This document discusses the structure of simple and complex tissues. Simple tissues are composed of a single cell type with a common origin and function. There are three types of simple tissues: parenchyma, collenchyma, and sclerenchyma. Parenchyma cells have thin cell walls, are living with large central vacuoles, and have intercellular spaces. They provide support, store food, and facilitate gas exchange. Collenchyma cells are elongated and living with small intercellular spaces. Their cell walls contain cellulose and pectin. Collenchyma provides flexibility to plant parts without breaking.