Answer is ground tissue.
Potato tuber is a modified underground stem. It stores food and becomes swollen. Histologically, potato stem is made of ground tissue. Ground tissue is composed of parenchyma cells. These cells store food and nutrients. Besides parenchyma cells it may also contain collenchyma and sclerenchyma cells that help support the stem. The ground tissue towards the interior is called pith, while the layer of tissue between the vascular tissue and the epidermis is the cortex.