The Asteraceae family

Remark The words or terms in red (actually dark orange) in the text are defined in a glossary.


The Asteraceae, a taxon of dicotyledonous flowering plants, is the second largest family in the division Magnoliophyta, with some 1,100 genera and over 20,000 recognized species, with around 350 genera and 2687 species in the U.S. and Canada, and, from the “Flore Laurentienne„ Frère Marie Victorin, 41 genera and 311 species in Québec. Only the orchid family (Orchidaceae) is larger, with about 25,000 described species.

The Asteraceae are herbs, shrubs, or less commonly trees. They show remarkable variation in growth form and general morphology because they occur in so many different localities and habitats.

The most common characteristic of all these plants, is that what is usually called a flower, is an inflorescence or flower head, a densely packed cluster of many small, individual flowers, usually called florets (meaning small flowers). The uniqueness of the Asteraceae family is that what first seems to be a single large flower is actually a composite of many smaller flowers. If one looks closely at a Black-eyed Susan (Rudbeckia hirta) in bloom, (picture below) one can see that there are hundreds of little flowers growing on a disk, the inner portion of the flower head, the brown center of the Rudbeckia, each producing just one seed. Each 'disk flower' has 5 tiny petals fused together, plus 5 stamens fused around a pistil with antennae-like stigmas. The outer perimeter of a flower head, the yellow 'petals' of the Rudbeckia, is composed of florets, the 'ray florets' or ray flowers, each flower with its petals fused together; each ray flower is also termed a ligule.

The three types of flower head
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Cichorium intybus
Ray florets only
Matricaria matricarioides
Disk florets only
Rudbeckia hirta
Disk and ray florets

(You can click the thumbnails above to get a larger view).

The Asteraceae were (and still are) also known as the Compositae. The alternative family name is derived from the Latin word compositus which means, in one of it many meanings, properly disposed, properly arranged. This refers to the collection of different florets arranged together in the inflorescence.

Remark
For family names, the modern tendency in taxonomy is to use the name of a familiar plant in a certain family and to add the suffix -aceae to the name. In this way the family name Asteraceae is made up by using the name Aster and adding -aceae. However, the long prevailing name Compositae is also authorized as an alternative family name.

Technically then, a species of the Asteraceae family is characterized by having the flowers reduced and organized into an involucrate pseudanthium in the form of a head or capitulum.

The flowers that compose the inflorescence of an Asteraceae are of two basic types:

and both types of flowers can often be found within the same head, with the central flowers being tubular as in the Rudbeckia hirta. If the head has only 'disk flowers' and lacks 'ray flowers' it is said to be discoid. The so-called disciform heads have bisexual central disk flowers surrounded by female flowers that have a very slender tube and an extremely suppressed or obsolete ligule. If the head has only 'ray flowers" and lacks 'disk flowers' it is said to be ligulate; it consists then of bisexual florets, with generally 5 rather than 3 distal teeth.

Depending on the species, either type of flower may be bisexual or unisexual. Where both types of flowers are found in a single head, the central flowers have tubular, usually with 4 times lobed or 5 times lobed corollas, and are generally bisexual, while the peripheral flowers have strap-shaped corollas generally with 3 distal teeth, but at times 2 or 5 teeth, and are usually female.

The head (capitulum) is then an inflorescence and a number of capitula are often aggregated together to form a secondary inflorescence or synflorescence. The capitulum is surrounded on the outside by one or several layers of involucral bracts resembling the calyx of other flowers.

In all cases then the flowers do not have a calyx, or it so highly modified as hairs, bristles or scales on the ovary summit that it is given the alternative name of pappus.

The pappus is an adaptation for wind dispersal of the fruit. Sometimes the pappus contains hooks, which help with animal dispersal of the fruit. In some Asteraceae the pappus is absent, in which case the fruits often have wings, which help with wind dispersal.

The bracts are mostly green (herbaceous) but can also be brightly colored like in Everlastings (Helichrysum spp.) or they can have a thin, dry, membranous texture (scarious). The involucral bracts are mostly free and arranged in rows, overlapping like the tiles of a roof (imbricate). When in one row, they are often fused to different degrees.

Bracts
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Cirsium vulgare Centaurea stoebe Symphyotrichum puniceum (Aster puniceus)

(You can click the thumbnails above to get a larger view).

In the tubular (disk flowers) the androecium nearly always consists of 4 or 5 stamens that are united by their anthers and are adnate to the corolla tube or epigynous zone, alternate with the lobes.

The gynoecium consists of a single compound pistil of 2 carpels, a single 2-cleft style, and an inferior ovary with one locule and one basal ovule. During maturation of a flower, the style grows through the anther column, and as it does, hairs on the outer surface of the closed style lobes brush the pollen that is released into the anther column to the distal opening where it is available for biotic pollinators. A nectary in the form of a scale or small cup is commonly found alongside or around the base of the style. The fruit is an achene which may have a persistent pappus that commonly functions in fruit dispersal.

Subtending and often partly enclosing the florets of the head is one or more series of usually green, free or variously connate bracts called involucral bracts or phyllaries. Another kind of bract called a receptacular bract or chaff may be associated with each disk floret throughout the head. The bracts are in some ways the sepals of the Asteraceae.

One of the best clues for identifying members of the Asteraceae family is to look for the presence of multiple layers of bracts beneath the flowers. In an artichoke, for instance, those are the scale-like pieces that are pulled off and eaten. Most members of the Asteraceae family do not have quite that many bracts, but there are frequently two or more rows. This is a common pattern of the Aster family. The genera are sometimes rather ill-defined, which can make the writing of efficient keys difficult. Members of the Asteraceae are more commonly in flower in late Summer and Fall in temperate areas; but in my area, 25 km north of Montréal, one of the first if not the first plant to flower in the Spring is an Asteraceae, the Tussilago farfara.

Plants belonging to the Asteraceae must then share all the following characteristics:

None of these traits, taken separately, can be considered synapomorphic.

The sesquiterpene lactones and alkaloids contained in the Asteraceae are thought to function as feeding deterrents. The sesquiterpene lactones can cause contact dermatitis. Members of the Asteraceae produce a wide array of toxic compounds, one of the most famous is Artemesia absinthium (Wormwood), which produces thujone. Extracts of Artemesia absinthium were mixed with alcohol to produce a drink called absinthe. This drink was popular in Europe for a time but has since been outlawed in most countries because of the toxicity of thujone. Vincent Van Gogh supposedly consumed large quantities of absinthe and his insanity and genius have both been attributed to absinthe.

Commercially important plants in the Asteraceae include the food crops lettuce, chicory and sunflower. Other well-known foods are Jerusalem and French artichokes, or herbal tea like camomile. The dandelion weed (Taraxacum officinale) is also used as food by many.

Familiar members of the Asteraceae used as cut or garden flowers are the Asters, Dahlias, Chrysanthemums, Cornflowers and Sunflowers, not to forget the Edelweiss (Leontopodium alpinum) of alpine gardens.

Members of the Asteraceae have been used for a wide array of medicinal purposes. The one that has probably attracted the most interest recently is Echinacea purpurea (Purple Coneflower), which produces compounds that stimulate the immune system.

Many members of the Asteraceae family are copious nectar producers and are useful for evaluating pollinator populations during their bloom. Centaurea (knapweed), Helianthus annuus (domestic sunflower), and some species of Solidago (Goldenrod) are major honey plants for beekeepers. Solidago produces relatively high protein pollen, which helps honeybees overwinter.

The Asteraceae of Québec

The numerous genera are usually divided into 3 subfamilies and about 18 tribes, as follows: (In the list below, only the the genera entries with a list of one or more species entries are found in Québec)

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