What is the term for undifferentiated cells between vascular bundles, extending from the central pith core?

The term that describes the undifferentiated cells located between vascular bundles and extending from the central pith core is known as pith ray. Pith rays are composed of parenchyma cells that are part of the plant's tissue system, which helps in the storage and transport of water and nutrients. They play a significant role in connecting the pith to the vascular tissues (xylem and phloem) and contribute to the overall functionality of the plant's internal structure.

In contrast, annual rings refer to the growth rings in the wood of a tree, reflecting the growth patterns over each year. Stolons are horizontal stems that grow along the ground, capable of forming new plants at nodes. Tendrils are specialized structures that aid in climbing and support for plants. While these other terms pertain to different structures or functions within plant biology, pith rays specifically address the undifferentiated cells in the area discussed in the question.