Good question, I haven't had the chance to type up a post on the different plant tissues. Hopefully soon.

To answer your question, I'm guessing you are referring to the structure that seems to take up most of the space. They are 'simple tissue' or 'ground tissue', the type you see there taking up most of the space is parenchyma. They help the plant repair and store nutrients. If you happen to see dots inside them, they are starch granules. Closer to the edge are sclerenchym and even fibre caps or phloem fibre.