More
Lamiaceae
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 axils.
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.
Above: details of flower of Ballota nigra. Clink the links below for the full-size images:
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Above: upper panel - The rim of the sepal funnel (calyx) is lined by glandular hairs. If you click on the links below to view the full-size images, 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 of lamiates.
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Above: lower panel - the upper (adaxial) surface of a young leaf of Ballota nigra.
Below: 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 insects.
Above: Ballota nigra var. foetida. Note the sepals form a funnel with 5 lobes ending in teeth/short-spines. In var. ruderalis (apparently much less common) the sepals teeth are more abruptly narrowed into a point, though there can be considerable variation even on the same plant. The purplish-rose colored corolla is variegated with white. The outside of the corolla, and the inside of the upper lip are hairy; the lower lip is 3-lobed with the middle lobe (itself ending in a pair of lobes) is the largest. The fruit are nucules (nutlets) characteristic of the Lamiaceae.
Ballota nigra is native to Europe and northern Africa, but has been introduced into parts of North and South America. It occurs in waste places and on hedge-banks.
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.
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.
The surface of many plant stems possess abundant hairs (trichomes) which may be particularly log and dense on the angles of the stems of Lamiaceae. These hairs have multiple functions but have been shown in some cases to make it difficult for potential insect herbivores to climb the stem. Such hairs may even be mechanosensory, allowing the plant to monitor insect traffic and invest resources into its chemical defenses accordingly. Pointed hairs may also pierce soft-bodied insects such as caterpillars whilst glandular hairs may trap insects with their sticky secretion that is often released upon touch (often by rupture of a thin and delicate membrane). Specific functions of trichomes have only been investigated in few plant species to date. In Lamium album, the stem angles are covered by long dense hairs (external link: https://fineartamerica.com/featured/sem-of-the-stem-of-lamium-album-dr-jeremy-burgessscience-photo-library.html) which probably makes it difficult for insects to pierce the tissues and reach the vascular bundles beneath to feed on the plant's nutrient transporting sap. The collenchyma along the stem angles, in addition to providing support, perhaps also protects the vascular bundles.
Salvia
pratensis (Meadow
Clary) and its pollination mechanism
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
visiting insect.
Half-flower diagrams for use in labeling exercises.
Salvia x sylvestris (Hybrid Clary)
This Salvia keys out as Salvia x sylvestris (Salvia pratensis x Salvia nemorosa) or Hybrid Clary with characteristic showy purple bracts (the leaf-like structures beneath each whorl of flowers) and colorful sepals without conspicuous white hairs (which would suggest Wild Clary, Salvia verbenaca, which also has green bracts that may be tinged violet-purple). This form may be confused with Whorled Clary, Salvia verticillata, which usually has much more crowded whorls with 15-30 flowers each, but occasionally only 8, whereas hybrid Clary usually has 1-6 flowers per whorl, but sometimes also 8. Salvia pratensis, Meadow Clary, is a much larger plant with larger flowers. This form was introduced to the British Isles and is now naturalized there and often found on maritime dunes. It developed in gardens and also as a natural hybrid in southestern Europe. It is partially fertile.
In Salvia the calyx tube has two lips, the upper lip is entire (as in Salvia reflexa) or has three teeth (formed from three fused sepals) which may be hard to see without a magnifying lens; the lower calyx lip is bifid (forks into two, consisting of two fused sepals). The corolla (petal tube) is also two-lipped, the upper lip vaulted and the lower lip 3-lobed, but with the middle lobe often notched (as above). There are two stamens, each containing one pair of anther cells (locules) which are separated by a long slender connective such that the flower appears to have 4 stamens united into pairs. Each such pair contains one dominant fertile anther lobe, the other lobe being reduced and usually sterile. Salvias are either herbs or small shrubs. Salvia x sylvestris is perennial.
Prunella
vulgaris (Self-heal)
and its anti-viral properties
Above and below: 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 vulgaris L.).
Close up view of the flowers (click image for full size).
Above: a white variety of Selfheal. This plant is lacking anthocyanin pigment in the petals and sepals. Prunella vulgaris occurs in a number of beautiful forms, such as these forms from Minnesota: (external link).