Thallophyta are the organisms among plants as they do not possess well differentiated roots, stems and leaves.
Vascular tissues are absent for conduction of food and water.
Their reproductive structures are inconspicuous.
Few examples of thallophyta are Ulothrix, Volvox, Chalmydomonas.
Final Answer-
Thallophyta is the simplest division among simplest organisms in plants, they do not possess well differentiated root stem and leaves.