The three plant structures for storing water are stems, leaves and roots. Modified plant structures are called tubers.
An example of a modified plant structure for a stem is a potato. We call a potato a stem tuber. The stem has been exaggerated to allow for holding more nutrients.
A root tuber is very similar to a stem tuber. An example of a root tuber is a sweet potato. It is a storage root. A bulb is also a modified stem.
A tendril is an example of a modified stem. Tendrils are used by plants to climb. For example, a plant may affix itself to a building or tree and climb it.