What part of the plant is primarily responsible for storage?

The part of the plant primarily responsible for storage is the tuber. Tubers are specialized swollen structures that function as storage organs for nutrients, particularly starches and other carbohydrates. They develop from underground stems and serve as energy reserves that can be utilized by the plant during periods of growth or when environmental conditions are not favorable.

In many cases, tubers can also help in vegetative reproduction, as they can give rise to new plants. This storage capability is crucial for the plant's survival, especially in perennial species that need to endure adverse conditions like winter or drought.

The other options, while related to plant structures, do not primarily function in storage. Tendons are not a plant structure; rather, they connect muscle to bone in animals. Stolons are horizontal stems that grow along the soil surface and help in asexual reproduction. Pith rays, found in the stem of a plant, facilitate the transport of nutrients and water but are not primarily storage tissues.