Seedless Vascular Plants

If you were to travel back in time 300 million years, you wouldn't be standing in any kind of forest you've ever seen before. In fact, the forests of the Carboniferous period were dominated by nonvascular plants and early vascular plants, known as the seedless vascular plants (e.g., ferns, clubmosses, and more).

We still find these seedless vascular plants today, but now they are overshadowed by their seed-producing counterparts (e.g., conifers, flowering plants, etc.). Unlike their seed-producing counterparts, seedless vascular plants do not produce seeds, but rather have an independent gametophyte generation through the production of spores.

Unlike nonvascular plants, however, seedless vascular plants contain a vascular system that supports them in the transport of water, food, and minerals.

What are seedless vascular plants?

Seedless vascular plants were the early vascular plants, predating the gymnosperms and angiosperms. They were the dominant species in ancient forests, consisting of nonvascular mosses and seedless ferns, horsetails, and club mosses.

Seedless vascular plants: common names and examples

Seedless vascular plants are mainly split into two groups, the lycophytes and the monilophytes. These aren’t common names, however, and might be a little confusing to remember. Below we go over what each of these names means and some examples of seedless vascular plants.

The lycophytes

The lycophytes represent the quillworts, spike mosses, and club mosses. Although these have the word “moss” in them, these are actually not true nonvascular mosses, because they have vascular systems. The lycophytes differ from the monilophytes in that their leaf-like structures are called “microphylls”, meaning “small leaf” in Greek. The “microphylls” are not considered true leaves because they only have a single vein of vascular tissue and the veins are not branched like the “true leaves” that monilophytes have.

Club mosses have cone-like structures called strobili where they produce the spores that will become haploid gametophytes. The quillworts and silver mosses do not have strobili, but instead have spores on their “microphylls”.

The monilophytes

The monilophytes are separated from the lycophytes because they have “euphylls” or true leaves, the plant parts we particularly think of as leaves today. These “euphylls” are broad and have multiple veins running through them. The common names you may recognize of plants in this group are the ferns and the horsetails.

Ferns have broad leaves and spore-bearing structures called sori located underneath their leaves.

Horsetails have “euphylls”, or true leaves that have been reduced, meaning they are thin and not broad like fern leaves. Horsetail leaves are arranged at points on the stem in a “whorl” or circle.

Still, the common factor linking the club mosses, spike mosses, quillworts, ferns, and horsetails is that they all predate the evolution of the seed. These lineages instead disperse their gametophyte generation by means of spores.

The life cycle of seedless vascular plants

The seedless vascular plants go through an alternation of generations just as the nonvascular plants and other vascular plants do. The diploid sporophyte, however, is the more prevalent, noticeable generation. Both the diploid sporophyte and haploid gametophyte are independent of each other in the seedless vascular plant.

Fern life cycle

The life cycle of a fern, for example, follows these steps.

1. The mature haploid gametophyte stage has both male and female sex organs- or antheridium and archegonium, respectively.

2. The antheridium and archegonium both produce sperm and eggs via mitosis, as they are already haploid.

3. The sperm must swim from the antheridium to the archegonium to fertilize the egg, meaning the fern depends on water for fertilization.

4. Once fertilization happens, the zygote will grow into the independent diploid sporophyte.

5. The diploid sporophyte has sporangia, which is where the spores are produced via meiosis.

6. On the fern, the underside of the leaves have clusters known as sori, which are groups of sporangia. The sori will release spores when they mature, and the cycle will restart.

Notice that in the fern life cycle, although the gametophyte is reduced and the sporophyte is more prevalent, the sperm still relies on water to reach the egg in the archegonium. This means that ferns and other seedless vascular plants must live in damp environments to reproduce.

Homospory versus heterospory

Most seedless vascular plants are homosporous, which means they produce only one type of spore, and that spore will grow into a gametophyte that has both male and female sex organs. However, some are heterosporous, which means they make two different kinds of spores: megaspores and microspores. The megaspores become a gametophyte bearing only female sex organs. Microspores develop into a male gametophyte with only male sex organs.

Although heterospory is not common in all seedless vascular plants, it is common in seed-producing vascular plants. Evolutionary biologists believe the adaptation of heterospory in the seedless vascular plants was an important step in the evolution and diversification of plants, as many seed-producing plants contain this adaptation.