Some parenchyma cells in stems and roots have colourless plastids that store starch. Collenchyma cells have unevenly thickened primary cell walls and are grouped into strands or cylinders. They lack a secondary cell wall. In comparison to sclerenchyma collenchyma cells are living and elongate with the stems and leaves they support. Collenchyma cells provide support to growing plant parts.
Sclerenchyma cells have thick secondary walls strengthened by lignin.
Mature cells cannot elongate and occur in regions of the plant that have stopped growing in length. There are two types of sclerenchyma cells fibres and sclerids . Fibres are long, slender and tapered usually occurring in bundles. Some plant fibres can be used commercially eg. Sclerids are shorter and irregular in shape.