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Saturday, July 30, 2011

Garden Nasturtium, Tropaeolum majus, Tropaeolaceae

I think garden nasturtiums Tropaeolum majus were probably the first plants that I ever grew when I was a child. They are perfect for kids - large seeds for small fingers, reliably fast-growing, tolerant of almost any soil, colourful, long-flowering and harmless (edible, even).

Tropaeolum majus comes from South America, from the Andean foothills from Bolivia to Columbia, and has been grown in Britain since around 1686. It naturally has orange flowers, as in the top photo here, and grows in the wild as a climber, with prehensile leaf stalks (petioles). Successive cycles of selection and possibly hybridisation produced numerous varieties including dwarf, compact (Tom Thumb) forms and double cultivars that were popular in Victorian and Edwardian Britain. Today it's almost always raised from seed but in the 19th. century numerous named varieties were bred and propagated from cuttings that were overwintered in heated greenhouses, and were said by William Robinson, in his English Flower Garden, to produce more flowers and fewer leaves if grown in this way.

There are various accounts as to how the plant acquired its generic name. All say that Tropaeolum owes its name to the martial connotations of its shield-like leaves and helmet-like flower shape, but there are differing opinions on whether the name was derived from Greek or Latin. In Roman times a tropeum was a trophy pole used to display the armour and helmets of vanquished enemies, and some say Linnaeus consequently used the derived name Tropaeolum for the genus, recognising that the leaves resembled shields and the flower, when tilted downwards, looked like an elaborate helmet. Others say that Linnaeus derived the generic name from the Greek tropaion, meaning a trophy, for essentially the same reason

Nasturtium nectar is hidden deep inside the long spur (see top photo) formed from the sepals and long-tongued insects have to force their way into the mouth of the flower to reach it, although I have seen bumblebees that have learned to go around to the back of the flower and bit through the nectar spur to reach the nectar. John Gerard, in his General Historie of Plantes, mentions that these flowers were also known as lark's heel, on account of the spur that resembles the claw on a lark's foot.

There was, until recently, considerable debate about the evolutionary relationships of the garden nasturtium family, the Tropaeolaceae, with other plant families and for a long time it was belived to be related to geraniums (Geraniaceae), but modern molecular biological techniques, using DNA sequencing, shows that it is related to the cabbage family (Brassicaceae) and is one of 15 related families that all contain compounds called glucosinolates.
Interestingly, the large white butterfly Pieris brassicae that breeds on cabbage also took to breeding on this South American plant when it was introduced to Britain. I suspect that the butterfly's sensitive antennae, attuned to detecting the volatile chemical signals that identify suitable food plants for its caterpillars, must have detected the biochemical similarities between garden nasturtium and cabbage long before biochemists came to the same conclusion.