Seedless vascular plants, also known as tracheophytes, are characterized by a dominant sporophyte life cycle, with relatively small gametophytes that are not microscopic. These plants primarily reproduce through spores rather than seeds or pollen, and their sperm requires water to reach the egg, similar to nonvascular plants. This dependency on water limits their habitat range.
Seedless vascular plants are divided into two main categories: lycophytes and monilophytes. Lycophytes, which include clubmosses, spikemosses, and quillworts, are unique for having microphylls—small leaves with a single vascular tissue strand. Monilophytes encompass ferns and horsetails, with ferns being a prominent example. Ferns possess sporophylls, modified leaves that bear clusters of sporangia called sori on their undersides, which are crucial for spore production.
These plants have played a significant role in Earth's history by removing carbon dioxide (CO2) from the atmosphere, contributing to global cooling and the formation of glaciers. The carbon fixation process also led to the creation of coal, highlighting an ironic cycle where the burning of coal today releases CO2 back into the atmosphere, contributing to current global warming concerns.
The life cycle of ferns illustrates the transition from sporophyte to gametophyte. The sporophyte, which is the familiar fern plant, produces spores in sporangia. These spores develop into gametophytes, which contain reproductive structures: the antheridium (male) and archegonium (female). While the sporophyte has true roots, the gametophyte relies on rhizoids for anchorage, as it is not the dominant stage of the life cycle. The sperm swims to the archegonium to fertilize the egg, resulting in the formation of a new sporophyte, thus continuing the cycle.