In multi-cellular organisms, the same specialised cells form a tissue.
For example, animals have epithelial cells that make a tissue which covers the linings inside the body, and muscle cells also make a muscle tissue. A plant also has many specialised tissue in the leaf like mesophyll tissue and epidermal tissue.
Groups of different tissues make up an organ. For example, a stomach has three layers of muscular tissue, the inner wallhas glandular tissue and epithelial tissue lines the outside.
Several different organs make up an organ system. For example, the digestive system contains the salivary gland, the stomach, the pancreas and the small and large intestine all for the breaking down of food.
Plants also have organs, one is a leaf with the specific function to absorb light for photosynthesis. Another is the root designed to absorb water and minerals. The last one is the stem which holds the leaves at a good position to absorb sunlight.
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