Tissues are group of cells which are similar in function , origin and shape. Every organism remains different in many ways such as the function they have to peform . Like plants are autotrophic they dont need muscles rather they need supportive tissue and same with animals they are heterotrophic and so they have to make their food . They are not capable of making their own food. Humans dont need phloem or xylem because they are not a requirement . Tissue are diffrent to help each organism differently.