Why these strange aquatic ferns are more like seed plants than their forest cousins

Gardening · Published 3 October 2026 Swipe more like this →

In short: Waterferns, or Salviniales, defy the traditional image of a fern. These aquatic plants are heterosporous, producing two distinct types of spores that develop internally. This unique reproductive strategy makes them biologically closer to seed-bearing plants than to the leafy ferns found in typical woodland environments across the globe.

The order Salviniales encompasses a diverse group of aquatic and semi-aquatic plants that vary significantly in appearance. While the family Salviniaceae consists of floating, natant species, the family Marsileaceae typically remains rooted in the substrate. Despite these differences, members of this order share specific anatomical traits, such as dimorphic leaves and anastomosing veins. Many also feature aerenchyma, a specialized tissue containing air spaces that helps the plant survive in waterlogged environments.

What truly sets Salviniales apart from other ferns is their reproductive mechanism. They are heterosporous, meaning they produce both megaspores and microspores. These spores develop into female and male gametophytes, respectively, which remain endosporic. Because these gametophytes never grow outside the spore wall, they are physically restricted by the size of the spore itself. This evolutionary path mirrors that of seed plants, marking a distinct departure from the reproductive habits of other fern lineages.

Beyond their unique biology, these plants have shaped the history of the planet. The genus Azolla is recognized as one of the fastest-growing plants on Earth. Its rapid proliferation is linked to the Azolla event approximately 50 million years ago, a period during which the plant's massive growth is credited with cooling the global climate. Furthermore, these ferns possess the smallest genomes of all known fern species, a genetic trait that continues to intrigue botanists studying their evolutionary development.

The classification of these plants has evolved through various phylogenetic studies. While early systems placed them within the Polypodiopsida, modern consensus—including the 2016 PPG I classification—places the order within the subclass Polypodiidae. Today, the order is recognized as containing two primary families, Salviniaceae and Marsileaceae, alongside the fossil family Hydropteridaceae, which includes the well-known extinct member Hydropteris. This group remains a fascinating subject for understanding how aquatic adaptations influence plant evolution.

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