Impatiens (Balsaminaceae)

Above: Impatiens glandulifera, Indian Balsam or Large Touch-me-not. The stem-branches have finger-like glandular stipules on their nodes, hence 'glandulifera'.

The Balsaminaceae (Balsam family) currently consists of just two genera: Impatiens and Hydrocera. Impatiens contains over 1000 species, however. Impatiens are generally referred to as 'touch-me-nots' due to their explosive seed pods which may scatter seeds for several meters and may be triggered, when ripe, by a light touch. This gives rise to their Latin name of 'Impatiens' as 'impatient'.

In Impatiens, the bracteoles (leaflike bracts subtending each flower) are absent in most species, but are otherwise tiny. The tiny leaf-like appendages visible in Impartiens glandulifera, at the end of the flower-stalk or pedicel, are the lateral sepals. Indian Balsam is also called (Ornamental) Jewelweed and Himalayan Balsam. The flowers are borne in racemes (stalked flowers around a central axis) on peduncles (inflorescence stalks) borne in leaf axils. The zygomorphic flowers (flowers that are monosymmetric, with one plane of symmetry) are colored pink or purple-red and may have purple spots. The flowers are hypogynous (superior ovary with petals, sepals and stamens inserted on the receptacle beneath the ovary).

Key: A = androecium; DP = dorsal petal; LLP = lower lateral petal (paired); LS = lateral sepal (paired); P = peduncle/pedicel; S = spur; St = stigma; ULP = upper lateral petal (paired). The flower is about 4 cm in length.

The main body of the flower is the posterior sepal (lower sepal, LS) which extends into a nectar-producing spur (the yellow protuberance at the rear of the flower as seen above). Thus the flower has 3 sepals (the anterior-lateral sepals are lost) but note that the flower rotates through 180^o on the pedicel axis to be held in the 'upside-down' position (it is resupinate).

There are 5 petals, the dorsal petal forms a hood over the androecium and the 4 lateral sepals are fused into 2 pairs. There are five stamens which are free but ventral appendages on the filaments that surround the ovary and touch one-another (they are connivent around the ovary) (the anthers remain attached around developing fruit capsule as the filaments detach). The filaments thus form a sheath around the ovary and stigma, reducing the likelihood of self-pollination (or auto self-pollination occurring without insect mediators). There is no distinct style, only a sterile region at the ovary tip. There are 5 carpels, each containing up to 2 rows of ovules.

The fruit capsule is elastically dehiscent (bursting open explosively upon drying due to the build up of elastic potential energy) and up to 2 cm long and contains many spherical black seeds up to 7 mm in diameter. The flowers are pollinated by bumblebees (at least in Europe) as they have long enough tongues to reach the nectar in the spur. In some populations they reportedly can not self-pollinate, in others self-pollination occurs at a low to moderate rate.

Above: the flowers remind me of little squids; note the developing fruit capsules.

Indian Balsam is an annual herb reaching with a prolific growth rate, reaching 2 to 3 meters in height. The erect to ascending stems may be branches and are tinged purple, somewhat woody but hollow for most of their length or with a central watery pith.

Fleshy adventitious roots develop at the stem base and extend outwards almost horizontally on or just under the soil to anchor the plant and may also develop from other parts of the stem where they contact the ground on slopes or if the plant is blown over. The numerous roots reach 15 to 20 cm in length with a strengthened lignified cortex and a watery pith. This may be similar to the situation in Impatiens pallida (see Meyer and Walker 1931) in which the primary root emerges from the seed and develops secondary lateral branches that become the functional roots of the young plant, but in the mature plant the adventitious roots take over root function.

The lanceolate (lance-head shaped) to elliptical leaves are sharply serrated (with 18 to 50 teeth on each side) and up to 15 cm in length and arranged opposite or in whorls of 3. Note the tiny bracts (bracteoles). The color of the flowers and the shape of the showy dorsal and lower lateral petals are quite variable, even within the same population.

 Impatiens glandulifera is native to the Himalayas but widely invasive across most of the Northern Hemisphere and in New Zealand (see: distribution map). It is perhaps the most invasive plant species in Europe. It is particularly invasive along the banks of waterways (the specimens here were observed along the River Severn in the UK) but can also extend into disturbed woodlands and into ungrazed grassland (as long as the grass cover is not too dense). It can form dense stands that overshadow shorter plants and this could potentially cause problems with increased flooding when the annual dies back in the Autumn to leaves swathes of riverbank without vegetation cover. This is compounded by the shading out of plants that aid in bank stability. (See: Cockel and Tanner, 2012).

Studies have examined the impact of Impatiens glandulifera on native ecosystems with mixed results. In some studies the impact was minimal, even where Indian Balsam became the dominant species, with changes in abundance of other species rather than changes in species diversity. Other studies have shown significant impacts on species diversity, with some species benefiting and others losing out.

Growing as an alien, Impatiens glandulifera is reported to reach greater stature than its native habitat and it has been suggested that this may be due to reduced parasitism and/or browsing where it proves unpalatable to the native fauna. It grows prolifically for an annual herb and has been declared 'Europe's tallest annual herb' (Cockel and Tanner, 2012). This will give the plant a competitive advantage as an alien against natives.

The success of this species as an invader depends on additional factors. The flowers compete successfully for pollinators (significantly reducing pollination in neighboring Stachys sylvatica, for example) by offering a large quantity of nectar high in sugar and a large quantity of pollen. Explosive pods disperse seeds for about 3 to 5 m from the plant, but human activity and water can transport the seed much further. The seeds do not float for long and are transported close to the river bed and are deposited on the flood plain during high waters. They require a period of winter chilling before germinating and some remain viable in the soil for more than a year.

Chromosome counts for this species are published as 2n = 20 in Indian populations, 2n = 18 in the Central Rhodopes Mountains in Bulgaria and the genome is 1127 Mbp.

Bibliography - useful general reviews on Impatiens glandulifera:

Clements et al. 2008. The Biology of Invasive Alien Plants in Canada. 9. Impatiens glandulifera Royle: https://cdnsciencepub.com/doi/pdf/10.4141/CJPS06040

Cockel & Tanner, 2012: a good review of invasiveness