Approximately 93 percent of all plants are vascular plants--which means they have the ability to circulate material from one part of the plant body to another. Specialized tissue in the plant, called the xylem and phloem, work similarly to the veins and arteries in our body.
Most vascular plants produce seeds, but the ferns, liverworts, club mosses and horsetails are seedless vascular plants. They all share characteristics that identify them as members of the same group.
Roots, Stem and Leaves
Seedless vascular plants are differentiated from seedless, non-vascular plants by the presence of true roots, stems and leaves. In order to have these complex structures, the plant needs a way to supply them with nutrients, minerals and water. The xylem carries nutrients and water from the roots to other regions of the plant. Their rigid cell walls give the plant structure. The phloem carries nutrients from one part of the plant to the other.
With the presence of a vascular system, the plants are able to grow larger and more complex than they could without. Roots, stems and leaves are possible with a vascular system.
Seedless vascular plants have a reproductive cycle that alternates between two forms with each generation. The gametophyte generation is haploid, having only one copy of the plant's genetic material. Since they only have one copy of cells, this is the generation that reproduces sexually by joining male sperm and female egg cells.
The sporophyte generation dominates the reproductive cycle--this is what we see when we are looking at a fern, for instance. It occurs when two haploids cells join during sexual reproduction, creating a diploid embryo that has two copies of genetic material. The plant embryo then is encased in a spore that is dispersed by the wind. When spores land in a suitable environment, they start a new plant.
The gametophye and the sporophyte generations are always separate in seedless vascular plants.
All seedless vascular plants reproduce through spores. On ferns, for example, the spores form small cases called sori, which are generally present on the underside of the leaves.
The spores that form in the sporophyte generation are haploid, or contain only a single copy of genetic material. Once these haploid spores find a moist environment, they sprout into small, heart-shaped plants where fertilization occurs. This small plant produces one egg and thousands of sperm. To prevent self-fertilization, the male and female cells of the same plant mature at different times.
Sperm cells have flagella to propel them through water to female cells on a different plant. After cell fertilization, the haploid plant grows a stalk that holds the diploid sporophyte spores--the blueprint for a new plant.
Near the end of the Paleolithic era, the seedless vascular plants were tree-sized. They were so abundant that they could not decompose fast enough. The excess plant waste accumulated and, over time, became the fossil fuels we use today.