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Droseraceae is the botanical name for a family of flowering plants. The family is also known under its common name, the sundew family.

Characteristics

The Droseraceae is a small family of carnivorous plants, which consists of three extant genera: Drosera, Dionaea, and Aldrovanda; each with distinct morphologies. The bulk of the members of Droseraceae are contained in Drosera, the true sundews; Dionaea and Aldrovanda both have only one extant species. Members of Drosera secrete a sticky substance from their leaves that traps prey. Dionaea and Aldrovanda both utilize quick-shutting snap-traps that close rapidly when the leaves are disturbed; however, Dionaea is terrestrial while Aldrovanda is strictly aquatic. Like carnivorous plants of other families, the Droseraceae are able to supplement their nutrient intake, especially that of nitrogen, by capturing and digesting small animals such as insects. In this way, these plants are able to thrive in nutrient-deficient areas, such as sphagnum bogs.

Drosera

Drosera is one of the largest genera of carnivorous plants and individual species vary extensively in their specific morphology. Common to all members of Drosera are highly modified leaves lined with tentacle-like glandular trichomes. At the end of each trichome, a bead of highly viscous mucilage is secreted, which resembles a drop of dew. The mucilage is a fairly pure aqueous solution of acidic polysaccharide with a high molecular weight, which makes the mucilage not only highly viscous but also very sticky.^[1] So much so that a single drop of mucilage may be stretched to lengths of up to a meter and cover one million times its original surface area.^[1][2] Insects and other prey animals are attracted to by the smell of this mucilage and become stuck in the viscous substance. Such snares are termed “flypaper traps,” however the trapping mechanism of sundews is often erroneously described as “passive.” In fact, sundew traps are quite active and sensitive, and the disturbance of one or a few trichomes quickly triggers an action potential that stimulates the rapid movement of other trichomes toward the prey. The leaf then curls in on itself, enveloping the prey for digestion.^[3]

Dionaea

Dionaea muscipula, better known as the Venus fly trap, is a globally famous carnivorous plant and according to Charles Darwin, “one of the most wonderful in the world.”^[4] The leaves of Dionaea are also highly modified and form a “snap-trap” that quickly shuts when a stimulus is detected. Three large trichomes extend outward on the inner surface of the trap. Two of these three hairs must be stimulated within a certain amount of time in order to trigger the trap. The trap closes as the result of a flipping of the trap lobes from a position where the exterior of the trap is concave to one where the exterior is convex. This movement can begin as soon as 0.4 seconds after stimulation and can be completed after one second.^[3]

Aldrovanda

Aldrovanda vesiculosa, also called the waterwheel plant, is less-well known than its relative Dionaea muscipula, but the two share similar trap structure. The trap of Aldrovanda is smaller, faster, and aquatic than that of Dionaea.^[3] In addition, while two stimuli are required to close a trap in Dionaea, only one is required in Aldrovanda. The trap of Aldrovanda closes approximately ten times faster than that of Dionaea.^[3]

Phylogeny

Despite some debate, taxonomists have tended to include at least two of the three genera, and, in general, all three, in this family since at least 1906.^[5] Separate families for Dionaea and Aldrovanda have been proposed in the past. These were Dionaecae, proposed in 1933, and Aldrovandaceae, proposed in 1949.^[6] Ultimately, molecular and morphological evidence support the inclusion of all three, and also shows that the two genera that have traps that snap shut (Dionaea and Aldrovanda) are more closely related to each other than to Drosera, suggesting that snap traps evolved only once.^[5]

The family Droseraceae is part of the order Caryophyllales in the clade core eudicots.^[5] The family totals nearly 200 species. Caryophyllales are divided into two major suborders: Caryophyllineae, which contains the “core” Caryophyllales like Cactaceae and Amaranthaceae and is sister to the Polygonineae – the “non-core” Caryophyllales. This non-core clade is where Droseraceae is placed.^[7]

In the past Drosophyllum lusitanicum has been included in this family. Drosophyllum, another monotypic genus, exhibits a flypaper-type trap similar to those of Drosera, but Drosophyllum does not actively curl its leaves to envelop captured prey animals. This important morphological distinction led researchers to question the validity of this taxon’s placement in Droseraceae. Other significant trait differences in Drosophyllum include pollen structure, trichome anatomy, and a woody stem with a deep taproot.^[6] Ultimately, it was shown that Drosophyllum is more closely related to the carnivorous liana Triphyophyllum and the non-carnivorous liana Ancistrocladus, and is, thus, classified elsewhere (to be specific, its own monotypic family Drosophyllaceae).^[5] Recent molecular and biochemical evidence (see the AP-Website) suggests that the carnivorous taxa in the order Caryophyllales (the families Droseraceae, Drosophyllaceae, Nepenthaceae, and the species Triphyophyllum peltatum) all belong to the same clade, which does not consist only of carnivorous plants but also of some non-carnivorous plants such as those in the family Ancistrocladaceae.

The fossil record of Droseraceae is the richest of any carnivorous plant family. Fossil pollen has been attributed to several extant as well as extinct genera, although some are of questionable validity.

References

1. ^ ^{a b} Rost, K.; Schauer, R. (1977), "Physical and chemical properties of the mucilage secreted by Drosera capensis", Phytochemistry 16 (9): 1635–1638 
2. ^ Zhang, M.; Lenaghan, S.C.; Xia, L.; Dong, L.; He, W.; Henson, W.R.; Fan, X. (2010), "Nanofibers and nanoparticles from the insect capturing adhesive of the Sundew (Drosera) for cell attachment", Journal of Nanobiotechnology 8 (20), doi:10.1186/1477-3155-8-20 
3. ^ ^{a b c d} Williams, S.E. (2002), "Comparative physiology of the Droseraceae sensu stricto – How do tentacles bend and traps close?", Proceedings of the 4th International Carnivorous Plant Conference: 77–81 
4. ^ Darwin, C. (1875), Insectivorous Plants, London, UK: John Murray, http://darwin-online.org.uk 
5. ^ ^{a b c d} Cameron, K. M.; Wurdack, K. J.; Jobson, R. W. (2002), "Molecular evidence for the common origin of snap-traps among carnivorous plants", American Journal of Botany 89 (9): 1503, doi:10.3732/ajb.89.9.1503 
6. ^ ^{a b} Williams, S.E.; Albert, V.A.; Chase, M.W. (1994), "Relationships of Droseraceae: a cladistic analysis of rbcL sequence and morphological data", American Journal of Botany 81 (8): 1027–1037 
7. ^ Cuénoud, P.; Savolainen, V.; Chatrou, L.W.; Powell, M.; Grayer, R.J.; Chase, M.W. (2002), "Molecular phylogenetics of Caryophyllales based on nuclear 18S rDNA and plastid rbcL, atpB, and matK DNA sequences", American Journal of Botany 89 (1): 132–144