Lycophytes, including clubmosses, spike mosses, and quillworts, form an ancient lineage within the vascular plant kingdom. They represent one of the oldest groups of extant vascular plants, with a fossil record extending back over 400 million years. Lycophytes do not produce true seeds. This reproductive method distinguishes them from conifers and flowering plants, the dominant seed-bearing plant groups today. Instead, they rely on dispersal involving microscopic spores.
What Defines a Lycophyte?
Lycophytes are characterized by a unique physical structure that separates them from ferns and seed plants. Their most distinguishing feature is the microphyll, a small, scale-like leaf containing only a single, unbranched strand of vascular tissue. This contrasts with the complex megaphylls, or true leaves, found in nearly all other vascular plants. Lycophytes possess a dominant sporophyte generation, meaning the plant typically observed is the spore-producing phase of the life cycle.
The body plan of most living lycophytes, such as clubmosses, consists of dichotomously branching stems that lack a single main axis of growth. They contain true vascular tissue, allowing for the efficient transport of water and nutrients. Although modern species are generally small, herbaceous plants, their extinct relatives from the Carboniferous period grew over 40 meters tall, forming vast forests.
Reproduction by Spores
Lycophytes reproduce through an alternation of generations, where the visible sporophyte produces spores rather than seeds. Spores are created inside specialized capsules called sporangia, which are often clustered on modified leaves known as sporophylls. In many species, these sporophylls are tightly grouped at the tips of the stems, forming cone-like structures known as strobili. The spores are released and dispersed primarily by wind.
When a spore lands in a suitable, moist habitat, it germinates to develop into a small, inconspicuous, free-living organism called the gametophyte. This gametophyte is the sexual generation, responsible for producing the sperm and egg cells. Because the motile sperm must swim through a film of water to reach and fertilize the egg, lycophyte reproduction remains dependent on external moisture. Following fertilization, the resulting embryo grows directly out of the gametophyte, developing into the mature sporophyte plant.
The Evolutionary Divide: Spores vs. Seeds
The difference between a spore and a seed lies in their structure, complexity, and resource allocation. A spore is a single-celled, haploid reproductive unit containing minimal protective layers or stored food reserves. Its success relies on mass production and rapid germination upon finding suitable environmental conditions. This simple design represents an ancestral strategy among land plants.
A true seed is a multicellular structure that represents a significantly more advanced evolutionary innovation. It contains a diploid embryo, a miniature, pre-formed plant, packaged with a supply of stored food (endosperm or cotyledons). This entire package is encased in a protective, tough seed coat, which allows the seed to remain dormant for extended periods. This dormancy grants protection from desiccation and harsh conditions, enabling dispersal to drier environments and greatly increasing the probability of successful establishment.
The Near Miss: Heterospory in Advanced Lycophytes
While most clubmosses are homosporous, producing only one type of spore, advanced lycophytes like Selaginella (spike mosses) and Isoetes (quillworts) exhibit heterospory. Heterospory is the production of two distinct types of spores: small microspores that develop into male gametophytes and larger megaspores that form female gametophytes. This differentiation of spore size and sex is considered a major precursor to the seed habit.
In these heterosporous species, the female gametophyte develops entirely within the protective wall of the megaspore, a process known as endosporic development. In some species, the megaspore is retained for a period within the megasporangium on the parent plant. This retention and internal development mirrors several stages in seed formation, providing the developing embryo with immediate resources from the parent sporophyte. However, because the megasporangium lacks the complete protective integument (seed coat) and the megaspore is eventually shed, this system falls just short of the definition of a true seed.