Protocarnivorous plant
[1] The demarcation between carnivorous and protocarnivorous is blurred by the lack of a strict definition of botanical carnivory and ambiguous academic literature on the subject.
[4] Later, in a 1981 review of the literature, Paul Simons rediscovered Italian journal articles from the early 1900s that identified several additional sticky species that digested insect prey.
[5] Debate about what criteria a plant must meet to be considered carnivorous has yielded two proposed definitions: one with strict requirements and the other less restrictive.
Upon further analysis of genera currently considered carnivorous, botanists widened the original definition to include species that use mutualistic interactions for digestion.
Plumbago, for example, possesses glandular trichomes on its calyces that structurally resemble the tentacles of Drosera and Drosophyllum.
There are also other plants that produce a sticky mucilage not necessarily associated with a tentacle or glandular trichome, but instead can be described more like a slime capable of trapping and killing insects.
Dr. George Spomer of the University of Idaho has discovered protocarnivorous activity and function in several glandular plant species, including Cerastium arvense, Ipomopsis aggregata, Heuchera cylindrica, Mimulus lewisii, Penstemon attenuata, Penstemon diphyllus, Potentilla glandulosa var.
[13] Other plants that are considered to be protocarnivorous have sticky trichomes on some surface, such as the flower scape and bud of Stylidium and Plumbago,[14] the bracts of Passiflora, and leaves of Roridula.
The trichomes of Stylidium, which appear below the flower, have been known to trap and kill small insects since their discovery several centuries ago, but their purpose remained ambiguous.
In November 2006, Dr. Douglas Darnowski published a paper describing the active digestion of proteins when they come in contact with a trichome of a Stylidium species grown in aseptic tissue culture, proving that the plant, rather than the surface microbes, was the source of protease production.
[15] Darnowski asserts in that paper that given this evidence, Stylidium species are properly called carnivorous, though in order to fulfill the strict definition of carnivory it needs to be proven that they are capable of absorbing nutrients derived from prey and that this adaptation gives the plants some competitive advantage.
Energy-dispersive X-ray spectroscopy spectra of the glands on P. auriculata and P. zeylanica found that the glandular secretions were composed mainly of the elements C, O, Si, Mg, and Al.[21] One such species, P. europaea, has also been noted to kill small birds by covering them in sticky calyxes, causing them to be unable to fly and subsequently die.
[25] While this has long been discussed as a defense mechanism, studies of Passiflora foetida have investigated them for potential carnivorous abilities.
A 1995 paper published in the Journal of Biosciences detailed the evidence that the glandular bracts played a distinct role in defense of the flower and were also capable of digesting captured prey and absorbing the nutrients.
[34] Observations have suggested that there may be a digestive mutualism between carnivorous insects and the sticky plant surface similar to Roridula.
[37][38] Darlingtonia californica[8] and several Heliamphora species do not produce their own enzymes, relying instead on an internal food web to break down the prey into absorbable nutrients.
It is able to attract and kill prey and the trichomes on the surface of the leaves can absorb nutrients, but so far no enzyme activity has been detected.
Only one major study has examined D. fullonum for carnivory and no evidence of digestive enzymes or foliar nutrient absorption was revealed.
[44] Mucilage production by seeds is fairly common in the plant kingdom and is typically associated with root and shoot penetration.
It is hypothesized that this, as well as dropping from the birds who lived amongst the leaves, are a source of nutrients upon decomposition and subsequent foliage absorption by the plant.
[45] Similarly, Puya chilensis was noted to ensnare livestock such as sheep who, unless rescued would degrade and feed the plant.
[49] Preliminary observations suggest that this particular species may have moved away from a solely (or even primarily) carnivorous nature and be adapted to "catching" the droppings of birds feeding at its nectaries.
The name "protocarnivorous plant" itself suggests that these species are on their way to carnivory, though others may simply be an example of a defense-related adaptation, such as that found in Plumbago.
