TRACHEOPHYTE

any of the vascular plants, members of the division, or phylum, Tracheophyta, numbering some 260,000 species and including all of the conspicuous flora of the Earth today. Tracheophyte, meaning tracheid plant, refers to the water-conducting cells (called tracheids, or tracheary elements) that show spiral bands like those in the walls of the tracheae, or air tubes, of insects. The division comprises a tremendous diversity of plants among its four subgroups: psilopsids, leafless and rootless primitive forms commonly known as whisk ferns (though not true ferns); sphenopsids, feathery leaved plants commonly called horsetails; lycopsids, low-lying plants called club mosses; and pteropsids, comprising the ferns, gymnosperms (pines, spruces, firs, etc.), and flowering plants. The vascular system consists of xylem (wood), concerned mainly with the conduction of water and dissolved minerals, and phloem, which functions mainly in the conduction of foods, such as sugar. Tracheophytes are believed to have originated from the green algae (Chlorophyta). The earliest fossils are from Silurian rocks more than 400,000,000 years old.