Carnivorous Plants Website
Carnivorous Plants in the Wilderness
by Makoto Honda



Carnivorous Plants
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Anatomy of Carnivory   
2017-03-15
Updated 2017-06-11

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 (visual/olfactory/food) that brings prey to physical proximity. Capture -- forced retention -- follows. Digestion 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 absorbed 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.  

 

Glands  The gland plays a significant role in carnivory: Glands are used for lure (nectar gland), digestion (enzyme secretion) and absorption of the digestion products. For adhesive traps, the glands  also secret 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. 

Repeatability  Generally, adhesive secretion can repeat many times, but can occur only once in Pinguicula’s 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 structure of the gland (presence or absence of conducting tissues to the gland).

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. Some traps do not possess any digestive glands and cuticular discontinuities provide passage of nutrients into the leaf tissue.    


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

- food (nectar glands)
 

2.  CAPTURE 

- pitfall trap  (pitcher-shaped leaf / wax / hairs...)

- adhesive trap  (adhesive glands - how many times)

- snap trap  ()

- suction trap  (glands – water expulsion)

- lobster-pot trap  (glands – water flow / hairs)

 

3.  DIGESTION

- digestive glands secrete enzyme - the Golgi apparatus (how many times.../how soon)

- commensals (bacteria / commensal bugs / mutualism)

 

4.  ABSORPTION 
- glands  (cuticular discontinuity)

- no glands (cuticular discontinuity)
 

 

EVOLUTION



Carnivorous plants (CP) emerged from non-carnivores at several distinct points in evolution. They only appeared as either an adhesive or a pitfall ... and then, only adhesives moved on to more sophisticated trap mechanisms... 

CP emerged as pitfalls in:
           Darlingtonia/Sarracenia/Heliamphora
           Cephalotus

CP emerged as adhesive traps in:
           Drosera  --- >> Snap traps (Aldrovanda/Dionaea)
                            --- >> Pitfall (Nepenthes) --- >> Adhesive again (Drosophyllum/Triphyophyllum)
           Pinguicula --- >> Suction traps (Genlisea/Utricularia)

Movement in Capture  We note that in the course of adaptation to carnivory, there are two distinct points at which physical movement was acquired, 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... 


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 where 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 taken place 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 mean 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. 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, we divide sticky-leaf carnivores into two groups. Drosera is the only member in Group 1, and the rest belongs 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, Drosera’s stalked glands (tentacles) are the only adhesive glands capable of secreting digestive enzymes and absorbing the products of digestion as well. In all the others, such as Drosophyllum, Triphyophyllum, Pinguicula, Byblis, etc., 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 Drosophyllum’s glue tends to peel off from the gland upon prey capture, exactly for this reason. This is in contrast to the Drosera’s 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, the stalked glands readily collapse upon prey capture, allowing the prey to sink down to the leaf surface.
 

Note that physical contact between prey and digestive glands may not be necessary for digestive process, as long as the small gap between them is bridged with fluids: For digestion, the 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….

 

In sundews, digestion is typically carried out in the center of the leaf blade where short, central tentacles grow. Depending on the species, it is conceivable that the glands of the long marginal tentacles are primarily for trapping purpose and only the central tentacles actively engage in the digestive process. Also, in sundews, the fluids containing the products of digestion may very well drip down onto the leaf surface from the short central tentacles, allowing numerous sessile glands on the surface to participate in the absorption as well.      

 

Under construction  For various CP genera below, we present the anatomy of carnivory in four steps showing the basic mechanisms involved. 

ANATOMY OF CARNIVORY 
                           LURE / CAPTURE / DIGESTION / ABSORPTION  (illustrations)

 

Poales                 Bromeliaceae                Brocchinia
                                                                      Catopsis

                              Eriocaulaceae                Paepalanthus

 

Oxalidales          Cepalotaceae                Cephlotus 

 

Caryophyllales Droseraceae                  Drosera

                                                                      Aldrovanda

                                                                      Dionaea

                             Nepenthaceae                Nepenthes

                             Drosophyllaceae            Drosophyllum

                             Dioncophyllaceae          Triphyophyllum

 

Ericales             Roridulaceae                  Roridula

                             Sarraceniaceae             Darlingtonia

                                                                      Heliamphora

                                                                      Sarracenia

 

Lamiales            Plantaginaceae              Philcoxia

                             Linderniaceae                Lindernia

                             Byblidaceae                   Byblis

                             Martyniaceae                  Ibicella

                                                                      Proboscidea

                             Lentibulariaceae            Pinguicula

                                                                      Genlisea

                                                                      Utricularia

 

  

Copyright (c) 2017 Makoto Honda. All Rights Reserved.

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