semillas la vida en cápsulas de tiempo - Clh.es

The force of the blast catapults the flat discoid seeds, of which there are as many as there are carpels, as far as 14m.The name sandbox tree dates back to the time before the invention of blotting paper and fountain pens, when thefruits served as containers for the sand used to blot the ink that ran profusely from goose quills.Active explosivesFleshy dehiscent fruits are able to build up sufficient pressure to explode by means of tissues of thin-walled cells,which increase their internal cell pressure (called turgor) by taking up additional water. As the tissues swell, at somepoint the pressure against neighbouring inelastic layers is so high that the slightest movement of the fruit sets offthe explosion that expels the seeds. A passing animal can trigger the mechanism and although the seeds of suchfruits are generally smooth and non-sticky, they may become entangled in its fur and be carried some distance.Classic examples of actively exploding fruits are the touch-me-not (Impatiens spp.) in the balsam family(Balsaminaceae), and the squirting cucumber (Ecballium elaterium) in the gourd family (Cucurbitaceae).Anyone who has played with the fruits of the touch-me-not, gently squeezing them between two fingers,knows that its name is appropriate. Its pendulous fruits are composed of five carpels, with pre-formed dehiscencelines between them. Pressure builds up in the outer layers of the pericarp and when it is strong enough to separatethe carpels, the slightest movement causes the fruits to explode. The outside of the pericarp expands more thanthe inside causing the fruit valves to curve inwards in a lightning-fast movement that hurls the seeds up to 5maway. The trigger for the explosion can be a passing animal, raindrops or water dripping off a tree, the wind, oreven seeds launched from a neighbouring fruit. As the fruits of the Mediterranean squirting cucumber ripen, theinner tissue layers build up turgor pressure against the thick, rather inelastic outer skin. The pre-determinedbreaking line at the base of the stalk points up in the air as the fruit turns almost 180º against the pedicel.Eventually, the slightest movement causes the stalk to pop out like a cork, leaving a hole through which bothwatery juice and seeds squirt out. The pressure inside the fruit is so high that the seeds can travel more than 10m.The true record breakers are dwarf mistletoes, which apply more or less the same principle as the squirtingcucumber. Predominantly native to North America, they are parasites on pine trees and belong to the genusArceuthobium in the Christmas mistletoe family (Viscaceae; recently united with the sandalwood family, Santalaceae).Whereas other mistletoes rely upon birds for the dispersal of their seeds, dwarf mistletoes (with the exception ofArceuthobium verticilliflorum) have evolved explosively dehiscent fruits. Just as in the squirting cucumber, the pedicelbends downwards as the fruit matures and a dehiscence line forms around its point of attachment. Once the fruithas reached maturity, the slightest touch dislodges it from the pedicel. The pressure that builds up inside the darkgreen, single-seeded berry is so high that it fires the tiny sticky seed through the hole in the rubbery fruit wall overa distance of up to 16m at the remarkable speed of 2m per second (97km per hour). A single ponderosa pineinfected with Arceuthobium campylopodum may be bombarded by more than two million seeds, which overwinter onthe surface where they land and ger-minate the following spring. If a seed is lucky enough to land on a compatiblehost tree, its embryo grows, supported by the photosynthetic endosperm tissue, and enters the host’s bark.Aceuthobium species can live inside their host for months or even years before producing a plant.Whether the seeds are catapulted from passively or actively exploding fruits, they are dispersed over nomore than a few metres. The advantage of ballistic dispersal is that it is cheap, requiring no animal reward andusually very little in terms of specialised structures. The preferred strategy of annual plants is to disperse theirseeds in a way that allows them to maintain and expand an existing population in the same place rather thancolonising new territories. For perennial plants the most important factor is that the seeds should avoidcompetition with the parent plant. However, the ejection of the seeds from their fruits is often only the first phaseof their dispersal history. Many seeds, such as the squirting cucumber, pansies, spurges and gorse, have an energyrich,oily appendage, which functions as an edible bait that lures ants into carrying them away from the parentplant. Apart from such appendages, ballistically dispersed seeds are usually small, smooth and more or lessspherical, ensuring low air resistance.Geocarpy – or how do peanuts end up underground?This is a legitimate question if one has never seen a peanut plant (Arachis hypogaea, Fabaceae). After all, peanuts arefruits and fruits develop from flowers, and flowers need to be pollinated, which mostly happens in the air. So howcan a fruit end up underground? The answer is that the plant itself buries them. After pollination and fertilization, theovary is pushed down into the ground, hence the name geocarpy for this type of self-dispersal. The single carpel of thepeanut flower sits at the end of a specialised stalk-like organ called the gynophore. As soon as the ovules are fertilised,the gynophore bends down and elongates until it has pushed the young, pointed ovary underground. Once the ovaryhas reached its subterranean destination, it swells to produce peanuts. Other legumes that bury their fruits in the sameway are the Bambara groundnut from West Africa (Vigna subterranea) and Astragalus hypogaeus from west Siberia.Geocarpy is mostly found in annual plants living in dry, hot climates such as deserts, grasslands and savannahhabitats. Here, burying the fruits is not only a safeguard against grazing animals, it primarily ensures that the nextgeneration is kept in a suitable location in an inhospitable environment.One rare temperate example of a plant that buries its fruits is the ivy-leafed toadflax (Cymbalaria muralis,Plantaginaceae), a common wild flower on walls and rocks in Britain, Europe and North America. When inbloom, the flowers face the sun. As soon as they have been pollinated, their pedicels turn away from the sunlightand seek out shade by growing longer and searching the substrate for suitably dark cracks and crannies in whichto deposit their fruit. When ripe, the capsules open to release a number of small, irregularly ornamented seeds.Careful planting by the mother plant and the rugged surface of the fruits prevent them from rolling out of theirprotected environment.Self-burying drillsSome diaspores bury themselves after dispersal. Apart from keeping them hidden from animal predators, this selfburialis regarded as an adaptation to dry soils as it allows the diaspore to reach the more humid layers just underthe soil surface. The rotating movement of their hygroscopic appendages, which twist and untwist with changesin humidity, allows the diaspore to drill itself into the ground. This behaviour is most famously found in speciesof stork’s bills, e.g. red-stem stork’s bill (Erodium cicutarium), and musky stork’s bill (E. moschatum). Their diasporesconsist of fragments (fruitlets) of a schizocarpic fruit with a long beak. Once the fruit has come apart, each singleseededfruitlet retains a share of the beak, which serves as a hygroscopically moving awn. The same mechanismis found in a number of species of the very distantly related grass family (Poaceae). The florets of wild oat (Avenafatua), wild barley (Hordeum vulgare ssp. spontaneum) and the needle-and-thread grass (Stipa comata) are equippedwith an awn, the basal portion of which twists and untwists with changes in moisture levels. The drilling floretsof the Australian corkscrew spear-grass (Stipa setacea) are so sharp that they are able to penetrate the wool andskin of sheep. Once embedded in the flesh of the animal, the curved hairs on the surface of the florets make themalmost impossible to dislodge. The muscle movements of the tortured animals drag the fruits deeper and deeperinto their body. Florets of Stipa setacea have allegedly been found in the heart muscle of dead sheep.Creeps and jerksHygroscopically moving appendages enable other diaspores to creep along the ground for short distances. Suchcreeping diaspores are found in some grasses, and in the sunflower family (Asteraceae) and teasel family(Dipsacaceae). Their fruits have a hygroscopically moving pappus (modified calyx). For example, the fruits(cypselas) of the cornflower (Centaurea cyanus, Asteraceae) are crowned by a tuft of short, stiff, scale-like pappussegments, which are too small to play any role in wind dispersal. Their function is rather different: with changinghumidity the pappus scales repeatedly move in and out, thus pushing the fruits a few centimetres over theground. The very short, forward-pointing teeth along their margins prevent movement in the opposite direction.The distances they creep are short but at least they move away from the parent plant; wind and rainwater maycarry them further. A more targeted additional strategy has given them an edible swelling at the base tospecifically attract ants, a mode of dispersal that will be discussed in detail.Specialised awns enable the fruits of certain grasses (e.g. Arrhenatherum elatius, Avena sterilis) to jerk and jumpfor short distances. The distal part of their long-kneed awns is straight whereas the lower part is helically twistedand extremely hygroscopic. With changing humidity the basal part winds or unwinds, turning the straight distalpart of the awn. Since each fruit has two awns, which turn in opposite directions, their distal parts eventuallymeet and become entangled. The tension that builds up between them is released when the pressure is strongenough to push the distal parts past each other. Within a split second, the jerky movement of the awns catapultsthe diaspore into the air.Dispersal by animalsAnimal movements are less haphazard than wind and water making animals much more reliable as dispersalagents. A plant that manages to develop a relationship with animals needs fewer seeds to guarantee the survivalof the species. The evolution of animal-dispersed diaspores clearly has many advantages and so it is not surprisingthat some fifty per cent of gymnosperms (Ephedra, Gnetum, Ginkgo, a few conifers and all cycads) use animals toassist in the dispersal of their seeds. Once again it was the angiosperms that perfected the shift from abiotic tobiotic dispersal agents by evolving a fascinating spectrum of strategies enabling them to travel either on or insideanimals. These close, sometimes highly specialised relationships between angiosperms and animal dispersersprovide another key to the understanding of the evolutionary success of this group.The evidence of such strategies in plants lies in every sweet, juicy fruit we enjoy. The sweet pulp of thefruit is a bait to lure potential dispersers into swallowing the seeds and dispersing them in their faeces. If they arefortunate, the seeds will land in a suitable place to grow well away from the shadow of the mother plant. Thisform of dispersal is called endozoochory, “dispersal inside an animal”. Endozoochory is a frequently encounteredphenomenon in many families with fleshy fruits, e.g. Ericaceae, Rosaceae, Solanaceae. Endozoochorouslydispersed seeds do not present any conspicuous adaptations to facilitate their dispersal. They are usually smooth,globose to ovoid, and either covered in a hard endocarp (if borne in drupes) or with a hard seed coat (if borneEnglish texts 281