Companion cells are characteristic element of phloem tissue associated with the sieve tubes in the angiosperms. Thus, they are the part of vascular tissue system. They are absent in pteridophytes and gymnosperms. Guard cells and subsidiary cells are associated with ...

Sclerenchyma tissues are long and narrow as the walls are thickened due to lignin (a chemical substance which acts as cement and hardens them).

Aerenchyma is a special tissue found in hydrophytes (e.g. Hydrilla, Potamogeton, etc.) and some land plants (e.g. petiole of banana, Canna), formed by parenchyma cells. They make a network by leaving wide air spaces (air cavities) for the gaseous exchange ...

Mixture is constituted by more than one substance in any ratio. For example, water, soil, polluted air, etc.

A pure substances is one which is made up of only one kind of atoms or molecules. They have the same composition throughout.