What structural feature allows Hydrangeas to last a significant period out of water, sometimes even being revived?

Answer

Large, clustered heads acting as a moisture reservoir

Hydrangeas possess large, clustered heads which can function as a moisture reservoir, contributing to their surprising ability to last out of water for a significant period.

What structural feature allows Hydrangeas to last a significant period out of water, sometimes even being revived?
plantFlowersurvivalwater