Composite Flowers - Asteraceae

The Asteraceae (asters, daisy or sunflower family) or Compositae
(composites) are a very large and diverse group of flowering plants
(angiosperms) with some 1620 genera and over 23 000 species. These
plants have composite 'flowers' which are actually flower-like inflorescences. In these composites, the individual flowers are also called florets. The inflorescence is a swollen axis, termed a receptacle, which is in the form of a flattened disc bearing the individual flowers. Such an inflorescence is called a capitulum. Typically the outermost circle of flowers around the edge of the capitulum each bear a single petal-like or strap-like extension of the corolla, called a ligule (this type of floret is ligulate) and these are ray florets and the capitulum is said to be radiate. (Think of rays of light radiating from a central star). The remaining disc flowers usually have no ligule, but instead have a fairly inconspicuous corolla tube (of fused or partially fused petals): they are tubular. Tragopogon pratensis (Goatsbeard) is a dandelion-like plant in which all the disc flowers also bear a ligule. In some forms the capitula are rayless or liguleless, meaning that the outer florets are not ligulate but tubular and in this case the outer florets are not ray florets but rather all the florets are disc florets, e.g. Tripleurospermum disciforme. Sometimes, as in some Centaurea, the outer florets are tubular but with extended radiating corollas and are sometimes called pseudoradiate.

Examples of composites include the asters and daisies, dandelions and
sunflowers, as well as burdock, thistles, knapweeds, ragworts, and others.
The bracts of these flowers typically form an involucre or cup below the
capitulum and are called involucral bracts. In the marigold (Calendula
officinalis), the several outer circles of ray flowers/florets are female, the disc
florets functionally male (the styles are non-functional). The pappus is
absent or reduced in Calendula.

The sunflower (Helianthus) is a classic example of an Asteraceae. The ray and disc florets are very distinct. The flower
bud exhibits heliotropism - tracking the Sun across the sky. Once the buds open one morning, however, they stop
these tracking movements (the flower itself is not heliotropic) and the flowers are usually fixed facing East. (Certain
other plants are also heliotropic, in some cases including the open flower).

Bellis perennis - The Common Daisy

Dandelions (Taraxacum)

Dandelions (dandelion = 'lion's tooth') are another very common and familiar example. Some say they are
so-named because their leaves resemble lion's teeth, however, I expect it is also because of the yellow
composite flower heads, resembling the Sun which was associated with the lion by the ancients, for obvious
reasons. The term 'dandelion' is most often applied to plants of the genus Taraxacum and Taraxacum
officinale in particular. In North America, Taraxacum officinale is triploid (has three sets of chromosomes) and
produces seeds without fertilisation, a process called apomixis. This form of asexual reproduction is known as
parthenogenesis, or more often in plants: agamospermy. In Europe, there are diploid forms which reproduce
by normal sexual means.

Above and below: the Dandelion, Taraxacum, is an aggregate of hundreds of micro-species. These have bright yellow ligules and lack receptacle scales.

Galinsoga

As an illustration of how floral features can assist taxonomy in Asteraceae, we shall consider Galinsoga
(Gallant Soldiers). Galinsoga has two kinds of flowers borne on a short, conical receptacle. In Galinsoga
quadriradiata there are 4-5 outer ligulate ray flowers, which are female. These have white ligules with a
3-lobed apex, forming the distinctive 4-5 trilobed 'petals' of these flowers. The inner 5 or so disc flowers are
tubular, with a 5-lobed 'dirty-yellow' coraolla tube and are hermaphrodite (having both male and female
reproductive parts). The pappus is reduced to scales (tiny leaflike appendages). Galinsoga parviflora is
similar, with very similar ray and disc flowers. These two species are hard to tell apart by eye, but under the
miscoscope, their minute pappus and receptacle scales look very different (usually, there is quite some
variation) as illustrated below:

Galinsoga quadriradiata has lanceolate (long and narrow with parallel edges, lance-like) receptacle scales
which are acute (with pointed rounded tips) which are rarely trifid at the apex (rarely divided into three lobes at
the tip).  The pappus consists of scales, 0.5 to 1.5 mm long, fimbriate (divided into a number of hair-like or
filament-like lobes at the tip) with a fine terminal awn (central filamentous projection at the tip).

Galinsoga parviflora has trifid (with three obvious lobes) receptacle scales which are obovate in contour
(widest towards the tip). Those subtending the ligulate ray flowers are about 0.4 mm long, those subtending
the tubular disc flowers about 1 mm or more in length. The pappus consists of 15-20 silvery, lanceolate (long
and narrow with parallel edges, lance-like) to spathulate (shaped like a spatula) scales which are deeply
fimbriate (divided into filament- or hair-like lobes called frimbriae, forming a 'fringe'). The pappus scales are
about 1.5 mm long, obtuse-acute (with blunt or sharp rounded tips)  and lack awns.

Fruit

The fruit of the Asteraceae is an achene (some define it specifically as a clypsela). An achene is a dray fruit, which is indehiscent, meaning that it does not split upon to release seeds. The achene is a bit like a nut, always encloses a single seed, but unlike a nut has a much thinner wall which is typically so closely shaped to the seed inside as to appear like part of the seed coat. Indeed, the fruit are often referred to as 'seeds' even in professional botanical texts.

Left: a 3D computer (Pov-Ray) model of an achene
of Taraxacum.

In many Asteraceae, the pappus scales of the
flower become parachute-like filaments, borne on
top of a stalk formed by elongation of the region
between the pappus and the ovary as the fruit
matures. This parasol of filaments makes the fruit
readily airborne as the slightest currents of air may
carry it far from the parent flower for dispersal. The
ovary wall, forming the dry fruit coat enclosing the
single seat, is variously ornamented with ridges,
pits, scales and spikes.

Note that although some pappus-borne fruit may be carried far on the air, most are only dispersed a few meters from the parent plant, according to measurements, and many forms lack the pappus altogether. Thus, the main objective is not the colonisation of new sites, that seems to be secondary, but the replenishment of populations at existing sites. For example, Carduus nutans (Musk Thistle) achenes mostly land within 50 m of the parent plant, with less than 1% traveling further than 100 m (see review by Desrochers et al. 1988). The achenes of Taraxacum form the familiar 'clock' (above right). When ripe, a single puff of air will easily dislodge many of the achenes, making them airborne. This raises the question why the 'parachute' consists of hairs rather than a continuous disc or funnel.  Cummins et al. (2008) have shown that air passing between the pappus hairs of dandelion is shifted sideways, creating vortices above the pappus (to fill the vacuum that otherwise be formed). These vortices reduce the pressure above the pappus, generating more lift than an equivalent solid disc. The number of pappus hairs is important, fewer than about 92 or more than about 100 results in instability. (The study is also summarised in Essex Botany 2018 issue 8). Dispersal distances vary considerably with species, according to strategy, with some tending to favour longer-range dispersal than others and some producing two types of achenes in the same capitulum with different dispersal characteristics. For example, Centaurea scabiosa (Greater Knapweed, see below) has much shorter dispersal ranges than Cirsium arvense (Creeping Thistle, see thistles) as illustrated in a study by Sheldon and Burrows (1973).

Not all Asteraceae achenes are wind-dispersed. The achenes of the sea Mayweed, Tripleurospermum maritimum, are often dispersed in the sea and lack a pappus (it is reduced to a short hairless rim called the corona or 'crown') but large corky ribs allow the achenes to float on water.

So you see, there is much more to the familiar daisies and dandelions and their relatives than meets the eye. This is true of all botany - often plants we think we know appear so different when viewed under the microscope, and many have their own special tricks to ensure pollination and dispersal of their seeds! Next time you look at a flower, any flower, why not try to find out what special tricks it has to help it propagate and survive and what hidden surprises it can reveal under a microscope. Note however, that if you are going to study wild flowers find out what you can and should not pick. Plants are very diverse, but this means that local populations are often unique to a small region and you wouldn't want to damage a rare species!

Nyctinasty

Many asteraceae have an interesting trick - they undergo nyctinastic movements. As nighttime approaches,
plants like Bellis perennis and Taraxacum officinale close their flowers - specifically the ligules of the ray flowers
close over the flower-head, protecting it. In this way, nectar evaporation is reduced at night when the right
pollinators are not about. Many Asteraceae produce copious quantities of nectar. They also close in low
temperatures. These movements are growth movements - each day cells in the ligules grow a bit: when those
on the outside grow more, the ligules open, when those on the inside grow more, the ligules close! Try to
observe this next time you walk past a grass verge or lawn with these flowers growing on it! See the article on
the sensitive plant to learn more about nyctinsaty and other plant movements.

Interestingly, the ligules of Senecio (ragwort) fold down at night, rather than closing up over the
flower. Perhaps this protects the ligules from damage when they are not being used to attract pollinators. Such movements may also help keep the ligules free of debris. This suggests that nyctinasty may serve to increase the lifespan of the flower's display by helping maintain the petals for longer.

The time at which flowers open and close varies with species and is probably timed to coincide with the times the preferred pollinator(s) is (are) most active. For example, Tragopogon pratensis (Goat's-Beard, Asteraceae) opens in morning but closes by midday, whereas Taraxacum (Dandelion) closes late in the afternoon. However, both will close earlier during overcast weather and it is thought that this may prevent pollen and/or nectar from becoming waterlogged or washed away by rain.

Why Composite Flowers?

What is the evolutionary advantage of composite flowers? It is clear to see how these flowers evolved - the
flower head is a compressed inflorescence; it even bears scale-like leaves (receptacle scales) in between the
individual flowers. One advantage is probably to increase the visual display to pollinating insects without
developing large expensive flowers. A single visiting insect can even pollinate several flowers in a single visit, so the plant is clearly economising on resources whilst maximising fruit-set. However, pollinating several flowers at once might mean that they are pollinated by pollen from a single plant, reducing the potential genetic diversity of the offspring. The fact that so many plants use this method, however, demonstrates its advantage in many cases.

Nectaries and Anthesis

Asteraceae are typically inset-pollinated and possess nectaries. These nectaries are multicellular outgrowths of
the inferior ovary, surrounding the base of the style. A very interesting and detailed study has been made of
Echinacea purpurea nectaries (Wist, T.J. and A.R. Davis, 2006. Floral Nectar Production and Nectary Anatomy
and Ultrastructure of Echinacea purpurea (Asteraceae. Annals of Botany 97: 177–193). The nectaries in
Echinace purpurea occur only in the disc florets and each is a pentagonal ridge around the base of the style,
bearing about 30 modified stomata through which nectar seeps into the corolla-tube of five fused petals,
which acts as a reservoir. This ridge is about 0.36 mm in diameter and 0.15 mm high and secretes a sugary
nectar containing glucose, fructose and sucrose. Each nectary consists of a single layer of epidermal cells,
bearing the stomata, and is supplied by phloem (branching from the carpel's own vessels) and secretory
parenchyma. (In some Asteraceae the nectaries also have xylem vessels supplying them, whilst in others they
are avascular). The companion cells next to the phloem are connected to the phloem sieve cells by
plasmodesmata, but also have wall ingrowths facing other parenchyma cells, increasing the surface area of their membranes for transport of materials, presumably sugars for nectar synthesis. Cells with such wall ingrowths are called transfer cells.

The ligulate ray florets of Echinacea purpurea are sterile and form a single whorl around several whorls of disc
florets. There are, on average, almost 300 disc florets, each accompanied by its own bract or palea. The disc
florets mature from the periphery inwards, with one floret whorl opening each morning. The disc florets are
hermaphroditic (bisexual) and are protandrous, meaning that the male parts ripen first. On the first day after
anthesis (opening of the corolla) the flowers are staminate, that is they possess stamens. The stamens rapidly
elongate, the five anthers are fused into an anther tube and the filament bases are adnate (fused to) the petal
bases. The five petals are fused together to form a bell-shaped tube. The anthers dehisce to release their
pollen. On the second day, the female organs ripen and the style elongates, pushing through the anther tube
with the stigma lobes closed. The stigma lobes then curve backwards and become receptive to pollen from
another individual. The flowers remain receptive for several days, when they either set fruit or senesce.

Oxeye Daisy (Moon Daisy, Leucathemum vulgare)

Feverfew (Tanacetum parthenium)

Senecio jacobaea, Common Ragwort.

Senecio jacobaea (common ragwort): close-up view

Above: a disc floret with pappus

Above: a ray floret with its ligule (bottom)

Above: barbs on the pappus hairs of Senecio.

Mayweeds and Chamomiles

Above: a Mayweed. There are several Mayweed genera and their species need some care in order to distinguish them (Kent, UK). The best characters are the achenes and phyllary scales (which are absent in some forms).

Above: this is probably a young Picris echioides (Helminthotheca echioides, Bristly Ox-tongue) which has characteristic blister-like bristles with whitish bases on the leaves. In older leaves these bristles are unmistakable and a few seem to be present in the young leaves shown here. Stiff bristles also cover the stem and the sepal-like outer bracts. (Kent, UK).

Achillea millefolium, Yarrow

Above: Yarrow, Achillea millefolium. So named because of its numerous feathery leaves ('millefolium' literally
means 'thousand leaves') and because Achilles was said to have used it to cure wounds.

Aster tripolium - Sea Aster

Sea Aster is usually found along coasts, sea cliffs and salt estuaries, and occasionally further inland. This one
was found about half a mile away from a salt estuary and a few hundred yards from salt marshes.

Sonchus oleraceus- Smooth Sow-thistle

Smooth Sow-thistle often grows by walls, on roadsides and in waste and cultivated land.

Thistles

Cirsium palustre - Marsh Thistle

Cirsium is the genus of Plume Thistles since the pappus (tuft of hairs) borne on their fruit (achenes) are distinctly branched and feathery. These hairs (modified sepals which persist on the fruit) facilitate wind dispersal of thistle seeds. In contrast, other genera of thistles have unbranched pappus hairs. The marsh thistle has continuous spiny wings running down its hairy stem. The stem and leaves are often flushed purple and the upper surfaces of the leaves are hairy. The flower heads (capitula) are in characteristic dense clusters. The florets are usually dark red-purple.

                        Less commonly, the florets of marsh thistle are white.

                        More on thistles

Centaurea nigra - Black Knapweed, a sensitive plant

Black Knapweed, Centaurea nigra. All the florets of knapweeds and thistles are tubular or disc florets as they all have a symmetrically developed corolla or petal-tube (no ligule). The outer tubular florets of knapweed (Centaurea) are sterile and serve only to attract pollinators. The inner florets have a special pollination mechanism - when touched by an insect the stamen filaments rapidly contract (in flowers with motile stamens this typically involves the generation of electrical signals in the base of the stamen followed by action potential propagation along the stamen). The anthers, which are fused into a tube around the style are pulled back and the fixed style pushing up amongst them pumps out the pollen onto the insect. To avoid self-pollination, the stigma only becomes receptive when all the pollen is shed, when the style extends out from the anther tube. The outer sterile florets of knapweeds and their relatives are often star-shaped, as in cornflower (Centaurea cyanus) as their corolla is expanded and so the flower tube opens up into radially symmetric flowers as in a 'typical' actinomorphic flower.

Florets of Centaurea nigra agg. Note the visible anthers in the florets at top right, fused together to form a tube around the central style (projecting pinkish filament in center of floret, ending in a pair of stigma). Bottom left: note the silvery pappus hairs (modified sepals of the calyx) around the base of each floret. These aid in wind dispersal of the achene. Although Centaurea lacks ligules, and indeed the specimen above has no rays, some varieties and species have the petal-tube of the outermost florets elongated in the manner of rays, but these floral tubes remain largely tubelike. Such petal tubes are called pseudorays and specimens possessing them are called pseudoradiate (sometimes simply radiate if the distinction with straplike ligules is not made). Black Knapweeds lacking pseudorays/rays are the eradiata normalis (normal non-radiate) form, seen here. Forms with shorter florets, called the breviflora form have female only flowers with non-functional stamens (the anthers of which occasionally produce a few pollen grains but the development of the stamens is largely suppressed). In quilled forms, the petal lobes (tips of the corolla or tube formed by petal fusion) the lobes are poorly developed or do not open properly, so the florets remain more-or-less closed (this is presumably an adaptation to favour self-pollination).

Above: Centaurea debeauxii, the Chalk Knapweed is a member of the Centaurea nigra aggregate and along with Centaurea jacea, the Brown Knapweed, forms an interbreeding complex in which all three species may hybridise, resulting in a complex of forms with varying proportions of characters from the three species. this can cause some headaches when attempting to determine these plants in the field. This is a problem I am currently working on. The form of the scale leaves or phyllaries covering the cup of the flower head are diagnostic. The Black and Chalk Knapweeds both have phyllaries with a brown or black appendage at the tip which bear finger-like projections on the margin, called fimbriae. These are borne on the central triangular disc of the appendage. The key to tell these two species apart is to look at the phyllaries in the third row from the base: if the fimbriae of the appendages on adjacent phyllaries are not or just touching (or overlapping on some phyllaries) then it is C. debeauxii whereas if they overlap greartly, such that they extensively overlap, often onto the central discs of adjacent phyllary appendages then it is C. nigra. The hybrid between these two species has an intermediate degree of fimbrial overlap, with the fimbriae usually overlapping but not onto the central disc.This specimen was growing on chalk grassland.

Movable Phyllaries and Pappus Filaments

Notice that the appendages are reflexed or recurved (curved backwards, and away from the main axis). Early attempts were made by botanists to distinguish forms with reflexed appendages and forms without reflexed appendages, however, others have observed that this depends on humidity: reporting that the appendages curve back when dry and flatten against the receptacle when damp. I have confirmed this observation and similar movements occur in the phyllaries of many Asteraceae, including Taraxacum (Dandelion). It has been shown that when the phyllaries are open the achene fruit are more easily dispersed. In those forms with a pappus, opening of the phyllaries makes room for the pappus to expand in preparation for becoming airborne. Nevertheless, the maximum degree of opening may depend on the form. The phyllaries possibly also protect the flowers from insect herbivory and pollen thieving insects that may otherwise clime the stem and burrow into the capitulum. The fimbriae would make it difficult for an insect to pass when the phyllaries are open. Perhaps the opening of the phyllaries also makes the florets and their nectar more accessible to pollinating insects in dry conditions. It has been suggested that closure of the capitulum prevents raindrops from tangling and damaging the pappus hairs (Sheldon and Burrows, 1973). Furthermore, the pappus itself also undergoes similar humidity-dependent movements, closing when humidity is too high and opening in lower humidity, ready for dispersal. The pappus hairs are either hinged at their base at a common pulvinar ring on the crown of the achene (e.g. Lactuca, lettuce), or each pappus hair has its own pulvinus (e.g. Cirsium thistles), or the hairs themselves bend (e.g. Tussilago, Coltsfoot) due to uneven thickening of their walls. The opening of the phyllary bracts and the pappus filaments separates each achene from its neighbours, facilitating dispersal.

Centaurea scabiosa - The Greater Knapweed

The butterfly above is the Marbled White butterfly, Melanargia galathea. The Greater Knapweed does not hybridise with the Black Knapweed species complex. Note the pseudoradiate capitulum, which is typical of this species.

Apart from its larger flower heads (capitula) the Greater Knapweed can be distinguished from the Black and Chalk Knapweeds, all of which may grow in the same locale, by the shape of the scale leaves or phyllaries covering the capitulum cup. Notice the phyllaries are triangular with a fringed margin and black tip. They do not have distinct appendages bearing fimbriae. The petal tube of florets at the margin are also more obviously rayed (projecting outwards from the center).

Above: a number of the florets in this specimen have the stigmas protruding from the corolla.

References

Cummins, C., Seale, M., Macente, A. et al. A separated vortex ring underlies the flight of the dandelion. Nature 562: 414–418 (2018) doi:10.1038/s41586-018-0604-2.

Desrochers, A. M., Bain, J. F. and Warwick, S. I. 1988. The Biology of Canadian weeds. 89. Carduus nutans L. and Carduus acanthoides L. Can. J. Plant Sci. 68: 1053-1068.

Sheldon, J. C. and Burrows, F.M. 1973. The dispersal effectiveness of the achene-pappus units of selected compositae in steady winds with convection. New Phytol. 72: 665-675.

Article updated:
28/2/2015
16/3/2015
11/4/2015
6th June 2015
19 July 2015
24 June 2017
11 June 2019
7th Dec 2019
13 Dec 2019

12 Sep 2020