Carnivorous Plants Website
Carnivorous Plants in the Wilderness
by Makoto Honda

Carnivorous Plants
Anatomy of Carnivory
Updated 2018-2-3

FOR CARNIVORY TO TAKE EFFECT, there must be contact between leaf and prey. If a fly alights on a leaf and a moment later flies away, the contact is not sufficient. A fly must be persuaded to stay for a period during which the digestion can take place. 

The first step in the sequence of events leading to successful carnivory is a lure (1) that brings prey to physical proximity. Capture (2) -- forced retention -- follows. Digestion (3) then starts that decomposes the prey -- protein in the animal's body is broken down into amino-acids. The digestion proceeds with plant's own enzyme or with the help of other commensal organisms.
The products of digestion are now absorbed (4) into the leaf and are carried to the growth site of the plant.

The main nutrients selectively absorbed by carnivorous plants are nitrogen (N) and phosphorus (P) -- major plant nutrients -- but some other elements required in trace amounts such as potassium (K: kalium) and magnesium (Mg) are also utilized by some species.

A gland is a structure which secretes fluids. Normally the walls of the epidermal cells are covered with a cuticle to reduce water loss, but the outer cells of a gland have a thin, perforated cuticle. Glands play a significant role in carnivory: Glands are used for lure (nectaries - nectar glands), digestion (enzyme secretion) and absorption of the digestion products. For adhesive traps, the glands are also used to secrete viscous mucilage (or resin in the case of Roridula). Depending on the types of traps, different kinds of glands perform different functions, or the same gland assumes multiple tasks. 

Digestion  In pitfall traps, the digestion can start in the preformulated fluids immediately upon prey capture. In other trap types, there is detection/perception of stimuli associated with prey capture, and the digestive process is initiated within a certain time frame. The absorption of the products of digestion is usually carried out by the same digestive glands. In the case of some traps that do not possess any glands for absorption, cuticular discontinuities provide passage of nutrients into the leaf tissue.

Repeatability  Generally, adhesive secretion repeats many times, but may occur only once, as in Pinguiculas adhesive glands. Likewise, the secretion of digestive enzyme may repeat several times, as in Dionaea, or can occur only once, as in Pinguicula
, depending on the gland anatomy (presence or absence of conducting tissues to the gland).


CARNIVOROUS PLANTS (CP) EMERGED FROM non-carnivores at several distinct points in evolution. They only appeared as either a pitfall or adhesive trap -- and then, only some adhesives moved on to more sophisticated trap mechanisms. 

CP emerged as pitfalls in:
           ----- In the order Ericales:
           Darlingtonia / Sarracenia / Heliamphora
           ----- In the order Oxalidales:

CP emerged as adhesive traps in:
           ----- In the order Caryophyllales:
           Drosera  >> Snap traps (Aldrovanda / Dionaea)
                            >> Pitfall (Nepenthes) >> Adhesive again (Drosophyllum >>Triphyophyllum)
           ----- In the order Lamiales:
           Pinguicula >> Lobster pot (Genlisea) >> Suction trap (Utricularia)
           Ibicella / Proboscidea --- x
           Byblis --- x
           Philcoxia --- x
           ----- In the order Ericales:
           Roridula --- x

Aquatic Environment Pushed Evolution

In sundews (order Caryophyllales) and butterworts (order Lamiales), both of which are adhesive carnivores, transition to aquatic environment seems to have exerted pressure for moving forward to more sophisticated trapping schemes --- provided the snap trap first evolved in the aquatic environment.

Movement in Capture

We note that in the course of adaptation to carnivory, there are two distinct points at which physical movement was acquired during prey capture, leading to so-called "active" traps in carnivorous plants. One is in the order Caryophyllales in the branch leading to Drosera -- namely the Drosera's tentacle bending (and leaf folding) that eventually led to the snap traps of Aldrovanda and Dionaea. In the same order, the branch leading to Nepenthes, Drosophyllum and Triphyophyllum either lost or did not acquire any such movement. The other is the leaf movement (curling and dishing) in Pinguicula upon prey capture that --- though not as direct or obvious a transition as in Droseraceae --- may have some connection to the workings of some aspect of Utricularia traps... 
Some comments on use of the term "active" : The active trap simply means some movement is visible to the naked eye. If water molecules move in the trap, it is not visible. So, a bladderwort trap may be considered passive -- until it blasts as a result of gradual expulsion of water. The distinction between "active" and "passive" traps may be somewhat artificial...


1)  Nastic Movement - movement in a pre-determined direction, regardless of the location of stimuli.

2)  Tropistic Movement - movement in a direction that has correlation to the direction/location of stimuli.

      Phototropism - movement toward light
      Gravitropism - movement in response to gravity
      Thigmotropism - movement in response to physical touch


Occurrences of Sticky Carnivores 

We often talk about the number of occurrences of carnivory in the course of angiosperm (flowering plant) evolution. In the order Lamiales, there appears to be multiple, independent points at which adhesive-trap carnivores emerged (Philcoxia, Byblis, Ibicella/Proboscidea, Pinguicula). But there are many non-carnivorous taxa in this order that possess glandular foliage, generally for defense purpose. These adhesive glands probably can be traced back to a very ancient common origin. Given what we do not know, once a reasonably efficient and workable gland structure has been established, creating a sticky carnivore out of a sticky defense seems a relatively small step. Therefore, even if this final step toward carnivory might have been undertaken independently in a handful of different families in Lamiales, considering these occurrences completely independent is a little misleading (or of little value). We should be cognizant of the fact that carnivory requires a lot of things and the ground work had started long ago, probably in the common ancestors of these families. This certainly does not suggest to imply that "carnivory" had been contemplated upon as a long-term goal when the gland structures were being developed eons ago and, from that point forward, having been worked on to finally come to fruition by this final step --- Evolution, by definition, does not have any "future" plan. What I am saying is that, once we understand what carnivory entails and how it might have come about, counting the number of occurrences of carnivory --- by just noting the final stage toward carnivory --- may not be as meaningful an endeavor as we might have initially thought.

Sticky" Difference

There are many adhesive-trap carnivores that use sticky mucus to capture prey. They are found in three orders: Caryophyllales, Ericales and Lamiales. It is interesting to note that all adhesive glands of carnivorous plants are stalked glands raised above the leaf surface by some length; none of them are sessile glands (stalkless).

Darwin noted some difference between Drosera and Drosophyllum in their mucilage characteristics. In order to highlight this subtle but crucial difference, let us divide sticky-leaf carnivores into two groups. Drosera is the only member in Group 1, and the rest goes to Group 2. This grouping has to do with the way the digestive process is carried out after prey are captured by the adhesive glands.




Among all sticky-leaf glands, Droseras stalked glands (tentacles) are the only glands that have acquired the "dual" ability to perform digestive process (secreting digestive enzymes and absorbing the products of digestion) in addition to prey capture. In all the others, digestion is carried out by separate sessile glands scattered on the leaf surface. This means, for all these carnivorous plants in Group 2, keeping the trapped prey high on the tip of the stalked glands is meaningless, for no digestion will take place there --- the prey must be dropped down on the leaf surface. As noted by some observers (including Darwin), the Drosophyllums glue tends to peel off from the gland upon prey capture, exactly for this reason. This is in contrast to the Droseras glue that typically remains on the tentacle head to hold the prey during and after prey capture.


By and large, similar trapping behavior can be observed in all adhesive traps belonging to Group 2. In Pinguicula, for instance, the stalked glands readily collapse upon prey capture. This will allow the prey to sink down to the leaf surface and touch the sessile glands. That said, physical contact between prey and digestive glands may not be necessary for digestive process, so long as the small gap between them is bridged with fluids: For digestion, secreted fluids rich with enzyme must touch the body of the prey; for absorption, the fluids containing the digestion products must reach the absorptive glands. Keeping prey on the tall stalked glands of Drosophyllum is not likely to achieve this.


As mentioned above, the mucilage tends to peel off from the gland upon prey capture in Group 2 carnivores. There may be some difference in mucilage chemical composition that explains this behavior. Comparing the shape of glands in Group 1 & 2, the gland morphology also seems to support the difference in behavior...   

Group 1 (Drosera) --- The entire, spherical surface of the gland secretes sticky mucilage (i.e., the entire surface has cuticular gaps). As a result of this, the mucilage tends to cover the entire gland in sundew tentacles.

Group 2 (Drosophyllum, Triphyophyllum, and Pinguicula) --- The gland itself at the stalk tip is flat-shaped, and only the upper surface of the gland secretes mucilage. As such, the mucilage tends to sit on a dish-shaped gland, rather than covering the entire gland. This may account for the easier detachability of the mucilage from the gland upon prey capture in Drosophyllum.



(illustration)  ------------ under construction

FOR VARIOUS CP GENERA BELOW, we present the anatomy of carnivory in four steps showing the basic mechanisms involved. 

(1)  LURE 
- visual (leaf color / leaf shape / dewy-look... UV absorption pattern)
- olfactory (fragrance)
- food (nectar)

- pitfall trap  (pitcher-shaped leaf / waxy surface / slippery hairs / retentive hairs...)
- adhesive trap  (adhesive glands - how many times)
- snap trap  (aquatic waterwheel plant / terrestrial Venus flytrap)
- lobster-pot trap  (glands water flow / hairs)
- suction trap  (glands water expulsion)

- enzyme secretion by digestive glands - the Golgi apparatus (stimuli detection / how many times...)
- commensals (bacteria / commensal organisms / inquilines)

- absorption by (digestive) glands.

- if no glands exist, cuticular discontinuity of epidermis allows absorption



Poales                 Bromeliaceae                Brocchinia

                              Eriocaulaceae               Paepalanthus


Oxalidales          Cephalotaceae             Cephlotus 


Caryophyllales Droseraceae                 Drosera



                             Nepenthaceae                Nepenthes

                             Drosophyllaceae            Drosophyllum

                             Dioncophyllaceae          Triphyophyllum


Ericales             Roridulaceae                  Roridula

                             Sarraceniaceae             Darlingtonia




Lamiales            Plantaginaceae              Philcoxia

                             Linderniaceae                Lindernia

                             Byblidaceae                   Byblis

                             Martyniaceae                  Ibicella


                             Lentibulariaceae            Pinguicula








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