Monday, 17 July 2017

In flower: Solanum pyracanthos

A couple of years ago I obtained at Malvern Flower Show a plant of Solanum pyracanthos, the Porcupine Tomato. The reason for the name is pretty obvious – it is heavily armed on the stem and even the leaves with serious spikes.

Solanum species include some of the world’s major food crops, including potato, tomato and eggplant, as well as crops of more local significance such as naranjilla. Not in the same genus, but still part of the Solanaceae, are the peppers (Capsicum) and various species of Physalis such as the tomatillo.
Note spines on leaves
The Solanaceae as a group often produce potent alkaloid toxins in their foliage to protect against herbivores, and to humans at least many wild species have toxic fruit. As a result Europeans were slow to take to using plants like potatoes or tomatoes after they were first introduced, although they came round eventually.

The Solanaceae have around 3,000- 4,000 species in around 90 genera, and a worldwide distribution. Perhaps 1,400 species are currently in Solanum, many in South America, but there are other centres of endemism in North America, Africa, Australia, and Madagascar, the home of S.pyracanthos.
With so many species, Solanum is divided into numerous subgenera and sections. Solanum pyracanthos belongs to section Melongena, the same group as the eggplant, a group mainly confined to Africa. The numerous Madagascan species of Solanum seem to be part of a mainly endemic radiation of this section. Although Madagascars native animals, especially the lemurs, are the most famous targets of conservation concern, but the plants are just as significant and indeed endangered – the massive deforestation of the island in the last 2,000 years or so since human beings colonised the island must have had a major impact. Loss of native plants would have had an equally massive knock on effect on the native invertebrate fauna, and very little published work is available on these.

The range and habitat of S.pyracanthos in its native range is unclear, but its impressive appearance means that it has been cultivated in many places. Although not frost resistant, in warmer climates it grows easily and can become an invasive weed. It appears to be a fairly typical “weedy” successional species spread by birds or mammals eating the fruit – which is allegedly technically edible although extremely distasteful to humans (check carefully first before attempting one).
Archaeolemur edwardsii
The amount of armament covering the stems and even leaves is plainly a deterrent against plant eating animals. What they might be is not certain. Those living lemurs which subsist mainly on foliage spend most of their time in trees and seldom come down to ground level, and S.pyracanthos seldom reaches more than 1m tall. Some extinct lemurs such as Archaeolemur were more terrestrial however, and several living species of lemur are surprisingly resistant to toxins. It seems more likely that the main threats were animals like the now extinct giant tortoises such as Aldabrachelys abrupta, a close relative of the living Aldabra Giant Tortoise.
(Images of Aldabrachelys, Archaeolemur from Wikipedia, rest are mine)

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