Carnivorous Plants: A Successful yet Unusual Strategy of Survival
Since the classic work of Charles and FrancisDarwin, back in 1875 and 1878, more than a century ago on the carnivorous habit of Drosera(sundew), considerable information has accumulated on the significance of captured animal prey in the nutrition of carnivorous plants. Carnivory in plants includes the catching and subsequent digestion of the freshly trapped prey. This is a common form of nutrition in the animal kingdom,but it is rather rare in plants, and even rarer in fungi. From almost 400,000 species of plants identified so far, only little over 700 species from 10 families are carnivorous plants. However, this numbers tend to increase as new species have been discovered recently which may change the idea that carnivory among plants is far more common than previously thought. Carnivorous Plants are distributed worldwide, but they are generally restricted to sunny, wet, and nutrient poor environments, such as acidic bogs and rock outcroppings, where light and water are not the limiting factors for their growth. This distribution pattern suggests that there are conditions where carnivory provides a major benefit for plant survival. However, the restricted distribution of carnivorous plants in its various habitats also suggests that the costs involved in the carnivorous habit exclude carnivores from most habitats occupied by non-carnivorous plants.
Rare but Diverse
Carnivorous plants invariably have highly specialized structures, modified leaves mostly, commonly called traps, made for capturing and digesting their prey. These traps are classified as active or passive whether or not they involve movement from the plant for capturing its prey. Basically, these traps can be divided in five different types, or categories:
- adhesive hairs that work as flypaper and use sticky mucilage, e.g. in Drosera (sundew) and Byblis (rainbow plants);
- bladder-like suction traps, as used by Utricularia (bladderwort) species;
- snapping traps, like the famous modified leaves in Dionaea (Venus' flytrap) that entrap the prey after it touches sensitive parts of the trap;
- pitfall traps found in the famous Nepenthes (tropical pitcher plant) and Sarracenia;
- and finally Lobster-pot traps or eel-traps that force prey to move towards a digestive organ through a pathway with inward-pointing hairs that prevent the prey to come out; these are found in the underground modified leaves of Genlisea species only, the corkscrew plants.
The pitfall traps mostly contain water and are an important ecological niche for numerous small animals, of which some (e.g., the larvae ofmany Diptera, i. e. flies) are exclusively associated with this particular habitat. In some species, e.g. Nepenthes, the plant produces its arsenal of digestive enzymes that digest the prey drowned in the fluid within the trap. Others, like some Sarracenia species, count with the aid of bacteria and other fauna inhabiting the pitfall that help the plant to degrade and digest the trap. Hence, we have a symbiotic relationship in which the nutrients are split between all parts and the plant takes what the other parts did not use. Usually these symbiosis is plant species-specific, meaning that the fauna and/or bacteria are not found anywhere else. Nepenthes plants, although capable of digesting their own prey, nonetheless establish other symbiotic relationships with specific animal species, e. g. ants, in which their mutual benefits go beyond nutrient sharing. These symbiotic relationships between Nepenthes and animal species can be in fact quite complex and diverse as several different animal species, called Nepenthes infauna or nepenthebionts, can be found in one single pitcher of Nepenthes.
An Adaptive Strategy
Adding to carnivorous plants there are about 300 plant species that exhibit some characteristics typical of carnivorous plants and are thus called protocarnivorous plants, that is almost carnivorous. However, this is not a topic of consensus among scientists as in most cases the adhesive hairs found in protocarnivorous plants seem more to act as a defence mechanisms against herbivores rather than a mechanism to catch prey which would then be digested. Also, in these species, there are no visible or obvious structures specialized on the digestion of these prey as they are found in carnivorous plant species. However, there is one curious and peculiar example of a plant that shows to be carnivorous at least at some point in its life. Capsella bursa-pastoris (shepherd's purse) has a mucous layer that surrounds the germinating seeds, known as myxospermy, which has the capacity to catch and digest nematodes, protozoa, and bacteria. This is still under debate as very little is known about this adaptive role of this particular seed coating. This a very rare example of a plant that is carnivorous at one particular stage of its life cycle, in this case at the very beginning of it.
Different Strategies for the Same End
Some carnivorous plants attract their prey by producting nectar at the edges of the traps or odours. Those plants take a considerable effort to secrete nectar which may be almost 10% of the plant's total carbon budget. In addition, there are other carnivorous plants that secrete adhesive substances by special glands in which their prey end up stuck in, e. g. Drosera (sundew) and Pinguicula (buttenwort). All these efforts and costs contribute to small to medium size carnivorous plants. Big or giant carnivorous plants are the exception rather than the rule and from those most of them are nepenthes (tropical ptcher plants). All carnivorous plants are green and capable of photosynthesis. Hence, carbon is unlikely to be a major element to be withdrawn from their prey, although it is certainly incorporated. As mentioned before, carnivorous plant species naturally occur on nutrient poor,wet, and acidic soils, with the exception of Pinguicula, which grows on a chalky substrate, and Drosophyllum (Portuguese sundew), which grows on very dry arid conditions. Most species have a poorly developed root system and at least some species (e.g. Drosera) are non-mycorrhizal; that is they do not associate with fungi in a symbiotic relationship as is mostly found in plants. It is generally assumed that carnivory is an adaptation to nutrient-poor soils and that inorganic nutrients are largely derived from the prey. Nitrogen is the major element withdrawn from the prey, especially in fairly tall, erect, or climbing Drosera species, which may derive approximately 50% of all their nitrogen from insect feeding. Carnivorous plants are likely to derive other elements from their prey as well. However, when insects are scarce (e.g., in the extreme nutrient-poor habitat of the tuberous sundew, Drosera erythrorhiza, in Western Australia) the input of nitrogen from the catch of insects by the glandular leaves may be very small. Carnivorous plants are in fact quite adaptive and evolved as plant species, which does not mean that they are particular easy to grow as some of you may know. However, as extraordinary beings as they are they sure deserve the ever growing popularity as fascination that they conquered. As Darwin once said they are "the most wonderful plants in the world".
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