

> There are several hypotheses as to how nectaries arose, and since they likely evolved independently multiple times, several explanations may be needed. One leading hypothesis is that hydathodes, which are secretory sites along angiosperm leaf margins and epidermis involved in alleviating positive xylem pressure (guttation), may be the evolutionary precursors to some nectaries. Lending support to this hypothesis, hydathode secretions do contain relatively dilute sugars, ions, and other metabolites, which may be attractive to insects 

> An alternative view is that some nectaries, particularly ones on flowers, may have evolved from other reproductive secretions, such as stigmatic exudates and gymnosperm pollination drops. Indeed, stigmatic exudates have been observed to serve as a nectar-like reward in some cases

Stigmatic exudates! The possibilities are endless ...

Peter L Borst

Nectar biology: From molecules to ecosystems
Plant Science 262 (2017) 148–164

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