Lamiaceae - Deadnettles and Mints
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.
(Common Bugle) thrives in damp woods, meadows and pastures. Here a
large number were found amongst bluebells (Hyacinthoides
primroses (Primula) and early purple orchids
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 labiates.
In Ajuga reptans, the corolla (petal tube) is dull blue, less often purple-pink or white. The calyx (sepal tube) has 5 short triangular teeth. The flowers are arranged in psuedowhorls called verticillasters (in which the flowers are arranged along a short spiral forming an apparent whorl around the stem). In Ajuga reptans there are 6 to 12 flowers per verticillaster.
Above: the uncommon white variety of Bugle. In some plants white forms arise due to genetic mutation blocking synthesis of anthocyanin pigments which give plants their red, purple and blue colours.
The stems have two opposite stripes of jointed white glandular hairs which possibly act as deterrents to insect herbivores climbing the stem. Ajuga reptans is perennial. The rootstock puts out runnerlike stolons along the ground, which extend for up to 30 cm and bear pairs of leaves and produce a terminal rosette of leaves at their apex. These stolons form roots in the autumn and are a means of vegetative propagation. The flowering stems are erect and do not root. They bear 1 to 2 pairs of leaves. The plant produces a basal rosette of stalked leaves which persist until flowering. As is typical of many lamiaceae the bracts are leaf-like but lack stalks (they are sessile). The bracts becomes shorter than the petal tubes towards the top of the spike and also become flushed with color, as can be seen here.
Common Bugle is a native of temperate Europe. The name 'Bugle' is said to be derived from the decorative glass bugle beads by Syme in English Botany (1867) but it has also been suggested that it was due to some association or confusion with Viper's Bugloss (Echium vulgare) (Harvey-Gibson, 1923 in British Plant Names and their derivations by A. 7 C. Black, Ltd, London). The derivation of Ajuga is unclear, but reptans means 'creeping' (as in 'reptile') and refers to the reproductive stolons.
Lamium album (White Archangel, White Deadnettle)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
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).
Lamium album is perennial with a creeping and branching rootstock that is generally below the surface of the ground. The basal leaves are stalked and the bracts (seen here) beneath the flowers are very similar to the leaves but with shorter stalks. Each tier of flowers (verticillaster) consists of 5 to 22 flowers. The corolla (petal tube) enclosed at the base by the calyx (sepal tube) which has 5 very long teeth at the apex. The calyx is spotted or stained dark purple (these spots superficially resemble blackfly from a distance!). The stems are procumbent at the base but the flower bearing parts are erect and may reach 60 cm in height. flowerless vegetative stems are also produced, which are short and upright.
The ovary consists of two carpels, each with two cells, containing a total of 4 ovules. Each of the 4 parts of the ovary develops into a nutlet or nucule. This fruit is characteristic of lamiates. There is a single common style with two stigma lobes.
The two photos above and the photos below are from the same population of Lamium album. These were growing in exposed non-shaded habitats (beside a salt marsh and estuarine river) and many of the plants were flushed red, due to increased anthocyanin synthesis, very strikingly so in many cases with the upper bracts in particular being a deep red color. However, green and reddened plants grew side-by-side, suggesting that this is not simply a response to increased sunlight (or salinity?) but also a genetic phenomenon. The red anthocyanins are stored in the plant cell vacuole and have been implicated in protecting the plant cell from UV damage, oxidative stress and high salinity. In particular, plants that are adapted to tolerate high salinity may increase anthocyanin production when exposed to increased salinity (e.g. in Atriplex portulacoides) but not all plants do. This habitat by an estuarine river was also exposed and brightly sunlit.
It is thought that anthocyanins primarily protect the chlorophyll from photo-oxidation (damage by bright sunlight). It has been suggested that their synthesis in response to other stressors may still be intended to protect the chlorophyll, perhaps under conditions of stress which impair the ability of the plant to regenerate damaged chlorophyll, or to protect the leaf itself when chlorophyll content is low (since chlorophyll harnesses light it reduces photodamage to other systems within the leaf). Young expanding leaves are also more vulnerable to photodamage as their chlorophyll reserves accumulate and in many plants young leaves are distinctly redder in color as they accumulate anthocyanins to protect the growing leaf. Once chlorophyll has accumulated the chlorophyll will harness the light and the leaf will be more resistant to photodamage.
The younger leaves at the top of the inflorescence in these specimens were redder than the more mature leaves lower down.
Lamium album is pollinated by long-tongued bees, especially bumblebees, which feed on the nectar and pollen. Note the yellow-green spots on the lower lip middle lobe which intensify towards the corolla mouth. these are thought to act as nectar guides (they resemble scattered pollen) (Sulborska et al. 2014). The lower lip consists of the prominent central lobe (a double lobe) and the two lateral lobes each of which is reduced to 2 or 3 teeth-like projections which allow the insects to remain on the flower (Sulborska et al. 2014). (A variety noted from Cirencester, UK, has enlarged lateral lobes).
The nectar is produced by a ring of tissue surrounding the ovary base (and more developed on the lower part) deep inside the corolla (flower tube). This nectary is covered by a layer of irregular epidermal cells and bears no obvious pores or obvious structures for nectar secretion. The name 'Lamium' is derived from the Greek laimos, meaning 'throat' in reference to the throat-like corolla tubes which require bees with long tongues to reach the nectar. Deep within the throat, above the nectary, is a partial ring of hairs that serves to keep rain out from the nectar chamber (Sulborska et al. 2014) and perhaps also creates a barrier to small insects who would otherwise rob the nectar without pollinating the flower. The flowers are nototribic, meaning that they preferentially deposit pollen on the back of the pollinating insect (there are 4 stamens in 2 pairs, the upper pair having the longer filaments).
The flowers secrete a subtle fragrance to help attract pollinators. The inner epidermis of the corolla bears conical trichomes and 'capitate' trichomes which secrete fragrant essential oils and the 4 stamens also bear multicellular trichomes and capitate glands that secrete fragrant essential oils.The anthers are black and very hairy. with long trichomes near the dehiscence slit which collect the released pollen and present it to visiting insects. The fragrant hairs also catch pollen grains with their secretions. (Sulborska et al. 2014)
Below: close-up views of flowers. Note the greenish-brown spots that indicate the landing platform, which signals to bees to 'land here'. Long-tongued bees, particularly bumblebees are the main pollinators. (Click each image for the full size version).
Below: the stamens can clearly be seen and also one of the white stigma lobes projecting downwards from inside the hood.
Close, D.C. and Beadle, C.L. 2003. The Ecophysiology of Foliar Anthocyanin. The Botanical Review 69(2): 149–161.
et al. 2014. Adaptations of Lamium album L. flowers
to pollination by Apoidea. Acta Sci. Pol., Hortorum Cultus
Lamium purpureum (Red Deadnettle)
Above: a large specimen of Red Deadnettle, Lamium purpureum. Red Deadnettle is often a much smaller plant than Lamium album or Lamium galeobdolon with smaller flowers, but can reach 30 cm (12 inches) in height.
Below: close-up views of the flowers, click each image to view the full-size version.
views of the flower of Lamium
The four anthers and two stigmas are clearly visible.
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).
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).
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