1. What Does It Mean to Call Ferns a Close Relative of Seed Plants?
Ferns are plants that reproduce by spores rather than seeds. In Japan, bracken and ostrich ferns are familiar examples, while there are said to be about 10,000 species worldwide [2]. When we say "close relative," it is like a cousin in a family tree. However, this is just an analogy. In reality, it means comparing groups of living plants to see which group is most closely related to seed plants.
This article looks at three clues: a 2001 study that compared many plants, the swimming sperm found in the fern life cycle, and fossils with leaves resembling those of ferns. We begin with the largest clue, the study itself.
2. How Did a 2001 Study in Nature Change Scientists' Views of Ferns?
In 2001, Pryer’s research team published a paper in Nature magazine. The study divided living vascular plants (plants with water channels in roots, stems, and leaves) into three large groups: lycophytes (clubmoss relatives), seed plants, and a combined group of horsetails, whisk ferns, and ferns [1]. This paper showed that the combined group of horsetails, whisk ferns, and ferns is the closest relative to seed plants among living plants. The analysis used maximum likelihood, a statistical method for choosing the tree shape that best fits the data [1].
This result rejected the previously popular view that saw ferns and horsetails as an intermediate stage between mosses and seed plants [1]. Instead of being a halfway form toward seed plants, the group combining horsetails, whisk ferns, and ferns was a close relative branch alongside seed plants. However, the conclusion is not fixed. A 2013 review notes that while shape and molecular analysis often place ferns as the sister group (closest branch) to seed plants, there are disagreements among paleobotanists who discuss the possibility that leaves evolved separately [9]. So what do fossils tell us about this closeness?
3. What Do Fossils of Seed Ferns Show About Ferns and Seeds?
In ancient forests, there were plants that had leaves very similar to ferns but possessed seeds. In the fossil world, these are called "seed ferns." Despite the name, they are not ferns; they are seed plants [7]. According to the University of California Museum of Paleontology (UCMP), the oldest known seed plant is Elkinsia polymorpha, a type of seed fern. It was found in West Virginia, USA, from the Late Devonian period (Famennian) [7].
Medullosa from the Carboniferous period looked like modern tree ferns but was a seed plant that produced seeds. Sphenopteris also had fern-like leaves but produced seeds [7]. These fossils show that leaf shape alone cannot distinguish between ferns and seed plants. However, they do not directly prove that "seed plants were born from ferns." The fact that plants with both fern-like leaves and seeds existed in the past is a clue suggesting the closeness of ferns and seed plants. Do living plants also have clues to this closeness?
4. Where Does the Sperm Swim in a Fern's Life Cycle?
Let us trace the life of a fern. When spores from a fern body (sporophyte) land in a wet place and germinate, a small body called a prothallus forms. This prothallus is the gametophyte. The large body known as bracken or ostrich fern and the prothallus appear alternately [3].
The prothallus is usually green, flat, and heart-shaped, about 1 cm in size. On its underside, structures that produce sperm (antheridia) and eggs (archegonia) form. Sperm move through water to reach the egg and fertilize it [3]. Because water is needed for sperm to swim, prothalli grow on wet ground. The fact that large ferns are often found in damp, shady places may be connected to this. However, there is no data in this article to say for certain that this connection exists.
By the way, mosses also have swimming sperm [6]. So what about seed plants? How are their sperm transported?
5. Which Seed Plants Still Have Swimming Sperm?
Most seed plants do not have swimming sperm. Their sperm is carried inside a pollen tube that grows from the pollen, and they do not have flagella (swimming hairs) [6]. However, it is said that among living seed plants, only cycads and ginkgo have swimming sperm. The same trait as mosses and ferns remains in them [6].
In 1896, Hirase Sakugoro of the University of Tokyo discovered ginkgo sperm. This became an important evolutionary clue linking mosses and ferns (which have sperm) with flowering plants (which do not) [4]. Ikeno Seiichiro also found cycad sperm, and dictionaries state he clarified the relationship between ferns and gymnosperms. Many sources say this was in 1896, but some dictionaries say 1895 [5].
Putting the three clues together reveals something. The core of judging closeness is research (phylogenetic analysis). Fossils show that plants with fern-like leaves and seeds existed, and sperm shows that some seed plants retain swimming sperm. These are materials for considering the results of research.
6. How Can You Look for Ferns and Think About Them Yourself?
What can you do today? Look for ferns in parks or gardens. Check if they are in sun or shade, and if the ground is wet. Compare about 3 places and record your findings. Prothalli are small and hard to find, but walking around will show you what ferns live in damp places.
What can you do when you go out? Look at fern specimens or fossil displays in encyclopedias or museums. If there is something called a "seed fern," look closely at the leaf shape and seeds to verify this article's story.
If you want to learn more, the UCMP fossil page (in English) summarizes records of seed plant fossils [7]. The 2001 paper [1] is worth reading and comparing as it examined the relationship among these groups.