...survive in the hydric conditions? I would really love some references for this!

Stagnant or standing water can keep the plant's roots from getting enough oxygen. Roots need oxygen to carry on cellular respiration to get energy to use.

I agree with the previous answerer that some plants have aerial roots that stick up into the air to get oxygen. Mangroves have these aerial roots and cypress trees have cypress "knees" for this purpose.

See cypress knees here:
http://images.google.com/images?client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hl=en&q=cypress+knees&btnG=Search+Im ages&gbv=2

Mangrove swamp trees have aerial breathing roots that protrude above the saturated silt.