The first electron source for plants was H2S, but now most modern plants use H2O as an electron source.
What is the advantage of using H2O instead of H2S?
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The major reason for this is because H20 can cause hydrogen bonding. Hydrogen bonding is what allows for plants to transport "this electron source" from their roots through their stems an further.
The stronger polarity of H20 also allows for stronger interactions. Such as ionic interactions (not fully bound but 2 polar atoms interacting). This kind of ionic interaction also allows for other types of material interactions such as cohesion and adhesion.
Overall, aside from the discussion of abundance as mentioned above (which is a very good point), it also allowed the plants to grow in directions that were more advantageous (darwinian) for their survival.