The concept of a plant that traps and partially consumes small animals was suggested and studied long before 1875, when Darwin's book Insectivorous Plants appeared. In that volume Darwin correlated a great deal of the knowledge of his day and added the results of his own numerous experiments and observations. Since then, interest in carnivorous plants has grown remarkably. Concurrently with this growth, new discoveries and ideas have arisen, and these have further increased interest in carnivorous plants. Imaginations have also soared, but stories of giant or man-eating plants have proved to be entirely mythical.
Green plants can derive a large part of their chemical requirements for sustaining growth and reproduction from some very basic but essential elements. These include energy from sunlight, oxygen and carbon dioxide from the air, and water and certain minerals from the earth or water in which the plants grow. Through photosynthesis, in which green chlorophyl plays so large a part by transforming sunlight into chemical energy, carbohydrates are built up from water and carbon dioxide. These various carbohydrates themselves are used as energy sources and building blocks for synthesizing myriad other chemical materials needed by the plant —products such as amino acids and proteins, vitamins and hormones, and even small quantities of fats.
During the entire complex process, which may be likened to an automated chemical factory that goes on and on as long as raw materials are supplied and end products removed, many minerals are absorbed by the plant's root system for ultimate inclusion in chemical end products. Examples of such minerals are compounds of nitrogen, phosphorus, potassium, calcium, magnesium, iron, manganese, boron, and several other elements needed in such minute quantities that they are seldom deficient in the environment.
During millions of years of evolution, plants have shown a clear capacity for adapting to different habitats which may be deficient in one or more of the basic requirements. For example, certain plants are able to grow and function very well in the deep shade of mature forests, where sunlight is quite reduced, because their photosynthetic apparatus functions more efficiently than that of other plants. Not only do such shade-loving plants thrive on the forest floor, but they often do poorly if exposed to full sunlight. As another example, in dry desert regions all over the world plants have adapted to very low water levels throughout much of the year. This is accomplished by various developments: deeper root systems, chemical monitoring of the spacing of plants by which no one plant absorbs too much water from a unit of earth, reduced rates of water loss by changes in leaf chemistry and shape, or the adoption of a life cycle that fits into the brief period of annual downpour when the desert blooms and plants are able to grow and reproduce very rapidly before the land dries out again. There are many more examples of adaptation to what for most plants might be termed difficult environments.
The kinds of deficient habitats that will mainly concern us throughout this book are the usually acid, mineral-poor bogs and swamps, and the freshwater marshes and savannahs. It is in such locations that plants that have adopted carnivory may be found.
Anyone who sees a freshwater bog or swamp is impressed by the great variety of plant life—including many mosses, ferns, and orchids —and by the seem ingly rich, black ooze of the wet soil in which the plants grow. But accurate chemical analysis of the soil in such areas frequently reveals that this initial impression is partly erroneous. First of all, the coffee-brown waters are very acid, and acid water along with frequent drenching rains quite efficiently leaches out many irreplaceable minerals. Second, in warm climates there is a very high rate of bacterial and other microorganic activity which in itself uses up the sparse supply of minerals that are needed by the larger plants. In cool climates such decay is delayed, but then the undecayed remains of dead plants and animals keep the much-needed minerals locked up. Third, a close examination in most cases confirms that the black ooze is little more than fine white sand along with a great deal of chemically sterile carbon or charcoal-like material, or the latter without sand. It seems that in such mineral-poor habitats there must have been considerable adaptation by all the plants in order for them to grow and reproduce.
To adapt to these environments deficient in many minerals and possibly in some cases to overcome the inability of their root systems to absorb required minerals, some plants retained the evolved capacity to trap and digest small animals. From the trapped animals, which are largely insects, the plants absorb what they need. As a result of selective adaptation, the plants that were able to trap animals survived and produced offspring with the same genetic characteristics.
The acceptable word "prey" is usually used in reference to the entrapped animals, but it is not literally correct in that the plants do not actively stalk and capture food in the sense that many animals do. Rather, the plant is able to lure or take advantage of incidental nourishing visitors by means of one of four kinds of trapping mechanisms, which will be discussed below.
After entrapment, the prey undergoes digestion. From a chemical standpoint the digestive process is quite similar in many respects to digestion in animals. Also, various microorganisms such as bacteria aid many carnivorous plant species in breaking down the prey into simpler, absorbable substances.
In the decades immediately before and after the turn of the century, many experiments were contrived to prove that digestive activity actually occurs in plant traps and to measure and define the nature of that activity. Some of these experiments were quite elaborate and their results still stand. For digestion to occur, certain enzymes must be present. Enzymes participate in the chemical reactions of biological organisms by causing the reactions to be completed rapidly at temperatures suitable for the maintenance of life. These reactions include the synthesis of more complex compounds as well as the reduction that occurs in digestion. The results of many experiments indicate that enzymes are responsible for digestion in the traps of carnivorous plants.
The next question concerns the source or sources of these enzymes: Did they originate from the plants themselves, secreted into the trap along with fluid as a response to entrapped prey of a suitable nature; or were they simply products of bacteria or fungi inhabiting the decaying detritus accumulated in some open traps? Like most questions of this nature in science, a categorical "either-or" answer is impossible, and it would be misleading to attempt to give one. It has been shown that some species of carnivorous plants have a complex enzyme-secreting system in small, specialized plant glands associated with the trap. Others with similar glands secrete practically no enzyme under sterile experimental conditions where the contribution of any microorganism can be discounted. And some plant traps function with no glands at all. At this stage, the answers are far from complete. Some plants seem to rely almost exclusively on their own enzymes, some seem to depend almost totally on bacterial action, and others take advantage of both sources.
Another question concerns what digestive products are actually absorbed by the trap of the carnivorous plant, which of these are truly required by the plant, and which are just passively absorbed. A second, related question is whether all the useful materials absorbed by the plant are simple minerals which may be lacking in the plant's habitat, or whether some are more complex, synthesized materials needed because, as a result of evolutionary change, the plant has lost the capacity to produce them. The individually studied cases are few and far from complete, so again we can give only some partial answers—merely clues in a highly complex problem that involves more than curiosity about carnivorous plants and actually cuts across the whole problem of the nature of adaptation.
Of all the mineral elements mentioned previously, the one that green plants need most consistently and in the largest amount is nitrogen, followed by phosphorus and potassium in more variable quantities. Acid soils are also quite deficient in calcium. All these elements are retained by "sweet" or basic soils —thus the gardener adds lime (a calcium compound) to "sweeten" or enrich soil that is too acid to permit most plants to grow well. Much research has centered on the idea that nitrogen is the limiting factor, or element most needed by carnivorous plants for sustenance and growth in their deficient environment, probably because nitrogen has long been prominent in soil and plant chemistry. But insufficient work has been done to establish the exact role that some other minerals or combinations of minerals may play in plant carnivory. For example, recent preliminary observations indicate that potassium levels in soils, plants, and prey greatly influence the amount and rate of nitrogen absorption by carnivorous plants.
Very early attempts were made to find out whether anything could actually be absorbed by carnivorous plants. Researchers utilized harmless dyes which could be followed visually in their course through the plants. The air surfaces of most plants are covered by a thick, waxy layer called cuticle. The absorption of watery materials through a waxy layer of cuticle varies from slow to impossible. It was noted quite early that the absorbent interior surfaces of the traps of carnivorous plants lack cuticle. It therefore was possible to follow the dyes visually in their course through the plants. These were important preliminary results. Of course the experiments had very severe limitations.
Later, with the advent of radioisotope tracers wherein various portions of a material can be tagged with radioactivity and followed through the plant and in actual chemical changes in plant tissue, it was possible to conclude that absorption of certain materials did take place and that these materials were actually used by the plant tissues—that is, the substances did not just passively enter the plant tissues. So far these studies have been limited to nitrogen compounds, and we have only the published reports of studies by one worker using one species of carnivorous plants out of the forty or so on this continent alone; but it is a beginning, and it is certainly indicative that carnivory must be of some benefit to the plant.
Additional work on more general levels suggests that some carnivorous plants can subsist without trapped and digested animals, or that minute quantities of suitable fertilizers can be substituted by applying them to the roots, the trap interiors, or even the external leaf surfaces. However, a common observation in such experiments is that the plants are not as vigorous as in nature: they grow more slowly and do not become as large; they are more prone to disease; and very importantly, they do not reproduce as well, as is indicated by the production of fewer flowers and seeds, a reduced rate of seed maturation, and less rhizome budding.
So far, we have looked at carnivory from the viewpoint of an isolated, experimental plant. But plants occur in nature with other similar and dissimilar plants, with animals, and with an inanimate environment as parts of a community. There results a complex interaction of so many factors that one is awed and baffled in one's first attempts to picture the situation in perspective. The picture is further complicated by the fact that biological communities are not static; they are always varying and responding to assault and change. When a prime environment for carnivorous plants changes from wetland to grassy field, scrub, or forest as a result of natural or man-made activities, carnivorous plants and many of their wetland companion species disappear somewhat promptly, often in a rather specific order. They are apparently crowded out by forms more vigorous and better adapted to what has become essentially a new environment. It seems that carnivorous plants require the poor soils of an acid wetland to be competitive, soils where many other plants that under different conditions would be strong competitors cannot grow. When dryland plants that demand richer soils are finally able to spread into a reduced bog or marsh, carnivorous plants become the disadvantaged forms and disappear.
This is not so difficult to understand or accept in broad terms if viewed from a simplified but largely valid evolutionary angle. We began this section by mentioning the adaptation of plants to differing sit uations. Not all plants able to grow in the environment of an acid, mineral-deficient wetland adopted carnivory. Evolution seldom narrows to one pathway or one structural adaptation to solve a problem. Variation and gradual migration are the keys to the continuation of some life forms in a continually changing environment.
CARNIVOROUS OR INSECTIVOROUS PLANTS?
I will not belabor the point as to whether these plants should be called "carnivorous plants" or "insectivorous plants," but I will mention it lest the reader become confused by the use of both terms in conversation or in other publications. When carnivorous plants were first noticed and studied, the most obvious prey was insects; hence the term insectivorous plants. Later, species with more varied appetites were found. Skeletons of small birds and amphibians were found in some traps, and aquatic plants trapped small water animals that were clearly not insects. Thus the term carnivorous plants was coined to be more general and inclusive, and more accurate. It is the preferred term and the one we shall use throughout this book.
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