This division includes club mosses, horsetails and ferns. The oldest known pteridophytes are fossils from the end of the silurian period, 380 million years ago. Pteridophyta constitutes the earliest known vascular plants. Vascular plants are those plants that contain the vascular tissue that is the conducting tissues of xylem and phloem. Sometimes all vascular plants are included in one division the Tracheophyta . This is to emphasise the advance nature of vascular tissue over the simple conducting cells of some Bryophytes and Algae. Tracheophyta includes pteridophytes and the more advanced spermatophytes (seed bearing plants) as two subdivisions.
Presence of vascular tissue is a feature of the sporophyte generation, which in the bryophytes is small and dependent on the gametophyte. The occurrence of vascular tissue in the the sporophyte is one reason why sporophyte generation has become the dominant one in all vascular plants. The vascular tissue of pteridophytes shows certain primitive features compared with flowering plants. The xylem of pteridophytes contains only tracheids rather than vessels and the phloem contains sieve cells rather than sieve tubes.
Vascular tissue has two important roles to perform. Firstly it forms a transport system, conducting water and food around the multi- cellular body, thus leading to the development of large, complex bodies. Secondly, xylem, one of the vascular tissues, supports these large bodies since xylem contains lignified cells of great strength and rigidity.