Till now we studied that all the organisms are made of cells.
On the basis of cell they are divided into unicellular (single-celled) and/or multicellular (two or more celled).
Example-Amoeba (unicellular), in case of amoeba all the functions are carried out by the single cell, whether it is digestion, excretion, respiration, but in case of human beings or plants (multicellular), all these functions are assigned to different cells rather different tissues or organs or organ systems.
Now the question here arises “How the cells form multicellular organisms? How they are connected to each others? What are the tissues? How they are formed? etc.
So, the answer is that the cells of multicellular organisms grow and divide and form small groups of cells, these groups of cell combine and specialize each other for specific function of the body. These groups or clusters of cells are called tissues.
Similarly, tissues form groups and combine to form organs and organs then form organ systems.