mayweed
Mayweeds and Chamomiles
mayweed
mayweed capitulum computer model
model of ray floret
model of disc floret
model of disc floret
The Mayweeds and Chamomiles encompass several genera of superficially similar plants which all belong to the
daisy family (
Asteraceae). They are common weeds, but closer study reveals them to be an interesting group of
plants. The most used species to make chamomile tea are
Matricaria chamomilla and Chamaemelum nobile
(sometimes described as the 'true chamomile').

These genera are (according to Stace 3rd edition):
Tripleurospermum, Matricaria, Chamaemelum and Anthemis.
Which species is shown in the above photograph? It is very hard to be certain without looking at the achenes (fruit)
under the microscope, but I suspect that this plant is either
Chamaemelum nobile or Matricaria chamomilla, and
probably the former. The receptacle appears too conical for
Tripleurospermum. After seeing hundreds of
specimens, it is often possible to tell by looking at the overall appearance of the plant, or by close examination of the
leaves, though this can lead to errors and microscopical examination of the achenes, along with the receptacle
scales is the only way to be certain. Consideration of the habitat can also help.

The following are the main species normally found in the British Isles:

Tripleurospermum inodorm (Scentless Mayweed)
Tripleurospermum maritimum (Sea Mayweed)

Matricaria chamomilla (Scented Mayweed)
Matricaria discoidea (Pineappleweed)

Chamaemelum nobile ('true' chamomile)
(
Chamaemelum mixtum is a scarce alien)

Anthemis cotula (Stinking Chamomile)
Anthemis arvensis (Corn Chamomile)
Anthemis tinctoria (Yellow Chamomile)
Anthemis punctata (Sicilian Chamomile)
Anthemis austriaca (Austrian Chamomile)
capitulum and tubular florets
ray floret
Tripleurospermum inodorum leaf and stem
leaf segment tip
phyllary
tubular florets
tubular florets
section through capitulum showing oil glands
achenes and oil glands
achenes and oil glands
tubular florets
achene and pappus
There are no receptacle scales in Tripleurospermum and Matricaria. In
Chamaemelum a receptacle scale closely ensheaths each floret. In Anthemis
the morphology of the receptacle scales help distinguish the various species.
The receptacle is hollow in
Matricaria, and solid to indistinctly hollow in
Tripleurospermum and much more conical in Matricaria and Chamaemelum
than in
Tripleurospermum.

Left: a ray floret with the two-lobed stigma. The ray florets appear to be
female, as is generally the case in Asteraceae, and produce full-looking (and
hence presumably fertile) achenes (these characteristics need verifying for
each individual species). The ligules are white, except in
Anthemis tinctoria in
which they are yellow, and the disc florets yellow. The ligule typically has two
or three teeth at its tip and in Asteraceae is often derived from three of the
corolla lobes, the other two being suppressed. The ligule functions to attract
pollinating insects.
Above: a tubular / disc floret. The corolla is yellow and when mature the five lobes
open out (the petals being fused further down). On the right-hand flower the style
bearing the bilobed stigma can be seen emerging from the cylinder of the five anthers.
Above: a capitulum of Tripleurospermum inodorum. The florets are arranged in a spiral on the disc-like
receptacle. The disc florets mature from the base towards the apex. Here, the basal florets are already
opening their corolla lobes (December, Kent, N. Hemisphere - unusually late flowering for this species
due to a mild winter and urban location).
Above: a ray floret of Tripleurospermum inodorum. Note the
two stigma lobes borne on the emerging style. Right: a
phyllary from the same plant: note the central green region
and the typical brown scarious (thin, dry and membranous)
margin.
Above: detail of disc florets of Tripleurospermum inodorum. The style bearing the bilobed stigmas can be
seen emerging in some of the open florets, carrying pollen with it. This makes the pollen more easily
accessible to pollinating insects. Mayweeds are probably largely self-incompatible, and the pollen is abaxial
(on the outer or underside of the spreading stigma lobes) not on the receptive upper (adaxial) surface.
Furthermore, the stigma will not become receptive until this pollen is removed. These mechanisms help
ensure cross-pollination.
Left: close-up view of part of a leaf of
Tripleurospermum inodorum. The leaves are
compound, greatly divided up into narrow
branching leaflets, giving the leaves a
variously feathery appearance. Note the
central gutters which allow rainwater to drain,
keeping the leaves clean and dry so as not to
impede photosynthesis, as well as funneling
water down the stem to the roots. Although
the leaves of
Tripleurospermum maritimum
are sometimes similar, they are usually
fleshier and more succulent, being more
xerophytic and tolerant to dehydration in line
with
Tripleurospermum maritimum's preferred
habitat on beaches and coastlines.
Tripleurospermum inodorum can occur
coastally, but is usually on more arable fields
slightly further from the shoreline, however,
their are places where both species occur
and hybridisation is known. Hybrids have
'intermediate' leaf characteristics.
A useful (though by no means
foolproof) diagnostic feature is
the tip of the leaf segments,
which often have a hyaline
(translucent) spine at the tip in
Tripleurospermum inodorum.
These are usually absent in
Tripleurospermum maritimum or
less frequent and less prominent
(shorter).
Tripleurospermum inodorum also typically has a very different habit (growth form) than Tripleurospermum
maritimum
. T. inodorum is annual and so often retains the light-weight primary stem, whilst T. maritimum is
perennial (or occasionally bienniel) and so, except when in its first year, acquires a woody stem (and can even
resemble a small bonsai tree!).
T. inodorum is more upright, whereas T. maritimum is often at least partly
procumbent (lying horizontal). Together with its fleshier leaves, this gives
T. maritimum a more scrambling and
more compact appearance whereas
T. inodorum tends to be more upright and open with more delicate leaves.
The stems of
T. maritimum are also often darker and redder stems (though T. inodorum can have reddish
stems), especially when growing along the southern coast of Britain. Those with obviousluy red stems are
considered to be the subspecies vinicaule. More northern populations give way to the classic form (subspecies
maritimum) and then to subspecies nigriceps (Northern Scotland and the Shetland isles) which has broader
and darker scarious margins on its phyllaries. There are gradations, however, for example many
Tripleurospermum maritimum specimens have brown or dark brown stems which grade into the very red stems
characteristic of subspecies vinicaule.
Despite the aforementioned differences, mistakes can happen when determining Tripleurospermum
maritimum
from Tripleurospermum inodorum. A more definite character is found by looking at the
achenes. In the photomicrograph above the achenes are still immature and a very pale apple-green
colour. However, each bears a pair of circular oil glands which determines this plant as
Tripleurospermum inodorum. The oil glands of Tripleurospermum maritimum are more than about 1.5
times as long as wide, and are often greatly elongated.
Above and below: immature achenes of Tripleurospermum inodorum (with the flower tubes still attached).
Below: another key difference in the achenes is seen when looking at the back (adaxial
surface) of the achenes. The achenes of
Tripleurospermum have three prominent ribs at the
back, and in
Tripleurospermum maritimum these ribs are very strongly developed with little
other surface tissue in-between and they join at the bottom of the achene (generally). These
are achenes of
Tripleurospermum inodorum which do not join at the base and leave wide
open strips of surface tissue between them. The achenes of
T. maritimum are also generally
quite a nit larger when mature.
Above: from the picture my guess is that this is either Matricaria chamomilla or Chamaemelum nobile. The
receptacles look too conical for
Tripleurospermum inodorum. However, in order to check one could see whether
the receptacles are hollow, as they would be in
Matricaria chamomilla (which is also usually more strongly and
sweetly scented than
Tripleurospermum inodorum) The achenes of Matricaria chamomilla also lack oil glands
and have 4-5 weak ribs (not 3 strong ribs as in
Tripleurospermum). Chamaemelum nobile is also typically
strongly scented, but it has prominent receptacle scales (visible under the microscope or with a hand lens).
Anthemis will also have receptacle scales and the shape of the scales can distinguish Anthemis from
Chamaemelum and also distinguish the species in each case.
The capitula (flower-heads or pseudoflowers) are typical of the Asteraceae: consisting of an enlarged receptacle,
subtended by
phyllaries (small bracts) and bearing a number of florets (small true flowers). The florets are
arranged in a helix around the receptacle; the angular positioning of the florets follows a mathematical sequence
called a Fibonacci series. The phyllaries form an
involucre (cup). The basal (outermost) ray florets usually bear
ligules (a ligule is a petal-like extensions of the corolla lobe) except in Matricaria discoidea and occasional ligueless
forms of the other species. The remaining disc florets have cylindrical (radially symmetrical) corollas (floral tubes)
formed by the fusion of the 5 petals which are still separate as 5 lobes at the corolla tip, except in
Matricaria
discoidea
in which it has only 4 lobes. Typical of Asteraceae the ovule in each ovary is erect (vertically upright) and
anatropous (inverted or upside-down with the micropyle pointing downwards). The ovary of each floret develops into
an
achene fruit. In the Mayweeds and Chamomiles, however, there is no pappus of sepals (except where the
pappus remains as merely a short rim, as in
Tripleurospermum). As in other Asteraceae the 5 anthers are introrse
(turned inwards) and united via their cuticles such that they form a cylinder into which the anthers open to shed their
pollen. As in most Asteraceae, they are
protandrous (the male organs ripen before the female organs). The pollen
is then pushed out by the piston-like style which later grows up slowly inside the anther tube. This secondarily
presents pollen to the outside, where it is more accessible to insect pollinators. When this pollen has gone, the two
stigma lobes expand and become receptive. Some Asteracea may later elf-fertilise (if cross-pollination has been
unsuccessful) as the floret wilts. However,
Anthemis cotula and Anthemis arvensis, at least, are known to be
self-incompatible.
Article created: 1 Jan 2016