Ok well first of all if you are looking for a simple answer
don't look here!!
A plant is made up of three different kinds of tissue systems,
dermal, ground, and vascular. Ground tissue is surrounded by the
dermal tissue, which makes up the outer layers of a plant, and
ground tissue is what makes up a majority of the inside of a plant.
Ground tissue will provide support for the plant and it also stores
materials in the roots and stems. Ground tissue in leaves are
packed with chloroplasts, which is where the photosynthesis process
makes nutrients for the plant.
The ground tissue system consists of all three of the simple
tissues: parenchyma, collenchyma, and sclerenchyma tissue.
Parenchyma is the most common tissue of the three and in cacti, the
ground tissue contains numerous parenchyma cells which store water.
Although those parenchyma cells store water, rigid parenchyma
actually make up most of ground tissue in cacti spines.