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 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.
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).
Depending on the types of traps, different kinds of glands perform different
functions, or the same gland assumes multiple tasks.
- 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)
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: Group 1 and Group 2. 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 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 is not necessary for digestive process, as long as the 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 central tentacles, allowing numerous sessile glands found there to participate in the absorption as well.
For various CP genera below, we present the
anatomy of carnivory in four steps showing the basic mechanisms involved.
Oxalidales Cepalotaceae Cephlotus
Caryophyllales Droseraceae Drosera
Ericales Roridulaceae Roridula
Lamiales Plantaginaceae Philcoxia
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