Plant carnivory is defined by the adapted ability of plants to attract, trap and digest animals with the aid of specialized structures (Barthlott et al., 2007). In this process they are able to kill their prey, digest soft tissues, and absorb some nutrients through highly modified leaf structures (Juniper et al; 1989). These specialized leaves, and additionally their stems, take over many functions typical to the roots of non-carnivorous plants, including: water and mineral uptake, vegetative propagation, anchorage, and food storage (Adlassnig et al., 2004).
While information is limited on the structure and function of carnivorous plant roots – botanists Wolfram Adlassnig, Marianne Peroutka, Hans Lambers and Irene K. Lichtscheidl published the article “The roots of carnivorous plants,” in the journal Plant and Soil in 2005, to pick up where past research has left off. Their article introduces research in the field and past open hypotheses alongside personal studies and observations – aiming to draw new conclusions and correlations. The compiled research covers morphology of the roots of carnivorous plants and their functions; how stems and leaves have adapted to take over root functions; exception cases in which roots still perform nutrient uptake; a comparison of nutrient absorption levels by traps and root systems; and how these adaptations have allowed for tolerance of harsh habitats.
As of 2007 some 650 species of carnivorous plants have been identified and subject to research (Barthlott et al., 2007). Many carnivorous plants can be compared by few similar features despite aesthetic and physiological differences. They are all poor competitors, tolerant of nutrient poor soils and prefer sunny habitats. They can be found living in diverse environments where harsh conditions can vary from a lack of oxygen in the soil, to high salinity, areas of excessive drought and flooding, extreme heat caused by fire, and high levels of heavy metals in the soil (Adlassnig, et al. 2004). Plants described in this article are broken down into sections depending on the environment they grow in – which subsequently illustrates a similar evolutionary lean towards certain types of root development.
A wide range of root types has evolved alongside carnivory, often morphologically categorized into three groups: plants with no roots, low-profile weak roots, or strong well developed roots. Aquatic carnivorous plants never mature roots, instead specialized shoots and leaves have replaced root function. Some examples include the genera Aldrovanda, Utricularia and Genlisea. Aldrovanda vesiculosa, the waterwheel plant, is a freshwater plant that catches its prey using snap traps. The seedling has a root, but it soon dies off. (Barthlott et al., 2007). The genus Utricularia, commonly known as the bladderworts, consists of some 233 species, in the Lentibulariaceae family, are recognized by their underwater suction or bladder traps (Barthlott et al., 2007). Both Aldrovanda and Utricularia species grow as horizontal stems (rhizomes) submerged in water, with their flowers rising above the surface. The leaves and free floating stems directly absorb the few available nutrients from the water (Slack, 2000). The genus Genlisea, part of the Lentibulariaceae family, also rootless, are submerged in water only during part of the year, and are found growing in loose sandy soil (Adlassnig et al., 2004). Species have developed two leaf types: aboveground – a rosette of green, narrow and linear leaves; and replacing roots underground – colorless snare leaves modified into trapping organs specialized for catching protozoans (Barthlott et al. 2007).
Features of low profile, faint, or weak roots are common to carnivorous plants found in wetlands, peat bogs and swamps. The soil is characterized by poor aeration, low nutrients, and often low pH levels. These roots are usually short-lived frail and thin. Hygrophilic carnivorous plants with weak roots, preferring moist habitats, include the genera: Brocchinia, Cephalotus, Dionaea, Pinguicula; select species of Drosera; and all species in the Sarraceniaceae family (Adlassnig et al., 2004). While water absorption and mineral digestion is reserved for the traps and modified leaves of the Pinguicula and Brocchinia species, the roots still serve anchorage functions (Barthlott et al., 2007). The Venus flytrap, Dionaea muscipula, found growing in continually moist sandy soil, has a few adventitious roots that supply the plant with water. Vegetative propagation is replaced by a rhizomatous stem and mineral uptake by the digestive glands found on the snap trap (Barthlott et al., 2007). Although frail and thin, the roots can still perform important tasks. The sparse roots of Drosera rotundifolia have extremely long root hairs impregnated with cutin. They will run horizontally along the surface of the soil forming root suckers for vegetative propagation (Adlassnig et al., 2004).
Although rare, strong, well developed and extended root systems can be found in carnivorous plants of varying moist and arid habitats. Drosophyllum lustanicum can survive 5 months without rainfall, it is presumed water uptake continues through its developed root system. Similarly, the species Nepenthes pervillei, has an extended root system that can reach into deep cracks to find moisture and nutrients. Some species of Drosera form specialized tubers that allow the plant to emerge in the rainy season after dying back from hot and dry summers (Adlassnig et al., 2004).
Harsh conditions eliminate competitors. The specialized physiological adaptations of carnivorous plants allow for them to thrive in otherwise inhabitable environments. Dionaea muscipula, described above, seems to need routine fires to eliminate competitors. The above ground organs, as with some species of Byblis and Sarracenia, are destroyed by the fire, while the underground organs are protected. Other adaptations have allowed for species to survive in extremely cold running water, high salinity environments, and levels of high calcium sulfate (Adlassnig et al., 2004).
A brief overview of a few species above has illustrated some functions of roots and the subsequent modified shoot systems that take over typical functions. Adlassnig et al. has summarized four different stem modifications including 1. Stolons: or horizontal stems, that set down new plants; 2. Anchorage stems: shoots that grow down into the soil to anchor plant; 3. Rhizoids of aquatic plants help anchor plant to other floating masses; 4. Stem tubers that store food in order to propagate new stolons after periods of drought or fire.
The process of plant carnivory is achieved by various types of traps adapted from highly specialized leaves attracting prey with scent, color, or nectar (Lloyd, 1942). Five types of traps have been identified by the author to include: adhesive traps, steel traps, pitcher traps, eel traps, and suction traps. In all carnivorous plants, primary nutrient absorption is achieved by these modified leaves, while other leaves can assist in water absorption from fog or humid air. Secondary nutrient uptake is achieved by the roots, although this is hindered by the lack of nutrients common in their found environments; contrastingly nutrient-rich soil may suppress growth. Nutrition plays an important role in the morphology of some species of Sarracenia and Nepenthes, nutrient poor soil increases the quantity of traps and more efficient pitchers; while increased nutrient levels produce flatter leaves more suitable for photosynthesis than trapping animals (Adlassnig et al., 2004). It has been found that many carnivorous plants can survive without animal prey, however they may lose their ability to tolerate harsh environments and can decrease their reproductive abilities. Nutrient uptake varies substantially between species, some have a limit to their root absorption and need to supplement with nutrients from animals. In some cases specific nutrients are only absorbed by the root or shoot system.
While the trapping organs of carnivorous plants are exaggerated and fetishized in art and literature in grotesque forms (Barthlott, et al., 2007), insufficient information is available on the morphology of the roots of carnivorous plants, quantitative data on levels of nutrient uptake, as well as their specific adaptations to extreme environments. Authors Adlessnig et al., provide a conclusive summary on past research and offer new insights. This is a significant compilation of information, and introduction to the roots of carnivorous plants, that will hopefully inspire new research in the field to help understand how these plants evolved and conserve their biodiversity.
Adlassnig W., Lambers H., Lichtscheidl I., and Peroutka M. (2004). The roots of carnivorous plants. Plant and Soil, Vol. 274, No. 1/2, Root Physiology- from Gene to function. pp. 127-140.
Bathlott W., Porembski S., Seine R., Theisen I. (2007) The Curious World of Carnivorous Plants. Portland, Oregon: Timber Press.
Lloyd F. E. (1942). The Carnivorous Plants. New York: Ronald Press.
Juniper B. E., Robins R. J., and Joel D. M. (1989). The Carnivorous Plants. London: Academic Press Limited. pp. 353.
Slack A. (2000). Carnivorous Plants. Yeovil: MIT Press. pp. 240.