31 July 2010


Small Balsam

Much is achieved by specialization in the study of natural history, but a more open and discursive approach is often more enjoyable and can lead to some unexpected byways, and, finally, to a deeper understanding of the subject as a whole. For one feature of the living world always holds pointers to another, and another, and so on, a chain of fascination and discovery without end.

The first link in the chain is usually forged lightly enough, with an idle speculation. On a walk through Cassiobury Park, for example, an unusual, weedy-looking plant may be found at this time of year. It grows abundantly in the gloomy woodland near the canal and River Gade in the southern end of the park, especially by the paths in the vicinity of the commercial watercress-beds.

With small yellow flowers and large, toothed leaves rather like those of the Dog’s Mercury, this plant is the Small Balsam, a close relative of the Busy Lizzie which makes such an easy and attractive pot-plant. It is a native of Siberia and Turkestan, and was introduced to this country in the last century, having first been found growing wild here at Battersea in 1851.

In Hertfordshire it occurs mainly in the vicinity of certain towns, including Hemel Hempstead and Watford. Its distribution in the county can be understood when it is known that one of its favourite habitats is timber yards and that seeds were presumably imported in the 19th century as stowaways in consignments of exotic timber from the east. The presence of Small Balsam in the Gade Valley may not be unconnected with the Corner Hall Wharf at Hemel Hempstead of W. H. Lavers & Sons, the well known local timber merchants.

Having been inadvertently introduced, the Small Balsam spread out as far as it could, being checked only by its own needs for moisture and shade. Most balsams are a bit like this, never being found far from water or damp ground. The Busy Lizzie likewise needs plenty of water to keep healthy.

A second alien balsam, the Orange Balsam or Jewel-weed, is also found in Cassiobury Park, as well as elsewhere along the line of the Grand Union Canal. In fact, a map of its distribution in Hertfordshire is tantamount to a map of the course of the canal. A larger, more handsome plant, with orange flowers spotted with crimson, the Jewel-weed got here from eastern North America as a garden introduction, first turning up in the wild in Surrey in 1822. It is now locally common along most of the waterways of the Thames basin and is apparently still on the increase.

Yet a third alien balsam, the Himalyan Balsam, variously known as “Policeman’s Helmet” (from the shape of the flowers), “Jumping Jack” (from its explosive seed dispersal mechanism), or “Nuns” (for no discernible reason!) can be found in the park. A big clump grows beside the third bridge over the Gade (counting downstream), adjoining the most luxuriant growth of the Small Balsam.

Policeman’s Helmet is an even showier plant than the Jewel-weed. It can reach a height of six feet or so, has thick and often reddish stems, and large flowers in every shade of purplish-pink. Again it was introduced in the 19th century as a garden plant, a native of India and the Himalaya, and was cultivated at first as a greenhouse annual. It soon escaped from captivity and by 1855 was to be found spreading rapidly along waterways.

Now these three foreign balsams illustrate a number of the laws by which nature seems to govern her affairs. The vigour with which they have taken over our waterways shows how alien genetic stock, freed from the competition at home, can have an unfair advantage over the home-grown organisms – for it is the same with fauna as with flora. The native stock has not had time to evolve defences against them: the checks and balances built up over tens of thousands of years no longer have any meaning.

The only native balsam, the Touch-me-not, is, in plant terms, something of a failure. It puts forth its flowers in damp woods in north-west England and north Wales, being very local even where it is found. It cannot compete effectively. Man’s mistreatment of the environment is going hard with it, while its foreign cousins are actually taking advantage of man and his activities.

The four British balsams, one native and three introduced, are all closely related and are obviously descended quite recently from a common ancestor. In size, though, they show a clear differentiation. Each species occupies a slightly different niche, has a slightly different role to play: this is how new species evolve. They rarely trespass on each other’s terrain. The Small Balsam, for example, is pollinated by hoverflies, the Touch-me-not by bees, and the Policeman’s Helmet by bumblebees. As for the Jewel-weed, it may dispense with pollination altogether, its flowers often all being of a type known as “cleistogamous”.

Cleistogamy (literally, “closed marriage”) is an unusual and somewhat degenerate response to the heavy cost of producing flowers as a means of reproduction. Cleistogamous flowers are of simplified form, with few pollen grains, and automatically self-pollinate at the bud stage. Among our group of balsams there is a pretty smooth gradation in this habit, from the Policeman’s Helmet, in which it is unknown, through the Small Balsam and Touch-me-not, in which increasing numbers of cleistogamous flowers are found, to the Jewel-weed, in which cleistogamy has assumed major importance.

A study of cleistogamy in these four plants would lead us deeper and deeper into the realms of genetics and evolution. A study of their habitat preferences draws us into the fields of plant physiology, ecology, and climatology. A study of their origins takes us to the history of human trade and commerce, 19th century tastes in timber and veneer, the exploitation of Russian and Asian forests to meet those demands ...

And so it goes. Many, if not all, the scientific investigations ever undertaken have had just such trivial beginnings as a stroll through the local park and an idle question like: “What’s the name of that plant?”

(Introduction to these pieces; see all)

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