CARNIVOROUS PARASITIC AND SYMBIOTIC PLANTS

Epiphytic plants, those which perch high on the branches of forest trees, certainly obtain more light for photosynthesis than their forest-floor cousins but they suffer the grave disadvantage of having little or no soil from which to get nutrient minerals and water. Some of the epiphytic orchids of tropical rain forests have overcome this problem by the use of aerial roots which absorb moisture directly from the humid atmosphere around them. Plants such as Nepenthes, the pitcher plant, have adopted a different approach to the problem.

Species of Nepenthes, whose range extends from Madagascar to North Australia, are to be found in wet jungles, growing either rooted in swampy soil or as epiphytes. The former species may send climbing shoots from their rhizomes in the soil to the tops of trees 15m (50ft) or more high. The basal part of each leaf of a pitcher plant extends into a tendril and ends in a vertically held pitcher. The pitchers range in size, depending upon the species, from structures that can hold 10 millilitres (0.02 pint) of liquid if filled to capacity to ones which can hold well over 1 litre (1.75 pint). The pitchers do contain liquid but only to about onethird of their total capacity and it is this pitcher liquid which holds the key to the plant’s nutritional success. The pitcher is an insect trap and the acidic liquid contains enzymes including trypsin which break down the proteins of the trapped insect to a form that can be absorbed by the plant and utilized as a source of nitrogen and phosphate.

Each pitcher is somewhat tubular, swollen below and constricted above. During development the mouth of the pitcher is sealed by a lid which is eventually raised slightly above the pitcher’s oblique mouth and serves both to keep out the ram and to attract insects to the nectaries on its underside. The mouth has a hard glossy ribbed rim which ends in sharply-pointed teeth on the inside of the pitcher. Between these teeth are large nectar glands and below them, inside the pitcher, is a zone covered with flakes of wax. The trap is set! The insect is attracted to the plant by the promise of nectar and, reaching for the nectaries just below the rim, but unable to grip on the glossy surface, it falls into the digestive liquid below. It cannot scale the waxy pitcher sides or the downward directed teeth and so it dies and is digested. So effective is this trap that, in the course of evolution, these pitchers have become the sole habitat of several species of spiders, mosquitoes, gnats, small crustaceans, protozoans and algae, all of which have evolved to overcome the problems of living in such a potentially hostile habitat.

Carnivorous plants

Plants which live on peat bogs have nutrient deficiency problems similar to those faced by epiphytes, for they live on very wet areas dominated by species of bog moss (Sphagnum) and often raised above the level of the surrounding land like a giant sponge with the result that their only water supply comes from the rain and snow falling directly upon them. The few nutrients that are brought in in this way are rapidly taken up by the Sphagnum leaving little for the other inhabitants of the bog. In such a situation the carnivorous (or insectivorous) habit has again developed.

The peatlands of North America support ten or so species of the genus Sarracenia, characterized by pitchers which differ from those of Nepenthes in that all the leaves emerge from ground level in pitcherlike form and have conspicuously coloured upright lids. Their mode of operation is very similar to that of Nepenthes although it differs somewhat in Darlingtonia, a relative of Sarracenia, where insects are attracted by nectar to a downwardly-directed mouth and then fall into the trap because they try to escape through translucent ‘windows’ on the far wall of the pitcher.

North America also provides one of the most spectacular of the insectivorous plants, Venus’ fly trap (Dionaea muscipula). This plant has a modified leaf blade consisting of two lobes each fringed with stiff spikes. Each lobe has a marginal zone of nectaries whilst the rest of the surface is covered by deep-red digestive glands and three stout ‘trigger bristles’ per lobe. Attracted by the nectar, insects land on the leaf lobes. The triggers are not as simple as they seem, for this is the only plant that can count! If one trigger only is touched—as it might be by a raindrop, or a small insect not worth bothering with-—nothing happens. But if one is touched twice in succession, or two of the three hairs are touched, the trap operates. Within a second the lobes have closed together and the marginal bristles interlocked and the insect is trapped, held close against the digestive glands which begin to secrete digestive enzymes.

Charles Darwin, who published a book on insectivorous plants in 1875, demonstrated the large quantity of digestive fluid produced by Venus’ fly trap by making a small hole at the base of a leaf lobe after an insect had been caught. He found that the digestive fluid was produced in sufficient quantity to flow down the leaf stalk for nine days.

Darwin also demonstrated, perhaps not unexpectedly, that the more food the common European sundew (Drosera rotundifolia) took in through its leaves the better it grew. This plant of nutrient-poor peatlands has leaves covered with 200 or so long, drumsticklike glands, usually called tentacles. At the tip of each tentacle is a drop of sticky secretion looking like a drop of dew (hence the generic name from the Greek, drosos meaning dew). Any insect landing on the leaf is caught on the sticky tentacles which bend over and envelop the unfortunate creature. It is then digested by enzymes secreted from the tentacles.

From the Northern Hemisphere and South America comes another group of insectivorous plants, the butterworts (Pinguicula species). These plants are to be found in a range of wet habitats and form small rosettes of sticky leaves. Insects become trapped when they land on the sticky butter-coloured substance on the leaf surface. They are ‘glued’ rather than held down while they are digested.

The genus Utricularia, the bladderworts, is related to Pinguicula but possesses a far more spectacular trapping device. Members of this genus can be found in aquatic and damp terrestrial habitats throughout much of the world, but particularly in the tropics. The bladders from which the plants take their name are scattered over the whole plant and are seldom more than a few millimetres long. Utricularia vulgaris, the greater bladderwort of Europe, Asia, North Africa and North America, is a widely studied aquatic member of the genus. In this plant the bladder, prior to trapping, encloses an empty flattened cavity apparently under a slight vacuum. At the mouth of the cavity is an inward-opening valve surrounded by sensitive hairs. If a small crustacean or insect larva happens to touch the sensitive hairs in passing, the valve immediately opens and the victim is sucked in, unable to escape because the valve closes after it. In time the larva dies and its body is broken down to be absorbed via starlike glands which line the bladder. The removal of water and minerals by these glands results in the bladder walls being sucked in so setting the trap again. Some tropical species of Utricularia make use of even temporary collections of water such as those which occur in certain flowers. They can be found straggling in a threadlike manner from flower to flower.

The use of animal traps by plants is not limited to the highly evolved flowering species. The soil fungus Dactylella bembicodes forms constricting rings, the cells of which swell as soon as an eelworm passes through. The worm is tightly held until the fungus invades and digests it. Other related species form sticky branches, to which the eelworms adhere, or spores, which stick to and germinate on the eelworm finally invading and digesting the animal.

The fungi as a whole are a group of plants that do not photosynthesize, so they must rely upon other organisms for food whether these be dead or living. They become parasites if the food is still living and saprophytes if it is already dead.

A little red club no more than a few centimetres high projecting through a mat of moss or grass is often worth careful excavation for below it may be the remains of a caterpillar, still recognizable as such but converted by the fungus Cordyceps militaris to ‘solid fungus’. Another fungus parasitic on insects is Aschcr-sonia. This has been used for the biological control of scale insects in Flo.rida and elsewhere where the climate is humid enough to allow it to grow in a matlike fashion over its host. Possibly the most familiar insect-parasitizing fungus is Entomophthora muscat. It is this fungus which causes the death of flies found adhering to windows that are seldom cleaned. A solid plug of fungus forms inside the fly and kills it. Shortly after the fly dies the fungus bursts through the body wall and large numbers of spores are released, forming a greyish ‘halo’ around the dead fly.

Saprophytes

Fungi are not the only plants to have given up photosynthesis. There is a liverwort, Cryptothallus mirabilis, which completely lacks chlorophyll, except in its spores, and lives saprophytically with the aid of its associated (mycorrhizal) fungi in plant litter. And among the higher plants quite a number have given up photosynthesis for a parasitic or saprophytic way of life.

One family of saprophytic plants, which is found throughout the north temperate parts of the world and also extends as far as Malaya, is that which contains the yellow bird’s-nest (Monotropa hypopitys). This plant has a yellowish waxy appearance and because it needs no light is found in the dense shade of coniferous and beech woodland. The below-ground parts are a nestlike mass of roots and associated mycorrhizal fungi.

Another plant, of similar common name and habit but unrelated, is the bird’s-nest orchid (Neottia nidus-avis) of Europe and Asia. Again a plant not dependent upon light because of its saprophytic habit, it is to be found growing on humus-rich calcareous soils of shady beech woodlands.

Parasites

Total parasites, dependent for all their nutritive needs upon a living host, are widespread in the flowering plant kingdom. Rafftcsia, a parasite on the roots of Cissus vines, is found in Malaya. It produces what is probably the biggest flower in the world, some im (3ft) in diameter and weighing around 6kg (131b). A European relative of Rafflesia is Cytinus hypocistis, the yellow cytinus, which is parasitic on the rock or sun rose (Cistus). It produces a globular head of bright yellow flowers below which are scales of yellow, orange or scarlet. The flowers emerge just above the soil surface near the roots of the host.

There are about 100 species of the parasitic plant the dodder (Cuscuta) in the world, many of which are of considerable economic importance because of the damage they do to crops. C. campestris from North America intertwines amongst the stems of lucerne or alfalfa to such an extent that harvesting is difficult. Except for support immediately after germination dodder has no roots but winds around the stem of its host sending out haustoria or suckers into the stem in order to tap the nutrients passing along in the phloem. The flowers frequently smell of rotting meat to attract the flies which pollinate them.

The broomrapes are a large family of parasitic plants found throughout the Old World, with one American species, Orobanche ludoviciana, which was at one time relished as food by the

Pah Ute Indians. They are, fortunately, less important as crop pests than they used to be due to modern methods of cultivation, but some species which attack crops such as clover and peas are still often found in less-developed countries, and there are some tropical species which are still extremely destructive.

Hemiparasites

There is a large group of plants belonging to the family Scrophulariaceae which have not given

Toothworts (Lathraea) are parasitic plants of both Europe and Asia and were once thought by taxonomists to be closely related to the previous example. They are now, however, considered to be near relatives of the hemiparasites described below. Whatever their affinities, tooth-worts are fascinating plants. L. clandestine! produces no aerial shoot and even some of its flowers may remain below ground level. L. sqitamaria has a stout erect flowering shoot with scale leaves bent over so as to enclose a chamber lined with water glands through which excess water is exuded. This exudation is necessary because the plant has no surface for transpiration and being a parasite predominantly on tree roots it needs to take in large volumes of water to be able to extract the food it requires. As waste water trickles down and evaporates the area around the plant often becomes white and encrusted with salts. up photosynthesis and look quite normal plants above ground. Below ground, however, they rely upon their attachments to the roots of other plants for water and mineral salts. Such plants are usually called partial or hemiparasites and include such widespread groups as the eye-brights (Euphrasia), red and yellow rattles (Pedicularis and Rhinanthus) and Bartsia. Mistletoe is another hemiparasite which has given up a root system and now attaches itself to the branches of its tree host, obtaining water and minerals from the xylem flow but producing its own sugars by photosynthesis. Mistletoes are very serious tree pests in many parts of the world.

Symbiosis

From a relationship where one partner gains at the expense of the other, as with a parasite and its host, it is but a small evolutionary step to a symbiotic relationship where two organisms live together to their mutual advantage. Such a relationship may be animal with animal, plant with animal or plant with plant.

Zoochlorellae (green algae, mainly freshwater) and zooxanthellae (yellow or brown algae, mainly marine) live in a symbiotic state with various aquatic invertebrate animals. From the relationship the alga gets some protection, exposure to the light and a source of nutrients from the waste products of the animal. But in some cases, as we shall see, it can lead to the plant’s ultimate destruction. The zoochlorellae in the tissues of Hydra is a symbiotic relationship that is well-known at the most elementary biological level. The symbiotic relationship in the marine flatworm Convoluta roscoffensis is less well-known. This flatworm, although only a few millimetres long, occurs in such numbers along the beaches of Brittany in France that it gives the sand a greenish tint. This is not due to the worm’s natural colour but to all the algae which the worm has eaten and which have then invaded its tissues. At maturity the worm is no longer able to feed so it begins to digest its accumulated algae, eventually using them up quicker than they can reproduce. The result is that soon after laying its eggs the worm dies having used up its algal food reserve.

Other animals with an algal symbiont include some corals, the giant clam (Tridacna derasa), tropical jellyfish such as Cassiopeia and some sea-slugs such as Aeolidiella glauca.

Plant with plant symbiosis has already been mentioned in referring to the mycorrhizal fungi which live in association with the roots of higher plants such as orchids and heathers.

Atlthocetos is a liverwort which has a symbiotic relationship with the blue-green alga Nostoc. It seems probable that the alga fixes nitrogen from the air for its partner but other than protection it is not clear what advantage Nostoc derives from the relationship. In a similar way bacteria of the genus Rhizobium fix nitrogen in the roots of leguminous plants such as peas, beans and clover and probably obtain carbohydrates in return.

The bacteria form small nodules on the roots which are clearly visible if dug up. Other bacterial root nodules occur in the madder family (Rubiaceae), where an unexpected phenomenon occurs—without the bacteria, the plants become dwarfed. In this case, what we call ‘normal’ growth is entirely dependent upon the symbiotic bacteria.

Symbiosis with fungi is particularly important to coniferous trees, where the association of fungus strands and tree roots creates a knobbly, sometimes corallike growth known as mycor-rhiza. The fungus forms a sheath of tissue around the tree roots and also penetrates between the cells of their outer layers. A large variety of ‘toadstool’ type fungi associate with trees in this way. This symbiosis is especially important to the trees early in their life: seedlings in fungus-free soil are stunted and grow very slowly. The fungus stimulates root penetration and enables the roots to absorb more food from the soil, while receiving carbohydrates from the tree in return. Lichens

One of the most successful symbiotic relationships is that found in lichens, those predominant colonizers of tree trunks and bare rocks. Here the symbionts are an alga and a fungus, the former providing the photosynthetic products and the latter the inorganic nutrients required for the maintenance of the lichen. Lichens are an interesting taxonomic problem because two separate organisms are being referred to by one scientific name. In spite of their compound origin the shrubby reindeer lichen (Cladouia) will always reproduce to form another Cladonia plant and the orange alga Xanthoria to form another Xanthoria and so on.

The body of a lichen is called the thallus and a thin section through this in a fairly typical species such as the grey dog lichen (Peltigera canina), which is found on grassy banks, will show an upper and lower area of fungal hyphae with a layer of green alga sandwichedjust below the upper surface.

Although a lichen may get some nutrient from the substrate on which it is living most of its inorganic requirements are obtained from rainfall and run-off water flowing over it. Lichens are, therefore, opportunists grabbing all they can from any water that flows over them. This has been their downfall. As the world becomes more polluted and the air carries more gases such as sulphur dioxide and the oxides of nitrogen, all of which form acids when dissolved in water, lichens are being poisoned out of existence and city centres and industrial areas are now ‘lichen deserts’ with few if any species to be found.

Lichens reproduce in two ways, asexually and sexually. Asexual reproduction involves the production of powdery soredia or slightly larger isidia which contain both algal and fungal elements. Sexual reproduction is by fungal spores alone and these will only form a new lichen if the normal algal partner happens to be present where they germinate.

Their ability to live on bare rock means that lichens are often the first colonizers of newly exposed areas, providing by their eventual decay the humus for new soil—the home for more conventional plants.

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