Savage Garden: Darwin on Drosera

In the world of carnivorous plants, Venus Flytraps often get all the love. But the sundew genus (Drosera) is pretty spectacular too. And you don't have to take our word for it: Charles Darwin himself was fascinated by these plants.

But I will & must finish my Drosera M.S... for at this present moment I care more about Drosera than the origin of all the species in the world. But I will not publish on Drosera till next year, for I am frightened & astounded at my results... Is it not curious that a plant should be far more sensitive to a touch than any nerve in the human body! Yet I am perfectly sure that this is true. (Letter to Charles Lyell, 24 November 1860, via the Darwin Correspondence Project)

Darwin was astounded by the sensitivity of the trapping mechanism of these carnivorous plants. Sundews capture prey using what is known as a "flypaper trap". The surface of the plant is covered with tiny tentacles, each of which secretes a small drop of very sticky mucilage (glue). Insects are drawn to the sweet scent of this mucilage, and are trapped on the plant. Digestive enzymes are then secreted to break down the insect and allow the sundew to absorb the nutrients. Simple enough, right?

Note the tentacles bent inward to help capture this fly.

Here's what really fascinated Darwin, though. Once an insect touches one of the sticky tentacles, the other tentacles in the area immediately react and bend in toward the trapped prey, sometimes causing the whole leaf to bend around the insect. This process is known as thigmonasty. Drosera tentacles are sensitive to the very slightest touch, so sensitive that Darwin worried his results were wrong (hence his hesitation to publish his findings). He observed that the legs of a single gnat were enough to cause the reaction to take place - a fact which astonished him and caused him to write to Daniel Oliver in 1860, "Are not these facts curious... if you can swallow them. Believe me that they are not stated without numerous & carefully repeated experiments."

Darwin also noted that the plants were able to differentiate between prey worth keeping until digestion had taken place (like gnats or ants), and other matter that was accidentally trapped (like leaves or other debris). Sundews release captured items that don't contain enough nutrients (Darwin noted it was specifically lack of nitrogen) without wasting energy trying to digest them. Darwin's experiments using a variety of liquids in various dilutions proved to him that, "With respect to Drosera, there is no doubt that the plant is acute enough & knows what is good." (Letter to Joseph Hooker, 1860)

About Sundews (Drosera sp)
Sundews are a large widespread genus (about 194 species), found on six continents in a variety of climates and growing conditions, mostly wet boggy areas. There are eight species of Drosera found in the U.S. and Canada, including five that are native to Florida. In the Savage Garden display at MOSI Outside, you'll find the following sundews:

• Cape Sundew (Drosera capensis), including variants 'Wide', 'Narrow', and 'Alba' 
• Thread-Leaved Sundew (Drosera filiformis) - FL native
• Southern Thread-Leaved Sundew (Drosera tracyi) - FL native
• Oblong-Leaved Sundew (Drosera intermedia) - FL native
• Spoon-Leaved Sundew (Drosera spatulata)

One great fact about sundews - while the Venus Flytrap is notoriously difficult for the home gardener to raise, many sundews are easy in comparison. If you're interested in growing carnivorous plants, try starting with a Cape Sundew (D. capensis). Learn more about growing carnivorous plants here.

What's in a name? The common name "sundew" is fairly obvious - it refers to the drops of mucilage on each tentacle that glisten like dew in the morning sun. The botanical name Drosera comes from the Greek "drosos" - meaning dew or dewdrops.

Interested in learning more about Darwin and his studies with carnivorous plants? Read his 1888 work Insectivorious Plants, or see more of his correspondence relating to the subject. The drawing at the left of D. rotundifolia leaf curled around prey comes from this book (Fig 5) and was drawn by Darwin's son George.