The Evolution of Carnivorous Plants

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Some of the most bizarre and fascinating plants in the natural world are undoubtedly carnivorous plants. Carnivory, defined as the consumption of animal tissue, is often only associated with the animal kingdom. However the existence of carnivory is widespread and diverse in the plant and fungi kingdoms as well. Specifically carnivorous plants, which originally descended from exclusively photosynthetic plants, have evolved elaborate, efficient, and diverse methods to capture, digest, and metabolize passing insects and microorganisms. Since Darwin’s landmark work Insectivorous plants, observers of carnivorous plants have tried to answer fundamental questions regarding their nature. Why would an exclusively photosynthetic plant expend valuable developmental resources to form structures for carnivory? How do these plants capture prey and why do they do it? This paper will explore characteristics, nature, and physiology of carnivorous plants as well as several possible reasons and methods for the evolution of carnivory in plants. Carnivorous plants are members of the in the angiosperm family (flowering plants). As the most diverse division of land plants, angiosperms have developed full carnivory six times in their phylogeny. There are approximately 600 species of carnivorous plants, representing eleven families and nineteen genera of angiosperms (Huebl et al., 2006). Carnivorous plants are widely distributed across the globe and can be found on all continents except for Antarctica. They are generally found in bog and fen conditions or any other habitat where the soil is very low in nutrients, slightly acidic, and or hypoxic (Academac, 1997). The most basic definition of carnivory in plants is the ability to absorb the products of decomposed organisms, either directly on the leaves or through roots in the soil, to increase nutrient absorption which ultimately increases seed production and overall fitness. While this definition includes most exclusively photosynthetic plants (pure autotrophs), a more specific definition of carnivorous plants is that they should have at least the ability to attract, capture, and digest their prey, which are typically arthropods or protozoans (Chase et al., 2010). It should be noted that there is a distinction between parasitic and carnivorous plants. Carnivorous plants are classified as photoheterotrophs, or organisms that depend on both light for photosynthesis and organic carbon from prey. Parasitic plants require a host plant to survive and draw nutrients from. While all trap structures of carnivorous plants are highly specialized leaves, each group is distinguished by the specific specialization of the leaf. The three primary categories of carnivorous plants, separated by capture method, are active traps, semi-active traps, and passive traps. These categories are refined further as plants are classified as either snap, pitfall, flypaper, bladder, or lobster-pot traps. The most dramatic and sinister of all carnivorous plants are active traps. Two widely recognized examples of active snap traps are Dionaea muscipula, the Venus Fly Trap, and Aldrovanda vesiculosa, the Water Wheel. Active traps most often use touch-sensitive hairs that trigger the plant’s capture mechanism when stimulated. For instance, the trap structures of the Venus Fly Trap are two broad, jaw-like leaves which form a symmetrical clamshell. The leaves are lined with interlocking spines around the edges and are held open at about 45° to 60°. When a prey insect touches a trigger hair, the trap will only close if another trigger hair is stimulated within approximately twenty seconds (Hodick & Sievers, 1988). This double action-potential requirement prevents the trap from wasting energy by closing unnecessarily due to random stimuli from objects the plant cannot digest. Dionaea muscipula is one of the fastest moving touch-sensitive plant species in the world, closing as quickly as 500 ms once a prey...
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