What is a Sundew?

  Drosera oblanceolata (Sunset Peak, HK)

       Drosera madagascariensis

   Drosera sp. "South Africa"

 

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.