Hydrostachyaceae: Enigmatic Rheophytes

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Today I would like to introduce you to an enigmatic family of aquatic angiosperms called Hydrostachyaceae. Though they kind of look like strange aquatic ferns or perhaps even lycopods, they are actually strange flowering plants. To find them, you need to hang out around waterfalls and rapids in either Madagascar or southern Africa.

Hydrostachyaceae is made up of roughly 22 species. This is a poorly understood group of plants and there is always a chance that more species await discovery. The various members of Hydrostachyaceae all take on a similar appearance. For much of the year they exist as a set of feathery, fern-like leaves that grow surprisingly large and look quite delicate, especially considering the types of habitats in which they grow.

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Their delicate appearance is deceptive. In fact, the feathery structure of their leaves is an adaptation to the waters in which they grow. These are plants that require fast moving, clean, fresh water. If they were to produce flat, unbroken leaves, the fast currents would quickly rip them to shreds. By producing long, feathery leaves, water simply flows right over them with minimal disturbance. However, their preferred habitats also make them extremely difficult to study. Hence we know very little about their ecology.

What we do know about these plants is that they need clean rock surfaces and clear water for germination and subsequent growth. Dump too much sediment in the stream and you can kiss these plants goodbye. When they dry season approaches and water levels begin to drop, these oddball plants go into flowering mode. To the best of my knowledge, nearly all members of this family are dioecious, meaning individual plants are either male or female. When it comes time to flower, each plant produces modest sized spikes densely packed with flower.

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The spikes themselves sit up and above the water line, which is how this family and genus got its name. Hydrostachys is Greek and roughly translates to “water spike.” I have not been able to track down any solid information on what might be pollinating these blooms, however, given their small, dense nature, and the extreme places in which they live, my bet would be on wind.

The ecology of Hydrostachyaceae isn’t the only mystery about these plants. Their position on the tree of life has also been cause for confusion ever since they were discovered. Morphologically speaking, aquatic angiosperms can offer a lot of confusion to taxonomists. Like whales, the ancestors of aquatic angiosperms lived out their lives on land. Making the move back into water comes with a lot of extremely specialized adaptations that can cloud our morphological interpretations of things.

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Some authors have put forth the idea that these plants belong to another family of highly derived aquatic angiosperms - the Podostemaceae. However, genetic analyses paint a much different story. When the Angiosperm Phylogeny Group got a hold of specimens, their molecular work suggested the Hydrostachyaceae were nestled in Cornales, somewhere near the Hydrangea family (Hydrangeaceae). Exactly where Hydrostachyaceae fits into this new classification is still up for debate but it just goes to show you how messy things can get when plant lineages return to water.

Sadly, like so many other plants, the various members of Hydrostachyaceae are under a lot of pressure in the wild. Basically anything that threatens the quality of streams and rivers is a threat to the ongoing survival of these species. Runoff pouring into water ways from agriculture and mining cloud up the water and bury available germination sites under layers of sediment. Things only get worse when hydroelectric projects are installed. The fate of these plants is unequivocally tied to water quality.

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Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3] [4]