They grow where some plants barely manage to survive and others merely wither and die. They are the carnivores of the plant kingdom. Carnivorous plants are adapted to soils that are lacking one or more essential nutrients, usually nitrogen. There are about 630 species divided into 12 genera plus about 300 speciesfrom several other genera that share some but not all the characteristics of the carnivores. Grouping them by their methods of trapping prey, there are five groups, plus a group for the borderline carnivores, as follows:
1) Trap prey in a rolled up leaf that contains a pool of digestive enzymes or bacteria : The Pitfall Trap. 2) Trap prey with a sticky mucilage : The Flypaper Trap. 3) Trap prey by rapid movement : The Snap Trap. 4) Trap prey by generating an internal vacuum and sucking them in : The Bladder Trap. 5) Force prey towards a digestive organ with inward pointing hairs : The Lobster-pot Trap. 6) The others, the borderline carnivores.
(Abridged and adapted from relevant wikipedia entries)
This is a convenient but contrived grouping, and results in some otherwise unrelated species being placed in the same group. The two genera of the snap-trap group is an uncomfortable example. Of course, the whole concept of carnivorous plants as a group is fuzzy at the edges. Darwin himself suggested many plants such as Erica tetralix , Mirabilis longifolia, Pelargonium zonale, Primula sinesis, and Saxifraga umbrosa, may be carnivorous, but he only mentioned it in passing. And some plants under stress or particular conditions may hark back to carnivory as though it were a momento of their ancestry.
Carnivory would seem to have a very ancient ancestry from which evolution has produced some very different plants, very distantly related.
No one but the enthusiastic amateur would dream of such a grouping of succulents, or such a grouping of caudiciforms. And if you do accept such groupings you are left with a knotty problem of definition. They are not scientific or botanical groupings; they are totally subjective.
Winter-hardy Aldrovanda (Group 3) form so-called "turions" as a frost survival strategy. At the onset of winter, the growth tip starts producing highly reduced non-carnivorous leaves on a severely shortened stem. This results in a tight bud of protective leaves which, being heavier and having released flotational gases, breaks off of the mother plant and sinks to the water bottom, where temperatures are stable and warmer. Here it can withstand temperatures as low as -15 °C (5 °F). In spring when water temperatures rise above 12-15°C, turions reduce their density and float to the top of the water, where they germinate and resume growth. Non-dormant turion-like organs can also form in response to summer drought. Temperate aquatic bladderworts (Group 4) generally die back to a resting turion during the winter months.
Turions of Utricularia vulgaris. Small pond near Swinoujscie, NW Poland. Author: Kenraiz Krzysztof Ziarnek
Turions :A survival strategy for aquatic species
Footnote: I have tried to produce a comprehensive overview of carnivorous plants, using the various Wikipedia entries as my source, and attempting to make these more readable, and occasionally throwing in my own revision or addition. I feel that I have fallen short of my objective and therefore will return to the subject several times henceforth. Anyone who would like to give me a hand will be most welcome.- Lez