What is a Sundew?

Drosera oblanceolata (Sunset Peak, Hong Kong) beautiful red sundew
  Drosera oblanceolata (Sunset Peak, HK)

Drosera capensis x spatulata
   Drosera capensis x spatulata (sterile)

Drosera madagascariensis
       Drosera madagascariensis

Drosera sp. "South Africa"

   Drosera sp. "South Africa"

Pygmy sundew, Drosera 'Lake Badgerup'
     Drosera 'Lake Badgerup', a                        pygmy sundew

  Sundew leaf anatomy and Leaf Structure including the Petiole, Leaf Blade, lamina, and tentacles
Basic leaf anatomy of a Drosera capensis leaf. The tentacles,
leaf blade (lamina) and petiole are labeled. The base of the
leaf would be below the
petiole and the stipule is
located at the end of the
leaf base, and it connects
the leaf to the stem).

Drosera regia
          Drosera regia

General description
Sundews are carnivorous plants. This means that sundews can trap and digest insects to obtain extra nutrients, such as nitrogen. This allows sundews to live where other plants can’t- in nutrient-depleted soils or peat bogs. Some sundews can obtain a sufficient amount of nutrients from the soil, which allows them to survive for long periods of time, even if they don’t catch any food (ie mature D. capensis). However, other sundews (such as Drosera glanduligera) are not very efficient at absorbing nutrients through their roots, so they must rely more heavily on catching prey. This means that if they do not catch prey after germination, they will only live a short time.

Depending on where you look (i.e. old books or Wikipedia), there are from 86-196+ distinguished sundew species. Since there are so many species, hybrids readily occur in nature, and many sundew growers have made their own unique hybrids. 

The leaves of sundews are covered with "tentacles". The tip of every tentacle contains a nectar gland, which produces a globule of a sticky digestive enzyme. When an insect lands on the leaf, they get stuck on the dew. As they struggle to get out of the dew, the tentacles/leaf start(s) to wrap around the insect (through a complicated biological process involving several action potentials). The dew eventually suffocates the insect, and it stops moving. The digestive enzymes absorb nutrients that the sundew needs. If sundews are fed (or capture their own food), they will generally grow larger and faster than plants that are not fed. See bottom of page for more discussion.

Sundews can be found all over the world. Because of their adaptability to many regions, there is much diversity in the genus Drosera. Sundews are divided into several categories by type and location. Here are a few examples: Tropical, Fork-leaved, Temperate, Tuberous, Pygmy (very tiny), Annual, South African, South American, Petiolaris (Australian tropical sundews), and Queensland sundews. 

A specific example of diversity in the genus Drosera can be seen in comparing Temperate and Petiolaris sundews. Temperate sundews prefer cooler to moderate temperatures, and grow well in low to medium humidity. Petiolaris sundews thrive only if given hot temperatures and very high humidity. While most sundews are very small (some smaller than a dime) "there are also climbing plants that reach a length up to 3 m". This shows the incredible diversity of the Drosera genus. Sundews are able to survive through hot dry seasons or cold seasons "with the aid of perennating buds, storage roots, or tubers" (Barthlott 97).

A fun fact: "European sundew species have been used in folk medicine since the Middle Ages, as remedies for coughs and respiratory ailments" (Barthlott 94).

Taxonomy (The scientific Name of the Sundew)
Sundews belong to the genus Drosera. The genus is a subdivision of the carnivorous plant family, Droseracae. So, to repeat, the Drosera genus belongs to the Droseracae family. As far as the origin of the name Drosera, according to James Pietropaolo's Carnivorous Plants of the World, the name "Drosera is derived from the Greek words "droseros" which means dewy and 'drosos' meaning dew, alluding to the resemblance of the drop of mucilage on the tentacle gland to dew" (Pietropaolo 75).

Scientific/ Biological Description
"Most Drosera species are perennial herbs; a few species are annual herbs or dwarf shrub" (Barthlott 97). The structure of the Drosera leaf corresponds to that of a normal foliage leaf and consists of a base, a stalk (petiole), and a blade (lamina). "The petiole connects the leaf base with the lamina, which is covered with tentacles (Barthlott 100). "Sundews have active adhesive traps, with the trapping device consisting of the leaves and their tentacles and sessile glands. The stalks of the tentacles consist of an outer cell layer (epidermis), a few cell layers lying beneath it, and a central string of tracheids that conduct water" (Barthlott 104) The leaf base generally bears a membranous, papery-thin appendage (median stipule) on the upperside that can be regarded as a product of fused stipules" (Barthlott 99). This trait is not found in any tuberous sundews. Median stipules are important for rosetted sundew species because it is responsible for allowing the sundew to form perreniate (perennial) buds.

"This bud helps the species of temperate and cold climates to overwinter and the species of subtropical climates to survive through tho the next rainy season. The median stipules, which develop long before the rest of the leaf, lie close together, and the space between them is filled with air. Due to the inclusion of this air, the bud often appears white and reflects sunlight well, while water is virtually unable to penetrate into the bud" (Barthlott 99).

William Barthlott provides a great description of the mucilage composition in sundews: 

"The mucilage is acidic and contains polysaccharides: for instance, Drosera capensis secretes a 4 percent solution of these polysaccharides with a pH value of 5. In addition, the mucilage contains calcium, magnesium, potassium, and sodium ions. Before digestion takes place, the mucilage does not contain any proteins” (Barthlott 106). 

Many growers wonder why their sundews don’t produce dew. If grown in a dim location, the plant will not be able to produce a sufficient amount of polysaccharides, which will reduce the amount of mucilage and sugars the sundew is capable of producing.

How Sundews Move Their Leaves and Tentacles around prey
While my knowledge of this process is limited, I will describe some of the basic information that has been published about the tentacle and leaf movement of Drosera. First, there have been several studies that tested the detection of molecules or ions by the tentacles of sundews in the absence of any major tentacle disturbance (physical stimulation). Darwin tested nitrates on the tentacles of sundew leaves and discovered the leaves moved in the abcense of tentacle disturbance. Different minerals also produce this response. While I haven't gone about testing different types of minerals, you can get a very fast response when adding a dilute solutions of potassium chloride, for example. It is likely that the sundew has different levels of detection depending on the mineral type, but I haven't looked into this. The minimal concentration needed to stimulate a response by the tentacles and leaves has been determined for nitrogen, and can easily be determined for other minerals by preparing different dilute concentrations of mineral solutions.

Chitinases are digestive enzymes produced by sundews, which are used to break down the exoskeleton of insects (which is comprised of chitin). When I placed a small amount of pure hydrated chitin (isolated from crab shells and softened in Deionized distilled water) gently onto the mucilage while avoiding disturbance of the tentacles, the tentacles and leaf curls dramatically, and the chitin was digested completely within a few days. Chitinases are able to cleave bonds between different portions of the chitin chains, providing the sundew with a good source of nitrogen.
Sundews also have the capability to detect and digest of proteins and fatty acids. Breakdown of proteins is carried out by proteases that are also produced by the sundew. While I haven't done this myself, I think by that adding pure amino acids or a purified protein to the mucilage (such as albumin, a very commonly available protein), the leaf should respond. There's likely a paper on that somewhere doing something of the sort, but I haven't been able to find that yet.
And then there's the actual mechanism for leaf/tentacle movement. While it isn't totally identified yet, the most commonly accepted method for the venus flytrap closure mechanism consists of the following:
1. An action potential is sent by the trigger hair to the cells at the base of the leaf.
2. The action potential causes ions to rush down a gradient into the cells at the base of the leaf, leading to an instant and rapid uptake water due to turgor pressure changes.
This process is very similar to the closure and opening of the stomata, which can occur at incredibly fast speeds.

For sundews, the mechanism may be similar to venus flytraps, but it is likely a bit more complex, since the leaf can bend in very different directions depending on the size and shape of the food, as well as where it's positioned on the leaf. Some people originally proposed that the backside of the leaf might actually be growing rapidly via auxins, but that does not appear to be the case, since the leaf can return to its original size and shape after it folds over a meal and "relaxes" back to its "resting" state.

Disclaimer: More info to come about tentacle/leaf movement when I have time to site my references and link the pages...In the meantime I apologize for any innacuracies, as I'm doing this all by memory at this point.

Works Cited

Barthlott, Wilhelm et.al. The Curious World of Carnivorous Plants. Portland: Timber Press, 2007.

Additional Questions or Suggestions?

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