Composite Family (Compositaceae)
Composite Family (Compositaceae)
Characteristics of the Asteraceae
Agricultural species of composites
Other useful species of composites
The composite or aster family (Asteraceae) is one of the largest families of plants, containing about 20, 000 species, distributed among more than 1, 000 genera, and occurring widely on all continents, except Antarctica. This family is commonly regarded by modern botanists as the most advanced of the plant families, because of the complex, highly evolved structure of its multiflowered, composite reproductive structures.
The members of the composite family display a remarkable range of growth forms, ranging from tiny herbaceous annual plants, to vinelike lianas, and tall, treelike perennials. For example, some species in the genus Senecio are small, annual plants, such as the widespread common groundsel (Senecio vulgaris ). In contrast, the giant senecio (S. adnivalis ) species found on a mountain in Uganda, is a perennial plant that grows as tall as 26 ft (8 m).
The most species-rich genera in the aster family are Senecio (about 1, 500 species), Vernonia (900 species), Hieracium (800 species), and Eupatorium (600 species). Various members of the aster family are familiar species in natural habitats, while others are cultivated plants in gardens, and some are grown as foods. Some species in the aster family are considered to have negative values as weeds of agriculture or lawns.
Characteristics of the Asteraceae
Members of the Asteraceae are most readily characterized by their unique floral structure. The flowers of members of this family are aggregated within a composite grouping known as an inflorescence, which in this family is known as a head. In the head, the small, individual flowers, called florets, are attached to a basal structure known as a receptacle. The latter is surrounded by one or more rows of bracts, that make up the involucre.
The heads may be present singly, or they may occur in various sorts of aggregated groupings. Typically, each head gives the visual impression of being a single, large flower, even though the structure is actually a composite of several to many, small florets. This visual display is best developed in insect-pollinated species of the Asteraceae, and is ultimately designed to attract pollinators.
In many cases, the individual flowers may occur as disk florets that have functional stamens and pistils
but lack petals, or as ray florets that have an elongate, strap-shaped petal known as a ligule or ray. In some species, the head is entirely composed of disk florets, and is known as a discoid head. These occur, for example, in the tansies (Tanacetum spp.). In other species the head is entirely made up of ray florets, and is referred to as a ligulate head, for example, in the dandelions (Taraxacum spp.).
In other species, the disk florets occur in the center of the head, and ray florets on the periphery, giving a particularly striking resemblance to a single large flower. The ray florets of these latter relatively complex inflorescences are commonly sterile, and are only designed to aid in attracting pollinating insects. One familiar example of this head structure is the oxeye daisy (Chrysanthemum leucanthemum ), which has a central packing of bright-yellow disc florets, and a white fringe of long, ray florets. The oxeye daisy is the species of wildflower that love-struck young people use to tell whether their adoration is returned by their sweetheart—the petals are picked off one by one, to determine whether “he/she love me, he/she loves me not.”
The seeds of plants in the aster family are borne in dry fruits known as achenes. In many cases, these are small, and have a fine filamentous attachment known as pappus, which gives the fruits aerodynamic qualities that favor their dispersal by the wind. This can be illustrated by the familiar dandelion, whose fruiting heads develop as whitish puffs of pappus-bearing seeds, which eventually disintegrate and blow about on the wind.
In some other cases, such as the sunflowers (Helianthus spp.), the seeds are encased in a relatively hard coat, and are dispersed only locally. The seeds of some other plants in the composite family are specialized to stick to the fur of mammals and are dispersed in this way. Two common examples of this hitchhiking strategy are beggar ticks (Bidens spp.) and the burdock (Arctium spp.).
Horticultural species
Many species in the aster family have very attractive inflorescences, and some of these are commonly grown as ornamentals in parks and gardens.
Many ornamental species in the aster family are annuals, and are used as bedding plants, in annual gardens, and in self-seeding gardens. Some common examples include the cosmos (Cosmos bipinnatus ), sunflower (Helianthus annuus ), summer chrysanthemum (Chrysanthemum coronarium ), blanket flower (Gaillardia pulchella ), strawflower (Helichrysum bracteatum ), ice-plant or living-stone daisy (Mesembryanthemum criniflorum ), marigolds (Tagetes patula ), (T. erecta ), and (T. tenuifolia ), and zinnia (Zinnia elegans ).
A few horticultural species are biennials, or species that can complete their life cycle in two years. Two examples are the daisy (Bellis perennis ) and oxeye daisy (Chrysanthemum leucanthemum ).
Many other horticultural species in the composite family are longer-lived herbaceous perennials, and can be used in perennial gardens. Some common examples include various species of asters (Aster spp., such as New England aster, A. novae-angliae ), black-eyed Susan (Rudbeckia hirta ), shasta daisy (Chrysanthemum maximum ), hemp agrimony (Eupatorium purpureum ), Scotch thistle (Onopordum acanthium and O. arabicum ), yarrow (Achillea spp., such as A. filipendulina ), yellow chamomile (Anthemis tinctoria ), knapweed (Centaurea montana ), blanket flower (Gaillardia aristata ), and goldenrods (Solidago spp., such as Canada goldenrod, S.canadensis ).
Wormwoods (Artemisia spp.) have rather unattractive, greenish inflorescences, but are commonly cultivated for their attractive foliage.
Agricultural species of composites
A few species of composites have been domesticated for agricultural purposes. The cultivated sunflower (Helianthus annuus ) is an annual plant native to Mexico and South America that is widely grown for its seeds, which are eaten roasted or raw. Sunflower seeds contain about 40–50% oil, which can be extracted as a fine edible oil, the remaining cake being used as animal fodder. This sunflower grows as tall as 11.5 feet (3.5 m), and can have enormous flowering heads, up to 14 inches (35 cm) in diameter. The long ray florets make this sunflower very attractive, and it is often grown as an ornamental. Safflower (Carthamus tinctorius ) is another species sometimes grown as a source of edible oil.
The Jerusalem artichoke (Helianthus tuberosus )is a perennial sunflower, native to the prairies of North America. The Jerusalem artichoke has underground rhizomes, on which grow starchy and nutritious tubers. The tubers can be eaten as a vegetable, or processed into alcohol.
The globe artichoke (Cynara scolymus ) is another perennial composite, originally from the Mediterranean region. The preflowering heads of this species are cut off before they begin to expand. These are boiled or steamed, and the thick, fleshy material of the involucral leaves of the receptacle (that is, the base of the flowering structure) are eaten, commonly by peeling them between the teeth. The fleshy interior of the receptacle itself, known as the artichoke heart, is also eaten.
The lettuce, or cabbage lettuce (Lactuca sativa ) originated in southern Europe, and is grown for its greens, which are mostly used in salads, or as a green decoration for other foods.
Chicory (Cichorium intybus ) is grown for its roots, which can be roasted and used as a substitute for coffee. A leafy variety of chicory is used as a salad green. Endive (Cichorium endivia ) is a related species, also used as a salad green. These plants are originally from western Europe. Species of dandelions are also used as salad greens, for example, the common dandelion (Taraxacum officinalis ).
Other useful species of composites
Several species of composites have minor uses in medicine. Chamomile (Anthemis nobilis ) is an annual European species that is collected and dried, then brewed into an aromatic tea that has a calming effect. The dried leaves and flowers of common wormwood (Artemisia absinthium ) of Europe are used to make a tonic known as bitters, while the flower buds are used to flavor vermouth. The seeds of the wormwoods Artemisia cina and A. maritima, species native to the steppes of central Asia, are given as a treatment against several types of intestinal parasites.
Some other species in the aster family have been erroneously ascribed medicinal qualities. This occurred as a result of a theory of medicine that was developed during the Middle Ages, known as the “Doctrine of Signatures.” According to this ideology, the potential medicinal usefulness of plants was revealed through some sort of sign, such as a similarity between their shape, and that of a part of the human anatomy. In the case of the herbaceous plant known as boneset (Eupatorium perfoliatum ), the leaves are arranged opposite each other on the stem, and they lack a petiole, and are fully joined to each other by a band of leafy tissue that broadly clasps the stem. This unusual growth form, or signature, was interpreted by herbalists to suggest that boneset must have therapeutic properties in helping broken bones to heal. As a result, boneset was spread as a moist poultice over a broken bone, which was then encased within a bandage, plaster, or splint.
Several species of chrysanthemums are used to manufacture an organic insecticide known as pyrethrum. Chrysanthemum roseum, C. coccinium, and C. cinerariaefolium of southern regions of Asia have been widely cultivated for the production of these chemicals. Sometimes, living chrysanthemums are cultivated with other plants in gardens, in order to deter some types of herbivorous insects.
The latex of the rubber dandelion (Taraxacum bicorne ) contain 8–10% rubber latex, and is potentially useful for the commercial production of rubber.
Composites as weeds
Some members of the aster family have become regarded as important weeds. In many cases, these are aesthetic plants, because they occur abundantly in places where people, for whatever reason, do not want to see these plants. For example, the common dandelion (Taraxacum officinale ), originally from Europe but now widely distributed in North America and elsewhere, is often regarded to be a weed of lawns and landscapes. This is largely because many people only want to see certain species of grasses in their lawns, so thatany dicotyledonous plants, such as dandelions, are considered weeds. As a result, many people put a great deal of time and effort into manually digging dandelions out of their lawns, or they may use an herbicide such as 2, 4-D to rid themselves of these weeds.
Interestingly, many other people consider the spectacular yellow displays that dandelion flowers can develop in springtime lawns and pastures to be very pleasing. Dandelions are also favored by some people as a food, especially the fresh leaves that are collected in the early springtime. Clearly, the judging a plant as an aesthetic weed is a matter of perspective and context.
A few aster species, however, are weeds for somewhat more important reasons. Some, such as the ragwort or stinking-Willie (Senecio jacobea, ) are poisonous to livestockbecause its leaves contain toxic alkaloids. Ragweed’s natural range is Eurasia, but it has become an important weed in pastures in parts of North America and elsewhere, possibly having been introduced as an ornamental plant. Recently, several insect species that are herbivores of ragweed in its native habitat have been introduced to some of its invasive range, and these show promise as agents of biological control of this important pest.
Some other species in the aster family are important weeds of pastures because they are very spiny, and livestock cannot eat them. These inedible plants can become abundant, displacing valuable forage species. North American examples include various thistles introduced from Europe, such as bull thistle (Cirsium vulgare ), field thistle (C. arvense ), nodding thistle (Carduus nutans ), and Scotch thistle (Onopordum acanthium ).
Some species in the aster family attach their seeds to animals’ fur for dispersal. Animals with large numbers of these seeds in their fur can become very irritated by the matting, and they may scratch themselves so much that wounds develop, with a risk of infection. Examples of weeds that stick to animals (and to the clothing of humans) include the beggar-ticks (for example, Bidens frondosa ), and several introduced species known as burdock (for example, the greater bur-dock, Arctium lappa ). Interestingly, the finely hooked bristles of the globular fruits of burdock were the inspiration for the development of the well-known fastening material known as velcro.
The several species that are known as ragweed (Ambrosia artemesiifolia and A. trifida ) are the major
KEY TERMS
Achene— A dry indehiscent one-seeded fruit, with the outer layer fused to the seed.
Bract— A small scale modified leaf that is associated with a flower or inflorescence.
Floret— A small flower, often with some reduced or missing parts. Florets are generally arranged within a dense cluster.
Head— A dense cluster of flowers attached to a common receptacle. This is the characteristic arrangement of the flowers of members of the aster family.
Inflorescence— A grouping or arrangement of florets or flowers into a composite structure. Paphairs, sometimes intricately branched to achieve aerodynamic buoyancy. The pappus is derived from modified tissues of the calyx.
Receptacle— The enlarged tip of a peduncle where the parts of a flower are attached. Four distinct whorls of modified leaves, the sepals, petals, stamens, and carpels make up the parts of the flower.
Weed— Any plant that is growing abundantly in a place where humans do not want it to be.
causes of hay fever during the summer and early autumn. The ragweeds are wind pollinated, and to achieve this function they shed large quantities of tiny spiny-surfaced pollengrains to the wind. Many people have an allergy to ragweed pollen, and may suffer greatly from hay fever caused by ragweeds.
Interestingly, at about the same time that ragweeds shed their abundant pollen to the air, some other more conspicuous species in the aster family are also flowering prolifically. For example, pastures,fields, and other habitats may develop spectacular shows of yellow goldenrods (Solidago spp.) and white, blue, or purple asters (Aster spp.) at that time of year. Because people notice thesebrightly colored plants, but not the relatively small and drab ragweeds, the asters and goldenrods are commonly blamed for hay fever. For this reason, fields of these attractive plants may be mowed or herbicided to deal with this perceived problem. However, the asters and goldenrods are insect-pollinated, and they do not shed their pollen to the wind. Therefore, these plants are not the cause of hay fever—they are merely implicated by their association in time with the guilty but inconspicuous ragweed. Indeed, even people who suffer badly from hay fever often do not recognize the rather plain green unobtrusive-looking ragweeds as the cause of their allergies.
Resources
Books
Hvass, E. Plants That Serve and Feed Us. New York: Hippocrene Books, 1975.
Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.
Klein, R. M. The Green World. An Introduction to Plants and People. New York: Harper and Row, 1987.
Bill Freedman