Tiny Plants: Draba verna

Draba verna is a small but memorable plant. Common names for it include early whitlowgrass, vernal whitlowgrass, and spring whitlow-mustard. Sometimes it is simply referred to as spring draba. As these common names suggest, Draba verna flowers early in the spring. It is an annual plant that begins its life by germinating the previous fall. While its flowers are minuscule, multiple plants can be found packed into a single section of open ground, making their presence more obvious. This and the fact that it flowers so early, are what make it so memorable. After a cold, grey winter, our eyes are anxious for flowers, and even tiny ones can be enough.

Draba verna

Draba verna is in the mustard family (Brassicaceae), which is easy to determine by observing its flowers and fruits. The flowers are about 1/8 inch across, with four, deeply-lobed petals. The fruits are oblong, “football-shaped,” flattened capsules that are divided into two chambers and hold up to forty seeds or more. Flowers and fruits are borne at the tips of branched stems that are leafless, hairless, and very thin. Stems arise from a small rosette of narrow leaves that are green to purplish-red and slightly hairy. The plant itself is generally only an inch or two wide and a few inches tall, easily missed other than its aforementioned tendency to be found en masse.

flowers of Draba verna via eol.org

Draba verna¬†occurs throughout much of eastern and western North America, but is said to be introduced from Eurasia. A few sources claim that it is native to North America, but as far as I can tell, that is unverified. Either way, it is naturalized across much of its present range, and even though many of us consider it a weed, it doesn’t seem to be causing too much concern. It’s too tiny and short-lived to really be a problem. It makes its home in disturbed and neglected sites – along roadsides; in fields, pastures, and garden beds; and in abandoned lots. The one place it may be trouble is in nurseries and greenhouses, where it might be able to compete with young plants in pots.

open capsule and seeds of Draba verna via eol.org

The flowers of Draba verna are self-fertile, but they are also visited by bees that have ventured out in early spring. The foliage might by browsed by rabbits and other small mammals, but otherwise this plant is of little use to other creatures. Being in the mustard family, it is likely edible, but again it is so small that harvesting it would hardly be worth it. Instead, maybe its best to leave it in place and enjoy it for what it is: a tiny, brave reminder that spring is on its way and an encouragement to get down low once in a while to admire the little things.

An attempt at sketching Draba verna fruits on a raceme.

See Also: Tiny Plants: Duckweeds

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Tiny Plants: Duckweeds

Obviously, a series about tiny plants must begin with duckweeds – a group of aquatic plants that holds records in a number of categories including smallest flowering plants, smallest vascular plants, and smallest fruits. They are so small, in fact, that they don’t even have true stems or leaves, but rather are composed of undifferentiated vegetative tissue known as a thallus. Some species have one or a few tiny rootlets; others form no roots at all. However, what they lack in their hyper-diminutive size, they make up for in their ability to form massive colonies, creating dense mats that can take up serious square footage in a pond or lake. Depending on the species present, a single square yard of a duckweed colony can contain hundreds of thousands¬†of individual plants.

Five genera make up the duckweed subfamily (Lemnoideae): Spirodela, Lemna, Landoltia, Wolffia, and Wolffiella. This group used to be considered the family Lemnaceae, but has since been placed in Araceae – the arum family. While they are considered flowering plants, not all species of duckweeds produce flowers, and those that do, do so only rarely. They mainly reproduce asexually through a process called budding, in which growth occurs at the base of the thallus (or frond) and eventually splits off from the parent plant. This process happens fairly quickly, which is why duckweeds are able to create substantial colonies.

 

Duckweed mats form atop the still waters of lakes and ponds, but can also form in very slow moving rivers and streams. Their presence is an indicator of high levels of minerals and nutrients, which is why they are commonly seen in agricultural and industrial wastewater ponds. Nutrients are absorbed through the underside of the thallus, so the rootlets of duckweeds likely function more for stabilization than for nutrient uptake. As duckweed mats expand and grow dense, they shade the environments below them. John Eastman writes about this phenomenon in The Book of Swamp and Bog: “Thick blankets of duckweed can shade pond bottoms, preventing adequate photosynthesis and making life difficult or impossible for submersed plants and animals…however, this is often a problem of only intermittent duration.” One potential benefit of such dense mats is that they can kill off mosquito larvae. Eastman points out that for this to be the case, the duckweed may need to be accompanied by other surface dwelling plants in order to create dense enough shade.

duckweed 1

Duckweeds overwinter by forming turions, small buds that act as storage organs. Eastman explains the process:

These tiny, kidney-shaped buds detach and immediately sink to the bottom, where they remain all winter. In the spring, each turion expels a gas bubble, which causes it to rise to the surface, where it rapidly develops into a new duckweed thallus. Turion formation requires a combination of bright sunlight and high water temperature.

Duckweeds colonize new areas either by moving downstream (if they have that option) or by finding themselves attached to the fur, feathers, or feet of animals that unwittingly transport them. The common name, duckweed, is likely derived from the fact that it is a major source of food for waterfowl. It is high in protein and rich in nutrients, especially when you factor in all the tiny critters growing on and among it. Muskrats and beavers occasionally eat duckweeds as well. Despite losses from herbivory by these creatures, being made mobile by their moving bodies is a major boon.

A collection of various duckweed species - photo credit: wikimedia commons

A collection of various duckweed species – photo credit: wikimedia commons

Duckweeds are also consumed by various species of fish, which is why they are commonly used as a food source in aquaculture. Frogs and other amphibians as well as various aquatic insects and microinvertebrates also consume duckweeds. The diversity of small animals and protists that use duckweeds and the environments they help create is incredible. Eastman writes:

Duckweed mats host a large variety of small fauna that feed, lay eggs, or shelter amid the plants. Many of them secure themselves to the thallus rootlets or undersides, where they snare and capture passing food organisms or particles. Protozoans, rotifers, insect larvae, and crustaceans are often abundant.

Humans have also been known to eat duckweeds. Duckweed farming is not a simple procedure, but a highly nutritious food source is the result when it can be done. A simpler alternative is to use the harvest as animal feed. Duckweeds are also used in bioremediation and are being considered as a source of biofuel.

Depending on the species, an individual duckweed can vary in width from 10 millimeters to less than 1 millimeter. They truly are tiny wonders of the plant world, and it is worth getting down to their level for a closer look (hand lens recommended).

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