Anatomy of Carnivory
2017-04-14 Under construction ------------------ Anatomy of carnivory
For carnivory to take effect, there must be contact between leaf and prey. If a fly lands 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 -- the 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) but some other elements required in trace amounts such as potassium (K: kalium) and magnesium (Mg) are also utilized by some species.
For various CP genera below, we present the anatomy of carnivory in four steps showing the basic mechanisms involved.
- food (nectar glands)
- 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)
- digestive glands secrete enzyme - the Golgi apparatus (how many times.../how soon)
- commensals (bacteria,,,, / commensal bugs / mutualism)
- no glands (cuticular discontinuity)
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 are also used for secretion of viscous mucilage (or resin in the case of Roridula). Generally, adhesive secretion can repeat many times, but can occur only once in Pinguicula’s adhesive glands. 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. Also, the secretion of the 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). The absorption of the products of digestion is usually carried out by the same digestive glands.
MOVEMENT in CAPTURE - We
note that in the course of adaptation to carnivory, there are two distinct points at which a 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
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...
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...
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 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. Give 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 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.
ANATOMY OF CARNIVORY LURE / CAPTURE / DIGESTION / ABSORPTION (illustrations)
Oxalidales Cepalotaceae Cephlotus
Caryophyllales Droseraceae Drosera
Ericales Roridulaceae Roridula
Lamiales Plantaginaceae Philcoxia
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