A Few Words About Sundews: Grow Your Own Living Flytrap!
| Polish version is here |
The following article was originally published in the journal for educators Biologia w Szkole (eng. Biology in School) (1/2016):
We usually think of plants as little more than parts of the landscape. While we know they are alive, the changes they undergo tend to be too subtle and too slow to notice over short periods of time. Whether consciously or not, we often contrast plants with animals because of the profound differences between the two kingdoms. For example, consider motility, which I discussed in the previous issue of “Biologia…” in reference to the sensitive plant Mimosa pudica. Plants seem to lack active movement, while animals are typically quite mobile [1]. Even more fundamental is the difference in nutrition: plants are autotrophic, while animals are heterotrophic.
A particularly fascinating group within the plant kingdom is made up of species commonly known as insectivorous. The term is not entirely precise, however; “carnivorous plants” is more accurate, for insects are by no means their only prey.
Carnivory in plants is less rare than one might assume; at least 600 species have been recorded. Nor is this a taxonomically coherent group: the ability to feed on animals has emerged independently at least five separate times across different plant orders [2]. All carnivorous plants share the ability to produce traps, in various forms, that lure and ensnare prey.
Why are carnivorous plants so fascinating? Perhaps it’s because they seem to combine characteristics we typically associate with plants and traits we usually attribute only to animals.
Carnivorous plants often inhabit highly specific environments, which can make them challenging to cultivate. Nevertheless, some species are quite easy to grow in controlled settings, even at home. One of the most adaptable is the Cape sundew Drosera capensis, the focus of this article.
Cultivation and Observations
The Cape sundew is native to South Africa, particularly the Cape of Good Hope region.
Its requirements are rather specific. It thrives in nutrient-poor substrates (ideally a blend of acidic peat and perlite or coarse sand) and needs ample sunlight. The pot should be relatively large and deep because the root system is robust.
While the sundew is not fussy about ambient humidity (it performs well at 40–70% relative humidity), the substrate must remain consistently moist, even saturated. In nature the plant grows in marshes, bogs, and along lake and river margins. The simplest method is to set the pot in a water-filled tray. A word of caution: the species is extremely sensitive to impurities commonly present in tap water. Rainwater is acceptable, but distilled water is preferable [3].
Vegetative propagation via leaf and root cuttings is possible, but of course one's first plants will come from seed. Sundew seeds are small (Photo.1A), spindle-shaped, and less than 1mm (0,03 in) long. Higher magnification reveals a characteristic sculpturing (Photo.1B).
Seeds of some sundew species require stratification, which involves a period of cold, moist conditions lasting 6 to 8 weeks before they will germinate [4]. Conveniently, D.capensis does not.
Sow the seeds directly onto the moist surface of acidic peat. Space them at least 1cm apart and do not cover them with peat. Never allow the substrate to dry. During germination, shade the seeds from intense direct sunlight [5].
Germination occurs within two to three weeks, occasionally a bit longer. The seedlings are very small and easily overlooked. Remarkably, the third leaf already functions as a trap!
At this stage the plants are too small to hunt effectively, so they can be hand-fed; growth then proceeds rapidly. Sundews photosynthesize perfectly well without supplemental feeding, but growth is noticeably slower. Once the juvenile plants reach a few centimetres in diameter, they can be transplanted so they do not crowd one another.
The Cape sundew is a perennial of relatively slow growth, reaching maturity about a year after germination. A mature rosette can be seen on Photo.3.
Cooler ambient temperatures may cause leaf loss, but new leaves appear once favourable conditions return.
Flowering usually occurs in summer. The plant produces inflorescence, typically about 30 cm (5.9–9.8 in) tall. Occasionally, twin inflorescences arise on a single scape (Photo.4).
The flowers open sequentially; typically only one is fully expanded at a time. Twin inflorescences, however, can treat the observer to two fully open flowers at once (Photo.5). Depending on the cultivar, their colour ranges from white, through violet, to deep purple.
Flowering noticeably weakens the plant.
The Cape sundew is self-fertile, so viable seeds set without intervention. After flowering, allow the scapes to dry and then shake them over a sheet of paper to collect the seed. Stored in a cool, dry, airy place, the seed remains viable for a considerable time.
From our perspective the leaves are the most interesting organs. In D. capensis they are linear-lanceolate, up to 15 cm (≈6 in) long and about 5 mm (≈0.2 in) wide, with a rounded or blunt apex. Younger leaves stand erect, whereas older ones lie more or less flat, parallel to the substrate. Younger leaves stand erect, whereas older ones lie more or less flat, parallel to the substrate.
Each leaf is coated with stalked glandular trichomes that secrete droplets of sticky mucilage containing digestive enzymes (Photo.6). The dew-like appearance gives the genus its common name.
The secretion is extremely sticky. Insects such as ants, attracted by the sundew, become firmly trapped. You can then observe the plant’s hairs slowly bending toward the prey, with the entire leaf gradually curling around it (Photo 7). This process takes several hours. The insect typically dies from suffocation or exhaustion.
Formic acid and proteolytic enzymes are secreted, dissolving the prey’s soft tissues, which the leaf then absorbs. Digestion typically lasts several days. The leaf subsequently reopens, and the remaining exoskeleton is shed (often blown away by the wind) leaving the trap ready for new prey. Each leaf functions for only a limited number of meals (typically three) before withering and being replaced.
Do not place dead insects on the leaves, as the plant does not respond to immobile prey. Nor should human food, such as cooked meat, be used; it can harm the plant. Ants, fungus gnats, and other small arthropods are suitable. The lesser fruit fly Drosophila melanogaster is also excellent delicacy for sundew.
Explanation
Why does the sundew hunt at all? As a green plant it is autotrophic: it can synthesize the organic compounds it needs from inorganic matter via photosynthesis.
However, D.capensis thrives in substrates poor in certain mineral nutrients, notably nitrogen. Nitrogen is an essential biogenic element found, for instance, in proteins.
How can any organism be nitrogen-limited when the element accounts for roughly 78% of the atmosphere? Unfortunately, the diatomic nitrogen molecule, N2, is chemically inert and biologically unavailable to plants and animals. Most nitrogen enters the biosphere only after free-living nitrogen-fixing bacteria, such as those of the genus Rhizobium, reduce it to ammonium ions (NH4+) or glutamine, an amino acid readily assimilated by plants and, consequently, by animals.
Some plants have entered into symbiosis with nitrogen-fixing bacteria into forms that plants can use. Most legumes (Fabaceae) form root nodules that house such symbionts [6].
In carnivorous plants a very different strategy evolved: nitrogen is obtained by digesting animal protein, a rich and readily assimilable source. It is therefore unsurprising that carnivory evolved in several unrelated plant families, including the Nepenthaceae, Drosophyllaceae, Sarraceniaceae, Droseraceae, and others. Prey also supplies additional nutrients.
The trap mechanism in sundews is particularly interesting. An insect, entangled in the sticky secretion, struggles and mechanically stimulates the nearest hairs. This elicits haptonasty, a type of movement triggered by a specific tactile stimulus, causing nearby hairs to bend toward the prey and increase immobilization. Unlike the seismonastic response of Mimosa pudica, where any touch suffices, the sundew’s reaction is finely tuned to a particular mechanical stimulus [1].
Chemically mediated stimuli (chemonasty) also play a major role [7]. Formic acid and proteases are secreted in parallel, dissolving digestible tissues, which the leaf then absorbs.
Other carnivorous plants employ different trapping strategies. The Venus flytrap Dionaea muscipula develops bilobed leaf traps with sensitive trigger hairs; the lobes snap shut when touched. Pitcher plants such as Nepenthes and Sarracenia form fluid-filled pitchers in which prey drowns. Butterworts Pinguicula resemble sundews: their leaves are coated with sticky mucilage and curl around small insects. Several aquatic species are also carnivorous, including the bladderwort Utricularia vulgaris and the waterwheel plant Aldrovanda vesiculosa.
Growing the Cape sundew can be a truly rewarding experience for anyone interested in the natural sciences. It offers a hands-on introduction to plant movement and unconventional ways of obtaining nutrients. Nurturing a sundew from a tiny seed into a mature, graceful, and endlessly fascinating plant brings a real sense of satisfaction. Since this species is relatively easy to care for, it can be successfully grown in a school biology lab, making it a great way to spark curiosity about the living world in students and other enthusiasts.
References:
- [1] Ples M., Wstydliwa roślina (eng. Sensitive plant), Biologia w Szkole (eng. Biology in School), 6 (2015), Forum Media Polska Sp. z o.o., pp. 52-56 back
- [2] Krasuska U., Glinka A., Gniazdowska A., Menu roślin mięsożernych, Kosmos. Problemy nauk biologicznych, 4 (2012), s. 635-646 back
- [3] Cheers G., Burnie G., Botanica, Könemann, 2005 back
- [4] Lewak S., Spoczynek roślin, in: Kopcewicz J., Lewak S., Fizjologia roślin, Wydawnictwo Naukowe PWN, Warszawa, 2002, s. 560-563 back
- [5] Haager J., Mieszkanie w kwiatach, moje hobby, Polska Oficyna Wydawnicza „BGW”, 1992, pp. 258 back
- [6] van Rhijn P., Vanderleyden J., The Rhizobium-plant symbiosis, Microbiological Reviews, 1 (1995), pp. 124-142 back
- [7] Schumacher W., Fizjologia, in: Strasburger E., Botanika: podręcznik dla szkół wyższych, Państwowe Wydawnictwo Rolnicze i Leśne, Warszawa, 1967 back
All photographs and illustrations were created by the author.
Addendum
As a complement to the article above, I would like to share a short video:
Marek Ples