|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 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, called ray flowers, each bear a
single petal-like or strap-like extension of the corolla, called a ligule.
(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). Some, however, like
Tragopogon pratensis (Goatsbeard) is a dandelion plant in which all the
disc flowers also bear a ligule. In these composites, the individual flowers
are also called florets.
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).
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.
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.
Knapweeds and Thistles
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.
The fruit of the Asteraceae is an achene. 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.
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 9usually, there is quite some
variation) as illustrated below:
6th June 2015
19 July 2015
Left: a 3D computer (Pov-Ray) model of an achene
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.
Photo by Greg Hume (Wikimedia Commons)
Photo by Greg Hume (Wikimedia Commons). This photograph of a Taraxacum
inflorescence clearly shows the outer ligulate ray flowers and the inner tubular disc flowers.
Note the prominent paired stigmas borne on the long stalk-like styles.
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.
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!
Bellis perennis - the common daisy.
Taraxacum officinale - the Dandelion.
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 teh 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) appear to fold down at night, rather than closing up over teh
flower. Perhaps this protects the ligules from damage when they are not being used to attract pollinators.
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
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.
Centaurea nigra: close-up view
Florets of Centaurea nigra. Note the visible anthers in the florets at top
right, fused together to form a tube around the central style..
Below: the silvery pappus hairs (modified sepals of the calyx) around the base of each floret.
Senecio jacobaea (common ragwort): close-up view
Above: a disc floret
Below: a ray floret with its ligule
Above: barbs on the pappus hairs of Senecio.
Above: a mayweed (there are several mayweed genera and species which need some care in order to
distinguish them - see mayweeds and chamomiles) (Kent, UK)
Above: Oxeye daisy (Moon daisy, Leucathemum vulgare)
Above: this is probably a young Picris echioides (Helminthotheca echioides, Bristly Ox-ongue) which has
characteristic blister-like bristles with whitish bases on the leaves. In older leaves these bristles are unmistable
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).
Left: Senecio (Ragwort)
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.
Above: Sonchus oleraceus, Smooth Sow-thistle, often grows by walls, on roadsides and in waste and cultivated
Below: The Greater Knapweed, Centauria scabiosa, growing on chalk grassland.
The butterfly above is the Marbled White butterfly,