Chapter 17: Seedless Vascular Plants

Seedless vascular plants represent a crucial evolutionary transition that established the foundation for plant colonization of terrestrial environments and shaped Earth's ecosystems for hundreds of millions of years. This chapter examines how the evolution of lignified conducting tissues—xylem for water transport and phloem for nutrient distribution—enabled plants to achieve greater height, structural complexity, and independence from aquatic habitats. The emergence of apical meristems driving primary growth and the subsequent development of secondary growth through cambial activity allowed these organisms to expand their ecological range and diversify into multiple lineages. Key morphological innovations included the differentiation of true roots, stems, and leaves, with the distinction between microphylls—simple, single-veined structures found in lycophytes—and megaphylls—complex, multiply-veined leaves characteristic of ferns and later plant groups. The organization of vascular tissues into different stele types—protosteles, siphonosteles, and eusteles—reflects evolutionary refinements in conducting tissue arrangement that influenced overall plant architecture and function. Reproduction in these plants maintained dependence on water for fertilization, as motile sperm required aqueous media for successful gamete fusion, yet the shift toward heterospory in certain lineages, producing distinct microspores and megaspores, foreshadowed the reproductive strategies of seed plants. The fossil record reveals an array of early vascular pioneers, including the simple, dichotomously branched Rhyniophyta with terminal sporangia, the laterally-sporangiate Zosterophyllophyta likely ancestral to lycophytes, and the more elaborately branched Trimerophytophyta that gave rise to modern ferns. Living seedless vascular plants divide into two major phyla: Lycopodiophyta, encompassing club mosses, spike mosses, and quillworts with characteristic microphylls, and Monilophyta, containing the diverse ferns, horsetails, and aquatic water ferns. During the Carboniferous period, giant tree-form lycophytes such as Lepidodendron and towering calamites dominated extensive swamp forests, accumulating vast deposits of organic material that became coal—a testament to the ecological dominance of seedless vascular plants and their profound influence on the global carbon cycle before seed plants achieved prominence.