What defines complex tissue structure in multicellular organisms?

Complex tissue structure in multicellular organisms is primarily defined by the presence of many differentiated cells that perform varying functions. In such organisms, different types of cells can be specialized for particular tasks, contributing to the overall functionality and efficiency of the tissue. For instance, in plants, vascular tissue includes xylem and phloem, which have distinct roles in transporting water and nutrients, respectively. In animals, muscle, nerve, and epithelial tissues demonstrate specialization that allows them to perform unique and essential functions.

Each type of differentiated cell contributes to the diverse and complex functions that tissues must carry out in multicellular organisms, allowing for greater adaptability and efficiency in response to environmental challenges. This specialization is key to the overall health and functioning of the organism, highlighting why many differentiated cells performing varying functions is a hallmark of complex tissue structure.