Seedless vascular plants have a waxy cuticle, stomata, and well-developed vascular tissue. Their vasculature allows them to grow to larger sizes than the nonvascular plants, but they still mostly occupy moist habitats. While this lineage is better adapted to drier habitats than are the nonvascular plants, they still require moisture for reproduction. Although the developing diploid embryo is dependent on the haploid gametophyte for survival (like mosses), the diploid sporophyte is more conspicuous and is the prominent generation of seedless vascular plants. Phylogenetically, seedless vascular plants are basal to the seed plants. The seedless vascular plants include species such as ferns and horsetails.
Like all plants, vascular plants have a gametophytic generation and a sporophytic generation. Recall, the sporophytic generation is the diploid part of the life cycle and produces haploid spores through meiosis. Remember that the moss life cycle is characterized by two types of haploid spores, male and female. We call this condition heterosporous ("hetero" meaning different and "sporous" referring to the spores). In this case, the sporophyte produces (via meiosis) megaspores and microspores. Haploid megaspores develop into haploid female gametophytes, which then produce eggs. Likewise, haploid microspores develop into male gametophytes, which then produce sperm. Haploid gametes then join to form sporophytes.
In seedless vascular plants, both the heterosporous condition described above and the homosporous condition ("homo" meaning same) result in a single type of spore that develops into bisexual gametophytes. The fern life cycle figure, which can be viewed on the next page, depicts this condition. Bisexual gametophytes can produce both male and female gametes (sperm and eggs). Note, sperm and eggs are still separate and must join during fertilization, just as in the heterosporous condition.
Plants II Part 1 VoiceThread Transcript
When one compares the life cycle of a moss to that of a fern (a seedless vascular plant), the most notable difference is the relative sizes of the sporophyte and gametophyte. In mosses the haploid gametophyte is the dominant generation, whereas in ferns the diploid sporophyte is the dominant generation.
What was life like when the early plants were colonizing land? Remember, one of the advantages of moving onto land was the new abundance of light energy that plants could access. However, light can damage DNA and induce mutations. Recall, most mutations are deleterious, and haploid organisms that suffer a lethal mutation have no wild-type copy to "rescue" them from lethal mutations. For this reason, the haploid stage is more sensitive to genetic insult than is the diploid stage. Likely, the transition from a prominent haploid stage to a prominent diploid stage was adaptive for the relatively high mutation rate suffered by terrestrial plants.
Take another look at the moss and fern life cycles. Both have flagellated sperm. This means that they are both dependent on water for fertilization. Mosses are already very small and low to the moist ground, but ferns have vascular tissue and are much taller. This could be another reason for the dominance of the sporophyte. Also, another major difference is that the sporophyte and gametophyte live independently for part of the life cycle. In the case of the fern, the gametophyte is photosynthetic, much smaller and lower to the ground, where moisture is more available for fertilization.
The other similarity between mosses and ferns is that both have antheridia and archegonia. Recall from the Plants I tutorial, these structures are the specialized gametophytic tissue where gametes are produced. The archegonium is also where the egg is fertilized once the sperm from the antheridium swims through water to reach it.
The seedless vascular plants can be divided into three groups: Club mosses (Lycophyta), Horsetails (Sphenophyta), and Ferns (Pterophyta). Lycophytes appeared during the Devonian period but split into two lineages during the Carboniferous period. One lineage became the tall extinct trees that thrived some 300 million years ago, and a good portion of the carbon they fixed was fossilized and is now burned as coal. The other lineage of lycophytes are small nonwoody plants. These extant lycophytes are usually found in either temperate forest floors or tropical areas. One species, Lycopodium , can be found in the forests around Pennsylvania.
Except for one existing genus (Equisetum ), the group whose members are commonly called horsetails is also extinct. Equisetum occurs in damp locations and is an example of a homosporous plant.
The third group of seedless vascular plants is probably the most familiar. These are the ferns or pterophytes. Most of us have seen ferns growing on a forest floor or as cut fronds in a flower arrangement. There are about 12,000 species of ferns in existence today, and they are found in tropical and temperate regions.
While the vasculature of seedless vascular plants has allowed them to grow to larger sizes than nonvascular plants, they still usually occupy moist habitats.