The sadistic dispersal strategy of the puncture vine
In his third 'Seed of the Month' blog series, Wolfgang Stuppy warns us of the 'dangerous' seeds of Tribulus terrestris.
Traveling with animals is surely the most successful dispersal strategy that plants have evolved. Some want their fruits and seeds to be swallowed so that they travel as stowaways in the gut of an animal, whereas others prefer to latch onto the outside of passers-by.
The puncture vine: flower with fruit below
After a walk through the countryside we often find some ‘sticky hitchhikers’ on our socks and trousers. The means by which they manage to attach themselves is simple and consists of small hooks that readily become entangled with the fur of mammals or the tiny loops of thread in the fabric of clothes. It was the microscopic structure of these diaspores that in the 1950s inspired the Swiss electrical engineer George de Mestral to develop the hook and loop fastener that we nowadays all know under the name Velcro®(based on the French velour = velvet, and crochet = hook). The ‘Velcro’ means of seed attachment is the gentle version and one very tenacious but harmless example is our native 'stickywilly' - also known as cleavers (Galium aparine).
Fruits of Galium aparine
Other plants are less gentle and some are, in fact, rather sadistic when it comes to achieving the dispersal of their seeds.
One example, the so-called puncture-vine, is found in warmer parts of Europe where its diabolical insidiousness has earned the fruits of this little plant the name 'devil’s claw' or 'caltrop' (See Note:1 below for Wikipedia's definition of a caltrop).
Puncture vine (Tribulus terrestris) fruits
The tribulation inflicted by the fruits on unwitting animals (and sometimes people) is even reflected in the plant’s scientific name (Tribulus terrestris). As the fruits of the puncture vine, a member of the Zygophyllaceae family, mature, they split into five ‘nutlets’. Each nutlet is armed with two large and several smaller spines. In whatever position the nutlets end up on the ground, some of their spines will always point upward, like a medieval caltrop, ready to penetrate the skin of an animal or even the soles of shoes. For example, on the Hungarian plains the prickly hitchhikers used to cause sheep farmers considerable trouble by inflicting suppurating wounds on their animals, which hampered their ability to walk.
Martynia annua mature fruit showing ferocious spines
Three mature fruits of Martynia annua showing their ferocious spines and with one fruit still with its soft green cover on it
Claws of the devil
Other ruthless and brutal examples of ‘devil’s claws’ occur in the dry tropical and subtropical semi-deserts, savannahs and grasslands of America, Africa and Madagascar. Like the fruit of the puncture vine, they bear sharply pointed spines, claws or horns which bury themselves into the flesh rather than gently ‘velcro-ing’ to the fur of animals.
The North American devil’s claws belong to species of the genus Proboscidea (esp. Proboscidea louisianica) and their smaller relative Martynia annua, and are both members of the Martyniaceae family.
Proboscidea louisianica mature fruit with its two sharp hooks widely spread ready to catch onto the hoof of an animal
In South America it is their carnivorous relatives in the genus Ibicella that produce very similar devil’s claws, or unicorn fruits as they are also called (e.g. Ibicella lutea). All of them have harmless looking green fruits which only reveal their true nature after their fleshy outer part has withered away. As the exposed endocarps dry out, their elongated beak splits down the middle to produce a pair of curved, sharply pointed, spurs turning the diaspore into a vicious contraption, poised to cling around the feet of hoofed animals and bore into their skin.
The old-world members of the sesame family (Pedaliaceae) honour their close relationship with the Martyniaceae by sharing the same ruthless concept of dispersal and produce even meaner traps. As the author can testify from his own experience, the fruits of the Malagasy genus Uncarina are undoubtedly the most tenacious fruits of all. With their radiating spines crowned by a pair of sharply pointed, recurved hooks, they not only rip into skin with great ease but are also impossible to untangle without using a secateur.
Uncarina species showing spines and hooks
The most infamous member of the sesame family is Harpagophytum procumbens, aptly called grappling hook, grapple plant or, like its New World relatives, devil’s claw. Used as mouse traps in Madagascar, the almost preposterously horrid looking woody pods can inflict gruesome wounds to animals with cleft hoofs or relatively soft soles.
Besides being cruel, plants can also be fraudsters, as you will find out in my next blog.
All above images by Wolfgang Stuppy and copyright Board of Trustees, Royal Botanic Gardens, Kew
Note 1: A caltrop consists of four spines arranged to point to the four corners of a tetrahedron so that however it falls, it will sit on three of the spines with the fourth one pointing up in the air. Caltrops were first used as a means to slow down pursuers on horseback but later proved to also work on pneumatic tyres in the motorised age. Iron caltrops were used as early as 331 BC at Gaugamela according to Quintus Curtius (IV.13.36). They were known to the Romans as tribulus or sometimes as Murex ferreus, the latter meaning 'jagged iron'. (from Wikipedia)
- Millennium Seed Bank partners in Botswana collect 'devil's claw'
- Image of devil's claw
- Understand the parts of fruits and seeds in Wolfgang's glossary (pdf)