Difference between parenchyma ,sclenchyma ,collenchyma

Parenchyma tissue consists of relatively unspecialised cells with thin cell walls. They are live cells. They are usually loosely packed, so that large spaces between them.  This tissue provides support to plants and also stores food. In some cases, it contains chlorophyll and performs photosynthesis, and then it is called as chlorenchyma. In aquatic plants, large air cavities are present in parenchyma to give buoyancy to the plants to help them float. Such a parenchyma type is called aerenchyma. The parenchyma of stems and roots also stores nutrients and water.

The cells of collenchyma tissue are living, elongated and irregularly thickened at the corners. There is very little or no intercellular space. It allows easy bending in various parts of a plant (leaf, stem) without breaking. Thus, provides flexibility to the plant. It also provides mechanical support to plants.  

The cells of sclerenchyma tissue are dead. They are long and narrow as the walls are thickened due to deposition of lignin. The walls of cells are so thick that there is no internal space inside the cell. This tissue is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts. It provides strength to the plant parts and makes the plant hard and stiff.