Clever Plans in Plants
Have you ever read The Enormous Crocodile by Roald Dahl? It is such a fun story! Even grown ups laugh about this nasty scheming reptile who keeps devising new and ingenious ways to catch children for his supper. He describes his schemes as “secret plans and clever tricks.” Various jungle animals however foil each sneaky plan and the nasty croc eventually gets launched by an elephant into outer space.
That crocodile reminds me of some interesting plants which are not particularly showy to look at but which have been endowed with clever devices. Consider the sundews for example. In Alberta you have to look in peat bogs for this tiny plant. Drosera rotundifolia consists of a leafy flat rosette smaller in diameter than a Canadian dollar coin. A rosette is a circle of leaves arranged close to the ground such as you see in dandelions or thistles in a mown lawn. Since the sundew rosette is so small, you have to look very carefully to find it. The best time to find it is when the plant blooms. It then sends up a flower stalk that may be as much as 25 cm tall. These plants don’t look threatening at all. They actually appear placid and pretty. But sun dews are carnivorous (meat-eating) plants.
The leaves of most sundews have a pinkish or even conspicuously red appearance. That comes from tentacles which bristle from the leaf surface. From the rounded red coloured tip of each tentacle the plant secretes a clear, bright, non-drying adhesive which smells like honey. Yum! Yum! Insects come from far and near. As a hapless victim touches a glob of glue, it becomes stuck. Drosera, the Latin name for this plant, means “dewy”, but insects quickly discover that appearances can be deceiving. Once the victim is stuck, all of the leaf’s other tentacles bend over it so that the victim suffocates in glue. Now new compounds ooze from the tentacle heads. This time it is enzymes that digest the soft tissue inside the insect. These nice nutritious insec t products are now absorbed by the same tentacle heads. The plant has thus helped itself to some badly needed nutrients in the form of nitrogen, potassium, phosphorus and sulfur. Once there is nothing more to be extracted from the victim, the tentacles pull back. With tentacles now upright again, the insect exoskeleton blows away and the leaf is ready for further action.
It may seem that sundews go to a lot of trouble to obtain nutrients that most plants simply absorb from the soil. The problem for the sundews however is that bogs are acid environments, extremely poor in soil nutrients. As one author reported “the average patch of bog inhabited by sundews has 27 times less nitrogen than the average patch of pine forest. Around the world, habitats of carnivorous species tend to fit the same pattern – plenty of water, plenty of sun, terrible soil.” (David Quammen. 1984. Outside, November issue p. 32). Sundews obviously are making the best of a bad situation. Rather than waiting for somebody to add fertilizer to the soil, which wouldn’t help much in such acid environments anyway, they catch their own nutrients which are “on the wing.”
The amazing capacity of sundews was noticed in England on a beautiful August day in 1911 when naturalist F. W. Oliver set out on a hike near the southern coast. He came across a hectare sized meadow covered in sundews. These however were not hungry sundews. Each plant had captured between 4 and 7 cabbage butterflies. Apparently a flock of these insect pests had just crossed the English channel from the continent. Happening soon after upon the festive-appearing field of sundews, the butterflies alighted, never to budge again. It appeared that this stand of carnivorous plants had finished off about six million butterfly pests.
Sundew plants grow on every continent except Antarctica. Of the 100 or so known species, more than half grow in Australia. Anyone familiar with the status of Australian soils will not be surprised. On that continent the good soils are those that need only be fertilized with superphosphate in order to grow crops. Most farm land requires addition of nitrogen and other mineral nutrients as well. This gives you an idea of the condition of Australian soils in general. It is little wonder therefore that sundews are so much more common there than in other parts of the world. When I and my husband were in Australia recently we saw many sundew plants. Naturally we took some pictures. See how you like them.
Another hungry plant has been in the news lately. This one, called Genlisea, has long been considered carnivorous because of some elaborate structures that could be traps. Nobody knew for sure, however. Hanging down from the leafy rosette, in a position where the roots should be but aren’t, instead are modofied leaves in the form of upside down Y’s. The two branches of the Y are each twisted in a spiral along the length of the branch. If you were to take a pipe cleaner in your fingers and twist the two ends in opposite directions until a tight spiral had been produced, you will understand what each branch of the upside down Y looks like. In addition along the spiral there is a ridge marking the location of a slit that runs the length of the branch. Tiny hairs inside the slit face inward. Laboratory experiments have demonstrated the function of this unusual structure. Apparently these plants release some kind of chemical that is very attractive to single cell animals (protozoans). They swim happily towards the traps. Once inside they swim only upstream because the hairs prevent easy escape. Their destination is a swollen digestion chamber. Soon nutrients from digested protozoans appear throughout the plant.
These small plants grow in nutrient poor white sands and moist rock outcrops in South America and tropical Africa. Obviously their traps are very precise and very dainty when they catch microscopic creatures and digest them before they can escape. Previously biologists had wondered why victims did not simply swim out again since there was no trap door to keep the victims in. The protozoans however simply take the route of least resistance and by the time danger is apparent, retreat is impossible because they are now too far inside the plant. (Besides protozoans are probably not too smart!)
Anyway our interest is in the ingenious methods that these meat-eating plants employ to supplement their nutrition. The tentacles in the sundew are not only pretty and sweet smelling, but they release a non-drying glue and they are able to move inward to further entangle the prey. These are unusual talents for a plant. Then not only do the tentacle heads next release enzymes to digest insect insides but they also are able to absorb the resulting nutrients. This is an all-or-nothing system (irred ucibly complex). If any component is lacking, that plant is not going benefit from any insect protein. Similarly Genlisea replaces roots with underground leaves changed into “extravagant trap-like structures” (Barthlott, Porembski, Fischer and Gemme. 1998. Nature 392 April 2 p. 447). In addition the plant releases chemicals attractive to protozoans, luring them into unique traps and upward to a distant dark digestion chamber. The finesse of these plants is truly awe-inspiring. Indeed they a re wonderfully designed. It is most appropriate to give praise to God, the maker of these organisms. What other wonderfully created organisms have you noticed recently?
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