These cell types are all found in ground tissue which forms most of the bulk of a plant. Parenchyma are the least specialized. They have thin walls, numerous plastids including chloroplasts, and can revert to meristem in response to injury. Collenchyma are elongated cells with thick walls. They provide flexible support. Sclerenchyma have very thick walls and are dead at maturity. They may form long fibers, as in flax, or be short as in nut shells and the gritty material in pear fruits.