Our Smallest Native Plant Floating Commuinities

Thursday, March 24, 2016

Our Smallest Native Plant Floating Commuinities
1:  The Lemnaceae

by Bob Thomas

Louisiana is blessed with wonderful biodiversity at all levels.  We tend to move through our wetlands and focus on the largest species, but by simply stopping, kneeling by the water, and looking closely at what might be floating on its surface, a whole new botanical world opens.

Of course, I'm speaking of the Lemnaceae, a family of plants that are generally tiny, yet they have all the features of flowering plants and are modified for floating.  Being minute they may disappear among densely growing large aquatic plants.

Back at the beginning of my career as a biologist, I was fortunate to have a professor, the late Dr. John Thieret, and fellow student, the late Dr. Howard Clark, who were experts on the Lemnaceae.  Howard's master's thesis was The Lemnaceae of Louisiana at then University of Southwestern Louisiana (now The University of Louisiana-Lafayette).  The fact that I was often in the field with these two botanists required that I get excited about these floating miniatures, and their enthusiasm guaranteed that even I would occasionally abandon my pursuit of amphibians and reptiles in deference to itsy-bitsy plants.

The genera of the Lemnaceae are rather easy to identify, but the species can be confounding.  Their anatomy consists of one or more small leaf-like fronds, or thalluses.

As a group, they are simply called duckweed.  That said, local lore provides a bit of diversity.   My friend, Gary Uhl, calls duckweed "peanut grass."  Botanist Charles Allen mentioned that some in our state call duckweed coverage of water "Louisiana snow."  He also has heard people call watermeal (Wolffia, see below) "grits," which is descriptive.  Jerald Horst, an extant naturalist of the highest order, told me that many in his circle call duckweed "water lily seeds," thinking they are the seeds of water hyacinths.

There are five genera, and despite all the local names, each genus has its own official common name:
Lemna - duckweed - one root per frond
Spirodela - duckmeat - two roots per frond
Landoltia - dotted duckmeat - two to five roots per frond
Wolffiella - mud midget - flat, often spiraled, without roots
Wolffia - watermeal - round, oval, or elliptical, without roots; the smallest form

Since they are so small, duckweed prefers water that is either still or slowly flowing in ponds, lakes, canals, and bayous.  All float at or near the surface, taking advantage of air sacs called aerenchyma.  They may be found mixed in with other plants in otherwise open water, or they may reproduce rapidly and cover the surface, with one or a combination of species and genera.  They typically are one plant thick on the surface, but winds may push them into thick piles at pond margins.  Eventually, these will either sink or spread back out to a thin layer.  They may live on moist soils if forced off the water surface by a boat wake or storm.

Lemnaceae are flowering plants.  In fact, Wolffia is the smallest flowering plant in the world and produces the smallest fruit.  They have simple inflorescences which extend from a cavity in the surface of the frond and consist of one or two staminate flowers and one pistillate flower.  They rarely set seeds for some reason.  When they do, each fruit, depending on the species, contains 1-5 seeds.  The seeds have no flotation, so they sink to the bottom.

Typically, they reproduce asexually.  The "mother" frond begins to bud from a meristematic-zone (a place in plants where undifferentiated cells are found and growth takes place).  The "daughter" frond exits laterally from a pouch and remains appended to the mother by a connecting stype until it is mature, at which time it breaks away.  By this time, the former daughter has probably already produced its own daughter fronds.

This process is rapid.  Various species can double their coverage of habitat every two days.  One species of Wolffia in India can bud a new daughter frond every 30 hours, meaning that one mature frond can, in four months time, produce offspring (with each offspring producing offspring) that are equal in volume to that of the earth!  Thankfully, this hasn't happened.

Each species can produce only a certain number of daughter fronds before becoming senescent.  These are easy to recognize because they yellow and appear desiccated.

A number of species produce turions.  These starch-filled, rootless morphs function in overwintering by sinking to the bottom and lodging in mud.  I'm not sure how common this is in our region, since it is normal to find various duckweed species on the surface in all types of winter weather.

Duckweed is eaten by many organisms that share their aquatic habitats.  Since they float, they don't need or have much fiber, and they are rich in nutrients, including amino acids that are animal-like.

In fact, it is always fun to find animals that live in duckweed-covered habitats that are densely coated with species of the Lemnaceae, especially alligators and turtles.

And speaking of being rich in nutrients, Nature Note reader Isabelle Cossart, proprietor of Isabelle's Orange Orchard in lower coast Algiers in New Orleans, shared that the only fertilizer she uses in her orchard of 550 trees is duckweed, which she harvests from a moat that surrounds her home.   That is a wonderful new approach to sustainability!

There are a number of nice duckweed sources of information available on the internet, but Wayne Armstrong's Treatment of the Lemnaceae and The Charms of Duckweed are excellent with many nice macrophotos.

If you would like to have a digital copy of The Lemnaceae of Louisiana (which was never published) by Howard Clark, let me know.

 

 

 

Alligators covered in duckweed at the Audubon Zoo.
Photo by Bob Thomas

Senescent (dying) duckweed, showing the yellowing that indicates it has budded asexually many times.  Near Willowdale, St. Charles Parish.
Photo by Bob Thomas

Mud midget, Wolffiella sp., at Shell Bank Bayou east of LaPlace.  The slender leaf-like fronds are interspersed with a few Lemna.  Collected by Tom Kane.
Photo by Shannon Fortenberry

Large Lemna surrounded by much smaller watermeal, Wolffia, in Manchac, Louisiana.
Photo by Bob Thomas

 

Lemna minor at the Louisiana Nature Center.  This is one of the most common species of duckweed in New Orleans.
Photo by Bob Thomas

Lemna in small mounds due to wind, with thin layers interspersed In Jean Lafitte National Historical Park & Preserve, Barataria Unit.
Photo by Bob Thomas