|Lamiaceae - Deadnettles and Mints
Above: yellow archangel or yellow deadnettle, Lamium galeobdolon (also
Lamiastrum galeobdolon) (Cromer's wood, Kent, UK).
Above and below: bugle, Ajuga reptans (Cromer's Wood, Kent, UK). Often
the square stem is only hairy on two opposite sides. Floral formula: K(5)
C(5) A4 G(2). All photos on this page can be enlarged by clicking on them.
Above: white archangel or white deadnettle, Lamium album (click image to enlarge). Below: a common feature of
lamiates, including both Lamium album and Ajuga reptans is the arrangement of flowers in tiers of about six or
more. However, in the deadnettle there are clearly only two leaf-like bracts per tier of flowers, so we have two
main branches off the central axis at each node. These bracts are sometimes considered leaves, they are
primary bracts subtending an inflorescence, rather than secondary bracts (bracteoles) subtending an individual
flower. The bracteoles are visible as the small green spikey leaflike structure beneath each flower unit. What we
have are sympodial branches off a monopodial axis, in which the branches each bear several flowers. (See
plant modules). The axis is really the vegetative axis and each branch is an inflorescence, though together the
tiers of flower-bearing branches and central axis create a composite inflorescence which is a flower spike. Each
inflorescence branch is a type of inflorescence called a cyme, meaning it is sympodial and, initially at least, it is
a dichasial cyme, meaning that each unit (node + internode) of the cyme produces two flowers. However, the
picture may be further complicated in some plants of this type as each unit may secondarily become
monochasial, bearing only one flower per unit. The resulting 'whorl' of flowers making up a tier is therefore
actually a false whorl called a verticillaster.
Ajugla reptans thrives in damp woods, meadows and pastures. Here a large number were found amongst
bluebells (Hyacinthoides non-scripta), primroses (Primula) and early purple orchids (Orchis mascula). The bugle
is perennial, growing to 10 to 30 cm in height from a short rhizome (underground stem). The inflorescence grows
from a basal rosette of leaves. The square stems are often hairy on two opposite sides only (in this case they
were more-or-less hairy on all four sides). These plants can reproduce asexually by means of long, leafy, rooting
stolons. Typical of lamiates the flowers have bilateral symmetry. The flowers are hermaphroditic, appear in
early spring and are blue (rarely pink or white) and usually appear in 6-flowered whorls in the axils of the leaf-like
bracts visible in the photo above. Protandry is common in lamiates. In protandry the male organs ripen first and
the female organs ripen later, so that the flower, although hermaphroditic, is functionally male first, female later -
a mechanism which reduces or prevents inbreeding. The stamens ripen first, whilst the stigma is held well above,
out of the way of pollinating insects, and later bends down to near the entrance to the corolla ( which forms a
petal tube) so as to receive pollen. There is a nectary at the base of the ovary and bees and other insects
pollinate these flowers.
The lamiaceae (labiatae) consists of plants such as the deadnettles and mints, which are called lamiates or
Above: a verticillaster of false-whorl (pseudowhorl) of flowers in Lamium
album. This whorl actually consists of two very short sympodial branches.
The white deadnettle is also protandrous - in the initial male stage the stigma is held well away from the
flower opening and out of the way of visiting insects. Later on the flower becomes functionally female,
and the stigma is brought near the entrance to receive pollen from visiting insects. The half-flowers,
floral diagram and floral formula of Lamium album are shown below.
Notice that although most parts of the flower occur in groups of five,
there are only four stamens. The missing stamen is sometimes
represented as a dot in floral diagrams (not shown here).
Above: the red deadnettle, Lamium purpureum, is a much smaller plant than Lamium album or Lamium
galeobdolon with smaller flowers. (Left: Cromer's Wood, Kent, UK; right: Milton Creek, Kent, UK).
Above: ground ivy (Gleochoma hederacea) is another labiate. (Borden
Nature Reserve, Kent, UK).
5th Oct 2014
16th March 2015
25 May 2015
4 July 2015
Above: hedge woundwort (Stachys sylvatica).
Above: Wild Marjoram, Origanum vulgare, growing on calcareous soil. Habitat: grasslands, scrub, hedgebanks, on
calcareous soils. Note the characteristic protruding stamens.
Ballota nigra (Black Horehound) - a microscopical study
Ballota nigra (Black Horehound) emits a
distinctive harsh musky or resinous odour when
damaged. It is a perennial herb and often a tall
plant, up to 80 cm in height. It is commonly
found on hedge-banks and roadsides. It is
described as hairy and under the microscope
the hairs on the floral parts are vividly apparent.
The upper lip is slightly concave and the lower
lip divided into small lateral lobes and a large
central lobe. The flowers are borne on the leaf
The rim of the sepal funnel (calyx) is lined by
glandular hairs. If you click on these
thumbnails to enlarge them, then you will see
capitate hairs (hairs ending in spherical heads
like tiny pins) around the rim of the calyx. This
morphology is typical of secretory hairs. Often
the function of such glandular hairs is
uncertain or completely unknown. They are
common on the aerial parts of Lamiaceae and
their distribution is of taxonomic significance.
Along with a second type of secretory hair
often found (peltate hairs) the capitate hairs
secrete the specific essential oil characteristic
The oil is secreted beneath the thin covering cuticle which swells like a balloon before eventually rupturing to release the volatile
oil. The essential oil of Black Horehound has been shown to have antimicrobial powers. Perhaps these glandular hairs are
positioned to protect the nectar from ants crawling up the stem. Ants reportedly often steal nectar from plants, often without
efficiently pollinating flowers intended for other types of insects.
Examining the flowers in detail, protandry is evident. In
the flower with freshly opened anthers (x 4) still packed
with white pollen, on the left, the stigma is not readily
visible. However, in the flower on the right which has
lost most of its pollen (presumably the anthers have
been dehisced for longer) the bifid (forked) stigma is
readily visible bending forwards (downwards when on
the plant) into position to intercept pollen from visiting
insects. In the top right is a flower with one anther in
prime position to intercept a visiting insect.
Left the upper (adaxial) surface of a young leaf of
Ballota nigra. Right: the undersurface (abaxial
surface) of the same leaf, showing hairs along the
veins. This is a common feature in many plants and
possibly protects the phloem from sap-feeding
Why a square stem?
A characteristic of lamiates (and some other herbaceous plants) is that the stem is distinctly squarish in cross-section. The main
vascular bundles are situated near the four corners (smaller bundles are often situated midway between each pair of corners). The
four corners contain ridges rich in collenchyma - cells with heavily thickened cellulose cell walls. This arrangement optimises
mechanical strength, whilst being more resource effective than forming a complete cylinder of collenchyma. Having the vascular
bundles situated towards the periphery makes the stem stiffer (the moment of bending is increased), since xylem is a strong tissue.
The four longitudinal ridges of collenchyma also add strength. Collenchyma forms slightly elastic but tough tissues to help the stem
resist bending forces. Collenchyma is also plastic and will flow and deform to accommodate growth of the stem.
The yellow archangel, Lamiastrum galeobdolon, has an interesting biology. Most forms can reproduce asexually
by means of stolons or suckers (a horizontal shoot which roots at intervals to produce new plants) which run either
beneath the ground or along it and which may bear leaves. Three main subspecies are naturally occurring in
Europe: subspecies montanum, galeobdolon and flavidum. In Britain only montanum and galeobdolon occur
naturally, flavidum (which unlike the others does not form stolons) occurs in mountainous regions on continental
Europe, such as the Central and Eastern Alps. In the 1980s another subspecies, Lamiastrum galeobdolon ssp.
argentatum escaped from horticulture and became naturalised and is now the most widespread in Britain and
Ireland, occurring in most regions. Montanum is quite widespread in england and Wales but very scattered in
Scotland. It grows in woodlands and hedges, apparently preferring woodland edges to deeply shaded areas and
occurs in beechwoods on limestone, and in oak-ash woods along with Bluebell (Hyacinthoides non-scripta),
anemones, arums, primulas, Ajuga reptans (Bugle), Dog's Mercury and Wood Sanicle.
Subspecies argentatum, also known as variegated yellow archangel, has 'variegated leaves' which actually have
colourless regions which contain air pockets (these are much less frequent in the other subspecies). This invasive
form was bred from native European forms. Telling subspecies montanum and galeobdolon apart is not so easy.
Subspecies galeobdolon is diploid (it has two copies, 2n, of n chromosomes) with 18 chromosomes (n = 9)
whereas montanum is tetraploid (4n) with 36 chromosomes. Montanum is more vigorous and is generally a taller
plant at 20-60 cm compared to 15-45 cm for galeobdolon and more flowers per whorl (more than 8 compared with
less than 8) and more whorls per inflorescence (4-7, sometimes 3-10, compared with 2-4, sometimes 5). The
upper bracts and stolon leaves of subspecies galeobdolon tend to have more crenate (rounded) teeth whereas
the teeth tend to be more serrate (pointed) in montanum. However, all these characteristics overlap, a montanum
growing in poorer conditions may resemble galeobdolon in vigour. However, a study by Wegmuller (Wegmuller, S,
1971. A cytotaxonomic study of Lamiastrum galeobdolon (L.) Ehrend. & Polatschek in Britain. Watsonia, 8:
277-288) showed a good separation of the two if one considers stomatal length together with bract length to width
ratio (l/w): ssp. galeobdolon has a bract l/w of about 0.9 to 1.8 and a stomatal length of 24 to 28 micrometres;
montanum has a bract l/w of 1.4 to 4 and a stomatal length of 28 to 36 micrometres.
It has been suggested that ssp. montanum is a hybrid of ssp. galeobdolon and ssp. flavidum as its characteristics
are intermediate. This would make it an allotetraploid - a tetraploid with two chromosome sets from one species
(or subspecies) and two from another.
Ssp. argentatum was favoured by gardeners partly because it easily spreads forms dense leaf cover. Although it
does not set seed it reproduces rapidly by stolon formation. this characteristic makes it a potential threat as an
invasive alien species, a potential problem being investigated by prof. Ian rotherham of Sheffield hallam University,
More Lamiaceae ...
Close-up views of the flower of
Lamium purpureum. The four
anthers and two stigmas are
The flower is, like Lamium album,
pollinated by bees and in both
species a significant quantity of
nectar can often be found in the
base of the flower.Studies in L.
album have shown that sticky
trichomes on the anther, near the
dehiscence slit from which pollen is
released, trap some of the pollen
grains. Observations suggest
these act as pollen presenters,
presenting pollen as a food reward
to visiting insects (sacrificing some
pollen so that other pollen has a
chance of successful pollination).
Above and right: a cross-section of the stem of
Lamium through an internode. The cortex and
medulla (pith) are composed of parenchyma
cells; smaller cells are found in the outer than in
the inner cortex and the largest parenchyma cells
occupy the medulla. The central pith has broken
down to form a central pith cavity, as it failed to
keep up with rapid stem elongation. In this type of
stem, central pith typically remains in the nodes,
forming nodal diaphragms. Collenchyma with
massively thickened walls occurs in the
longitudinal ridges or angles of the stem.
Collenchyma with thinner walls also occurs in the
vascular bundles, forming the vascular sheath.
The vascular cambium, consisting of several
layers of mitotically active parenchyma cells,
forms secondary xylem and phloem. In some
lamiates, the interfascicular cambium (layers of
parenchyma in between vascular bundles which
is continuous with the vascular cambium)
produces sclerenchyma as the stem matures.
Both the petals and stamens of L. album are thought to secrete scent (epidermal glands on the filament and multicellular
trichomes on the anther are thought to secrete this, along with secretory trichomes and papillae on the petals, though
establishing the function of individual secretions is not easy). The nectary of L. album consists of four lobes surrounding the
base of the ovary. 'Teeth' on the sides of the lower lip in Lamium album have been shown to support the legs of visiting insects
and the lateral teeth seen here in Lamium purpureum possibly perform the same function. (See: Sulborska, A., M. Dmitruk, A.
Konarska, and E. Weryszko-Chmielewska, 2014. Adaptations of Lamium album L. flowers to pollination by Apoidea. Acta Sci.
Pol., Hortorum Cultus 13(6): 31-43).
Above and left: Salvia pratensis (Meadow Clary or Meadow
Sage) (Queendown Warren, Kent, Britain). This plant is
threatened in Britain.
The curious viper-like flowers employ a staminal-lever
pollination mechanism (see the half-flower diagram below).
The two anthers lie side-by-side and both have the same form:
a short filament forks into a connective which joins the two
anther lobes. The topmost anther lobe is fertile and produces
pollen (it is unilocular) whereas the lower lobe is sterile. The
connective pivots on a hinge with the filament. Together the
two sterile lobes form a shield.
When a pollinating insect, visiting a young flower, forces its way
to the nectar at the base of the floral tube (corolla) it pushes
the shield forwards, causing the connective to pivot, swinging
the fertile lobe downwards to dab pollen on the insect's back.
Older flowers enter a female stage, in which the style has been
brought down into position to receive pollen from the back of a
Half-flower diagrams for use in
Left: Selfheal, Prunella vulgaris. This plant was used
as a cure-all in Medieval times. Science has since
shown that the Lamiaceae have a wide variety of
medicinal uses and Selfheal is no exception.
Aqueous extracts of this plant have been shown to
greatly inhibit the ability of HIV-1 (the most
aggressive strain of Human Immunodeficiency Virus
which causes AIDS) to infect cells in culture. (See:
Oh et al., 2011. Virology Journal 8:188 Inhibition of
HIV-1 infection by aqueous extracts of Prunella