Lycophytes are an ancient lineage of spore-producing plants that include the club mosses, spike mosses, and quillworts. These plants are often mistaken for true mosses, which are non-vascular, due to their small stature and common names. Lycophytes do, in fact, possess the complex plumbing necessary for life on land, setting them apart from simpler, non-vascular plant groups.
The Presence and Function of Vascular Tissue
Lycophytes are classified as tracheophytes, meaning they belong to the group of plants that have true vascular tissue. This tissue is a defining feature that allowed early plant life to grow vertically and colonize terrestrial environments more effectively than their non-vascular ancestors. It functions as an internal circulatory system, facilitating the efficient movement of resources throughout the plant body.
The vascular system consists of two components: xylem and phloem. Xylem is composed of tough, lignified cells that conduct water and dissolved minerals upward from the roots. Phloem tissue transports sugars and other organic nutrients produced during photosynthesis to non-photosynthetic parts of the plant. The development of these tissues gave lycophytes the structural integrity and resource-delivery mechanism needed to reach larger sizes. Extinct species in this group, such as the scale trees of the Carboniferous period, grew to heights exceeding 35 meters, a feat only possible with a robust vascular system.
Distinguishing Structural Characteristics
Although lycophytes possess true vascular tissue, its arrangement is distinct from that in more recently evolved plants like ferns and flowering plants. A key identifier for this group is the presence of microphylls, which are small leaves characterized by a single, unbranched strand of vascular tissue. This simple vein structure differentiates them from the complex, multi-veined megaphylls found in most other vascular plants.
The microphyll’s vascular strand is connected to the stem’s central cylinder without leaving a gap in the vascular tissue. This structure is thought to reflect a separate evolutionary origin, arising from simple outgrowths on the stem rather than from the flattening of entire branch systems. Furthermore, the arrangement of the vascular tissue within the lycophyte stem is typically a protostele.
A protostele is the most primitive type of vascular cylinder, characterized by a solid core of xylem tissue surrounded by a layer of phloem. Unlike the stems of many other vascular plants, the lycophyte protostele lacks a central core of ground tissue called pith. Variations of this solid core, such as the star-shaped actinostele or the plate-like plectostele, are commonly observed in club mosses. These specific structural features, including the microphylls and the protostele, mark lycophytes as one of the oldest extant lineages of vascular plants.