Unless otherwise noted, information contained in each edition of the Kansas School Naturalist reflects the knowledge of the subject as of the original date of publication.
Volume 30, Number 4 -
ABOUT THIS ISSUE
Published by EMPORIA STATE UNIVERSITY
Prepared and Issued by THE DIVISION OF BIOLOGICAL SCIENCES
Editor: ROBERT F. CLARKE
Editorial Committee: Gilbert A. Leisman, Tom Eddy,
John Parrish, John Ransom
Online edition by: Terri Weast
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Kansas 66801. The Naturalist is edited and published by Emporia State University, Emporia, Kansas. Editor, Robert F. Clarke, Division of Biological Sciences.
by Gilbert A. Leisman
"People were surprised and a little disgusted, to learn that the species was carnivorous, and that the flies and other insects caught in the cups were actually digested by the sticky substance there. We in temperate zones were not ignorant of insectivorous plants, but we were unaccustomed to find them outside special hothouses, and apt to consider them as in some way slightly indecent, or at least improper. But actually alarming was the discovery that the whorl topping a triffid's stem could lash out a slender stinging weapon ten feet long, capable of discharging enough poison to kill a man if it struck squarely on his unprotected skin...
But there were a number of not unobvious characteristics which escaped comment for some little time. It was, for instance, quite a while before anyone drew attention to the uncanny accuracy with which they aimed their stings, and that they almost invariably struck for the head. Nor did anyone at first take notice of their habit of lurking near their fallen victims. The reason for that became clear when it was shown that they fed upon flesh as well as upon insects. The stinging tendril did not have the muscular power to tear firm flesh, but it had strength enough to pull shreds from a decomposing body and lift them to the cup on its stem."
The Day of the Triffids
John Beynon Harris
Man-eating plants? Rubbish! Science fiction? Obviously! Pure poppycock? Not entirely. There are, indeed, plants which are capable of digesting trapped insects, as indicated in the first sentence of the above quotation, and which can even digest other very small animals as well. In view of their somewhat varied diet, it is perhaps more appropriate to refer to them as carnivorous rather than insectivorous.
Most flowering plants, of course, carry on photosynthesis and this is usually the only process for producing energy-rich compounds, like sugar. In such plants the various nutrient elements needed for metabolism are typically absorbed from the soil along with water. Carnivorous plants carry on photosynthesis, and most of them (Bladderwort is an exception) are anchored to the soil and absorb nutrients and water from the soil just as ordinary plants. Why then do carnivorous use animal protein as a part of their diet? Are animal proteins essential to such plants? Do these plants grow in some special type of environment that requires
a protein supplement? Questions such as these have intrigued scientists for years; even Charles Darwin, in 1875, published a book entitled Insectivorous Plants.
Most of the answers to the above questions are probably contained in the characteristics of the habitats in which these plants grow. Typical habitats include acid, mineral-poor bogs and swamps, and freshwater marshes and savannahs. The waters of such habitats are usually dark brown in color and are very acid. This acidity greatly facilitates the leaching out from the soil of many irreplacable minerals. Temperature may also play a role in nutrient availability. Warm temperatures encourage the growth of bacteria and fungi and hasten the breakdown of organic matter to available nutrients, most of which are utilized by the bacteria and fungi themselves, with the remainder subject to rapid leaching. In cool temperatures, decomposition slows down perceptibly and the nutrients remain bound and unavailable. The key factor in all of this is obviously poor nutrition.
The next question is which nutrients are in short supply. Since nitrogen is a critical component of protein, and an element most frequently in short supply in soils, a great deal of attention has been focused on it. And, indeed, numerous experiments have confirmed that the addition of nitrogen adds considerably to the vigor of the plants. However, current research seems to indicate that other minerals are perhaps equally as important as nitrogen, especially phosphorus and potassium. Both have been shown to play an important
role in the rate of absorption of nitrogen from animal protein.
Research has also shown that carnivorous plants can be grown in the absence of animal protein. It has also been shown that suitable fertilizers can be supplied to the roots, leaves, and even to the traps themselves, and be absorbed by the plants. However, in both cases (complete absence or artificial fertilization) growth of the plants is slow, flowering and fruiting is greatly reduced, and budding (vegetative reproduction) is lessened. Apparently, there is something in the insect or other animal that we have not identified that is essential for optimum. The search for that elusive something is the stimulus for continuing research on these unusual plants.
There are about 250,000 species of flowering plants on the earth. Of these, about 400 are known to be carnivorous. They belong to six families and 13 genera. In all cases it is the leaf that is modified to form the "trap". With 400 different plants being carnivorous, it would seem logical to find structural variation in the leaf trap. And, indeed, there is. However, most of them share two things in common: there is some mechanism or device to secure the insect in or on the trap and there is the secretion of digestive enzymes to break down the animal protein.
Basically, carnivorous plants can be classified as active or passive trappers (Figure 1).
Fig. 1. Active (A, B) and passive (C, D) trappers among carnivorous plants. A is bladderwort which has bulbous suction traps. B is Venus' fly trap in which the halves of the leaf dose together. The pitcher plant (C) has a pool of digestive juices at the bottom of the funnel-like leaf. Sundew (D) has sticky glandular hairs on its leaf surface. (From Carnivorous Plants, Y. Heslop-Harrison, copyright 1978 by Scientific American, Inc. All rights reserved.)
These are plants which employ a rapid plant movement to entrap and secure the insect.
1. Closing traps - We have only one species in the western hemisphere that qualifies under this heading, the Venus' flytrap (Dionaea) (Figure 1B). It has a very restricted range, being found only in the savannahs of southeastern North Carolina and northeastern South Carolina (Figure 2). Each leaf consists of two halves hinged together like a clamshell. Around the margins are numerous rigid and pointed hairs or teeth. The upper surface of the leaf blade is covered with glands which often serve two purposes. They frequently become bright red in color, thus attracting insects, and they also secrete digestive enzymes. Also present on the upper surface are trigger hairs, usually three per half. When two of these hairs are touched in succession (usually by an insect), the two halves of the leaf quickly close, the marginal teeth intermesh, (Figure 3) and the insect is trapped inside to be slowly digested by the glandular enzymes. Once the insect is fully digested, the leaf opens up and the indigestible skeleton is blown away.
Fig. 2. Distribution map of Venus' flytrap.
Fig. 3. A. General view of Venus' flytrap plant.
B. Close up of open leaf on left and closed leaf with insect trapped inside on right.
2. Trapdoors - The only representative that we have of this category is the bladderwort (Utricularia). However, it is a large genus, containing some 250 species and most, if not all, are carnivorous. They are worldwide in distribution and, while they do grow in a variety of habitats, most are aquatic. The modified leaves or traps are bulbous, very small (up to 3.0 mm) and have a trapdoor at one end. The opening and closing of this trapdoor is quite complicated, and several of the details will be omitted here. The door is surrounded by a ring of tactile hairs which are very sensitive in their response. During the resting stage, the door is closed, and much of the water inside the bladder is absorbed so that a negative pressure or suction results. If a water flea, for example, were to swim along and touch the ring of hairs, the door would spring open inwardly, water and the flea would be sucked into the bladder, and the door would slam shut (Figure 1A). Amazingly, all of this takes place in only 1/460 of a second! Once inside the bladder, the flea is slowly digested by glandular enzymes secreted by the inner lining.
This fantastic speed of the bladderwort trapdoor and the rapid closure of the Venus' flytrap have been the subject of much speculation. No normal metabolic process would seem to account for such rapid response. Current ideas center around an electrical action potential which can trigger an instantaneous change in turgor pressure of the cells involved in the response.
These are plants that, for the most part, do not require any motion as part of their trapping response. Some, like the sundew, do respond, but only very slowly, and the response is really not all that essential to the trapping process.
1. Pitfalls - These are the so-called pitcher plants, including such genera as Sarracenia and Darlingtonia. Here the leaves are modified to form tubes or closed funnels with a mixture of water and (usually) digestive enzymes forming a pool at the bottom. The prey is lured by one method or another to the mouth of the pitcher, enters or falls in, and is digested in the pool at the bottom.
The genus Sarracenia is by far the more common of the above two genera, being found in bogs and marshes in much of northern and eastern North America (Figure 4). There are eight generally recognized species; unfortunately, they have a tendency to hybridize readily which makes identification a bit sticky especially in areas of overlapping species.
Fig. 4. Distribution map of most widely distributed species of pitcher plant (S. purpurea).
Darlingtonia is a western genus found in the coastal bogs and mountain slopes of Oregon and northern California (Figure 5); there is only one species.
Fig. 5. Distribution map of Darlingtonia.
The general structure of Sarracenia pitcher plants is quite similar from species to species. The tube or funnel-like leaves are topped by a lid or hood. Except in one species, this hood is reflexed over the pitcher opening and may prevent the entrance of excessive rainwater. Running down the length of the outer surface of the leaf and facing the center of the cluster of leaves is a fairly prominent wing or ala whose exact function is not known.
There are a number of interesting adaptations to both lure and entrap prey. The pitchers are often brightly
colored - green, various shades of red, yellow, and white (Figure 6). The margin of the hood has numerous nectaries which yield a sweet-smelling liquid. Once inside the opening the prey finds it virtually impossible to get out. The inside of the hood is covered with stiff downward-pointing hairs which prevent egress. Inside the pitcher itself the inner lining of the upper third or half consists of a smooth waxy cuticle which prevents any kind of a foothold. As a result, the only possible consequence is an unwelcome dip in the enzymatic pool at the bottom (Figure 1C). Special glands lining the pool area secrete proteinaceous enzymes, and the amount of enzymes appears to be directly proportional to the number of prey. To make matters worse for the prey another ring of downward-pointing hairs is present near the bottom, ensuring permanent entrapment.
Figure 6. Pitcher Dark veins on leaves are bright red in nature, presumably to attract insects.
Figure 7. Darlingtonia, with flower stalk at left.
Darlingtonia pitchers are similar to those of Sarracenia except for a couple of features. At the top of the pitcher is a prominent globose hood which is reflexed so that the opening to the pitcher faces downward. From the outside edge of the opening a two-lobed or forked appendage extends downward (Figure 7). Viewed from the side, the whole pitcher and its appendages looks somewhat like a cobra, hence the common name cobra plant. Finally, the pitchers of Darlingtonia twist 180 degrees in either direction so that they wind up facing away from the center of the cluster.
Internally, the major difference is the absence of glands any place in the pitcher. Presumably the prey is decomposed by microorganisms and the nutrients absorbed through the lining of the pitcher.
Fig. 8. Pitchers of Nepenthes borne at end of tendrils.
There is at least one other pitcher plant worthy of mention since one frequently sees it growing as a curiosity in greenhouses. This is the tropical pitcher plant Nepenthes. It is a vine-like plant, growing to 50 or more feet in height. The vine produces long flat leaves which produce very elongate tendrils from their tips. In turn, the tips of the tendrils frequently, but not always, produce pitchers (Figure 8). Each pitcher has a hood or lid which is usually brightly colored and covered with nectaries, undoubtedly to attract insects and other prey. The mouth of the pitcher has a prominent ribbed collar; at the base of each rib is a downward-projecting tooth. Below the teeth is a smooth waxy area, and below that a digestive zone with water and glandular-secreted enzymes, just as in most other pitcher plants (Figure 9).
Fig. 9. Longitudinal section of Nepenthes.
1. "Flypaper" or adhesive traps - Sundew (Drosera) is the best known of this group of carnivorous plants (Figure 1D). Since many sundews are small in size (rosettes are frequently less than 5 cm across), they are frequently overlooked in their habitats. The upper surfaces of the leaves are covered with numerous sticky glandular hairs which immobilize any small prey that comes in contact with them. Following entrapment, many of the surrounding glandular hairs bend toward the prey and the leaf itself may fold so as to bring as many glands as possible in contact with the prey. This is a slow process, however, and is part of digestion,
not entrapment. The glands, of course, secrete enzymes, with those in the center of the leaf being most productive. There are about 100 species of sundew, and they are worldwide in distribution (North America - Figures 10 and 11).
Fig. 10. Distribution map of one of the northern species of sundew, the roundleaf sundew (D. rotundifolia).
Fig. 11. Distribution map of the southeastern species of sundew, the shortleaf sundew (D. brevifolia).
Another much less familiar flypaper plant is butterwort (Pinguicula). There are 48 known species, and in North America our species are mainly confined to Canada (Figure 12) and the southeastern coastal area of the United States (Figure 13). A typical butterwort produces a small rosette of flattened or slightly fleshy leaves (Figure 14). Like the sundew, the upper surface of the leaves are covered with glands, both stalked and sessile. The stalked gland is primarily responsible for entrapment and the sessile gland for digestion (Figure 15).
Fig. 12. Distribution map of northern species of butterwort (P. vulgaris).
Fig. 13. Distribution map of southeastern coastal species of butterwort (P. pumila).
Fig. 14. Rosette of leaves of butterwort (Pinguicula).
Fig. 15. Scanning electron micrograph (SEM) of surface of butterwort leaf showing large, stalked, entrapment glands (black arrow) and small, sessile, digestive glands (white arrow).
CULTIVATION OF CARNIVOROUS PLANTS
Contrary to popular belief, a majority of these plants lend themselves readily to cultivation with a minimum of time and effort. Space does not permit going into detail, but an excellent source book is Carnivorous Plants, Adrian Slack, 1979, MIT Press. Also contrary to belief, you don't have to "feed" these plants hamburger or filet mignon. Even the most fastidious of households has enough insect life to satisfy most of these plants. As a matter of fact, why pay an exterminator to come in each spring and fall to get rid of the "bugs"? Put a few carnivorous plants around the house, they'll do the job efficiently, and they won't charge even a farthing!
One final word of caution. Do not collect carnivorous plants from the wild. This is important because these plants play a special role in a special type of environment. The plants are not common, in many places they are rare, and in many states they are protected by law. Man seems to have a special talent for disrupting and even destroying natural environments. And a carnivorous plant bog is an especially fragile one. Please, let's leave this one alone.
There are several nurseries that offer carnivorous plants for sale. Some are listed in the book by Slack
above; others are listed in Schnell, annotated in the reference list below.
These nurseries, of course, had to start with wild plants, but all propagation of them has been within the greenhouse. A particularly promising propagation technique that is being used today is tissue culture. This is a cloning procedure in which small bits of tissue from a parent plant are induced to grow into completely new mature plants, all just like the parent. Venus' flytrap is particularly well-adapted to this technique, which accounts for its frequent appearance in stores as a novelty plant.
OTHER SELECTED REFERENCES
Heslop-Harrison, Yolande. 1978. Carnivorous Plants. Scientific American 238: 104-115.
Lloyd, F.E. Carnivorous Plants. Dover Books.
Mellichamp, T. Lawrence. 1983. Cobras of the Pacific Northwest. Natural History, April issue: 46-51.
Schnell, Donald E. 1976. Carnivorous Plants of the United States and Canada. Blair, Publisher.
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