A protocarnivorous plant (sometimes also paracarnivorous, subcarnivorous, or borderline carnivore), according to some definitions, traps and kills insects or other animals but lacks the ability to either directly digest or absorb nutrients from its prey like a carnivorous plant. The morphological adaptations such as sticky trichomes or pitfall traps of protocarnivorous plants parallel the trap structures of confirmed carnivorous plants. Some authors prefer the term "protocarnivorous" because it implies that these plants are on the evolutionary path to true carnivory, whereas others oppose the term for the same reason. The same problem arises with "subcarnivorous". Donald Schnell, author of the book Carnivorous Plants of the United States and Canada, prefers the term "paracarnivorous" for a less rigid definition of carnivory that can include many of the possible carnivorous plants. 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. Many examples of protocarnivorous plants exist, some of which are counted among the ranks of true carnivorous plants as a matter of historical preference. Further research into these plants' carnivorous adaptations may reveal that a few protocarnivorous plants do meet the more rigid definition of a carnivorous plant.
Historical observations
Historical observations of the carnivorous syndrome in plant species have been restricted to the more obvious examples of carnivory, such as the active trapping mechanisms of Drosera (the sundews) and Dionaea (Venus flytrap), though authors have often noted speculation about other species that may not be so obviously carnivorous. In one of the earlier publications on carnivorous plants, Charles Darwin had suggested many plants that have developed adhesive glands, such as Erica tetralix, Mirabilis longifolia, Pelargonium zonale, Primula sinesis, and Saxifraga umbrosa, may indeed be carnivorous but little research has been done on them. Darwin himself only mentioned these species in passing and did not follow through with any investigation. Adding to the small but growing list, Francis Lloyd provided his own list of species suspected of carnivory in his 1942 book on carnivorous plants, though these species and their potential were only mentioned in the introduction. 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. Simons was surprised to find these articles lacking in the literature cited sections of many modern books and articles on carnivorous plants, suggesting that academic research has treated Lloyd's 1942 book as the authoritative and comprehensive source on pre-1942 research on the carnivorous syndrome.
Defining carnivory 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.
The strict definition requires that a plant must possess morphological adaptations that attract prey through scent or visual cues, capture and retain prey (e.g., the waxy scales of Brocchinia reducta or downward facing hairs of Heliamphora prevent escape), digest the dead prey through enzymes produced by the plant, and absorb the products of digestion through specialized structures. The presence of commensals is also listed as strong evidence of a long evolutionary history of carnivory. By this definition, many sun pitcher plants (Heliamphora) and the cobra lily (Darlingtonia californica) would not be included on a roster of carnivorous plants because they rely on symbiotic bacteria and other organisms to produce the necessary proteolytic enzymes. The broader definition differs mainly in including plants that do not produce their own digestive enzymes but rely on internal food webs or microbes to digest prey, such as Darlingtonia and some species of Heliamphora. The original definition of botanical carnivory, set out in Givnish et al. (1984), required a plant to exhibit an adaptation of some trait specifically for the attraction, capture, or digestion of prey while gaining a fitness advantage through the absorption of nutrients derived from said prey. Upon further analysis of genera currently considered carnivorous, botanists widened the original definition to include species that use mutualistic interactions for digestion. Both the strict and broad definitions require absorption of the digested nutrients. The plant must receive some benefit from the carnivorous syndrome; that is, the plant must display some increase in fitness because of the nutrients obtained from its carnivorous adaptations. Increased fitness might mean improved growth rate, increased chance of survival, higher pollen production or seed set.
Degrees of carnivory
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