semillas la vida en cápsulas de tiempo - Clh.es

that it can only be split with an axe. In its natural habitat, the Brazilian rainforest, the only animals able to gnawtheir way into the seeds are scatter-hoarding agoutis. When, after much hard work with its sharp, chisel-like frontteeth the agouti has managed to gnaw a hole into the hard pericarp of the Brazil nut fruit, it is presented withmore seeds than it can eat at one sitting. Some of the seeds are therefore hidden and buried underground. It isonly from seeds forgotten or left behind by the agoutis that new Brazil nut trees can grow in the wild. Withoutthe help of these animals the seeds of the Brazil nut tree (the familiar Brazil “nuts”) would never escape fromtheir fruit.Fruits in the bed of ProcrustesThere are many more capsule-like fruits that simply do not open and so refuse to fit into the botanical definitionof a capsule. Bell pepper (Capsicum annuum, Solanaceae), cocoa pods (Theobroma cacao, Malvaceae) and theferocious devil’s claw (Harpagophytum procumbens, Pedaliaceae) are examples of indehiscent capsules. Mankind haslong known the nature of this so very human conceptual dilemma. A Greek myth tells the story of a legendaryvillainous innkeeper called Procrustes who considered his bed the ultimate standard in the world. Moreunsettling than his narrow mental attitude, however, was a rather gruesome habit he cultivated. He would inviteinnocent travellers to rest at his house in Eleusis and, once they lay down, tied them to his bed. If his unfortunateguests failed to fit perfectly into the bed, he would stretch or amputate their limbs until they did. It must havecome as a great relief to ancient travellers when Theseus forced Procrustes to lie on his own bed and made himfit by cutting off his head and feet, thus defeating him by his own methods.It would hardly be worth telling the story of Procrustes if the creation of the paradoxical expression“indehiscent capsule” was the only example of its kind. Other oxymoronic terms given to fruits in a Procrusteanattempt to provide conformity by violent means are “dry drupe” (for the coconut), “dehiscent drupe” (for thealmond) or “dehiscent berry” (for the nutmeg). To accommodate these and many other – especially tropical –types of fruits in a more logical and scientific classification, botanists created a plethora of technical terms anddefinitions to accompany them, resulting in more than 150 scientific terms for fruits.False fruitsJust as it is not a simple task to classify fruit morphologically, it is difficult to define scientifically what a fruit isin the first place. In the seventeenth and eighteenth centuries, an era long before the detailed structure of thegynoecium was taken into account, defining a fruit appeared to be a simple task. In 1694 Joseph Pitton deTournefort (1656-1708) defined a fruit simply as the product of a flower. Less than a century later, Carl vonLinné (1707-1778) in his Species Plantarum (1754) and Joseph Gärtner (1732-1791) in his famous book Defructibus et seminibus plantarum (“On the fruits and seeds of plants”, published 1788-1792) concluded that a fruitis a mature ovary.Although the simplicity of these definitions may seem both plausible and appealing, they come with a setof rather implausible consequences. For example, compound fruits that develop from more than one flower donot qualify as fruits according to these definitions, and nor do the seed-bearing organs of the gymnospermssince they lack the carpels that constitute an ovary. It is therefore surprising that many botanists still follow thetraditional (out-of-date) seventeenth and eighteenth century concepts. They believe that true fruits must bederived from either a single flower or, worse, solely from the gynoecium of a single flower. If any parts of theflower other than the carpels are involved in the formation of the fruit, many textbooks claim – in goodProcrustean tradition – that it is a false fruit or pseudocarp. Being Procrustean, the term pseudocarp is, of course,contradictory, in that it refers to a fruit type that is not supposed to be a fruit. In a rather heroic attempt tobring order into the chaos of fruit classification, Richard Spjut (1994) suggested the more appropriate termanthocarp for fruits in which attached floral parts persist and develop to form an integral part of the mature fruit(Greek: anthos = flower + karpos = fruit). Spjut also deserves credit for providing a precise scientific definitionof the term “fruit” that for the first time allows botanists to address the seed-bearing organs of the gymnospermsas “proper fruits”.The strawberry effectStrawberries are the favourite textbook example of a false fruit (or anthocarp) because most of the edible part isproduced by the axis of the flower into which the numerous separate carpels (multiple fruits) are inserted. Theindividual carpels of a ripe strawberry form single-seeded nutlets, visible as tiny brown granules embedded on thesurface of the fruit. What are perceived as hairs or bristles are the remains of the styles, one attached to each nutlet.Other anthocarps include apples, rose hips and pomegranates (all three with fleshy floral tubes), thepineapple (fleshy inflorescence) and everything that qualifies as a cypsela. Some magnificent examples ofanthocarps are produced by the members of the meranti family (Dipterocarpaceae), giant trees that form thedominant component of lowland rainforests in South-East Asia. The five persistent sepals of the flower surroundtheir often large, single-seeded nuts. Depending on the genus, two (Dipterocarpus, Hopea, Vatica), three (Shorea) orall five (Dryobalanops) sepals greatly enlarge during fruit maturation, allowing the wind-dispersed fruits ahelicopter-like flight on their protracted journeys to the ground. Since their wings are not formed by the ovary,as in true samaras, they are called pseudosamaras.Much of the morphology of a fruit is determined by the inherited structure of the flower, especially thegynoecium. Nevertheless, there are many more kinds of fruits than there are gynoecia. The sheer diversity of fruittypes is proof of the enormous flexibility with which angiosperms evolved and diversified. Among all fruits,anthocarps demonstrate best how they developed almost every conceivable modification and combination oforgans to achieve their goal: the successful dispersal of their seeds.The dispersal of fruits and seedsUnlike animals, plants are rooted in the ground and tied to one place. For most plants, therefore, the seed is theonly phase in their life when they are mobile. Travelling as a seed gives a plant the unique chance to escapeunwanted competition and other unfavourable conditions and hazards such as predators attracted by the parentplants. In most cases it is not advantageous for a seed to germinate in its place of origin. The young seedlingwould have to compete for light, water and nutrients, not only with its siblings but also with the mother plant.For this reason, fruits and seeds have often developed special adaptations that allow them to travel. Thesefunctional adaptations can be obvious and aesthetic, creating structures that resemble sophisticated pieces ofengineering. It is therefore not surprising that the dispersal of fruits and seeds has long fascinated both biologistsand the general public.Depending on the type of fruit, the nature of the dispersal unit (the diaspore), varies. In dehiscent (capsular)fruits, which open to release their seeds, it is the seed itself that functions as the diaspore. Fruits or fruitlets thatremain closed (berries, drupes, nuts and nutlets) are dispersed with the seed. In tumbleweeds like the Russianthistle (Salsola kali, Amaranthaceae) and the tumble pigweed (Amaranthus caudatus, Amaranthaceae), the entireplant functions as a diaspore. Irrespective of the nature of their diaspores, plants pursue four principal strategiesof dispersal: they can rely on natural processes (wind or water dispersal); have fruits that actively disperse theirseeds themselves (self-dispersal); or entice and sometimes also enslave animal couriers into their service (animaldispersal). The enormous diversity of diaspores found among seed plants is predominantly the result ofadaptations to these four dispersal mechanisms. Which strategy a diaspore uses, is usually reflected in its “Gestalt”and displayed by a specific syndrome of characters involving colour, texture and size.Wind-dispersalIn all climates, many plants entrust their diaspores to the wind. Since wind dispersal is such a common practice,the diversity of wind-dispersed diaspores is enormous and an entire volume could easily be devoted to them.Structurally, it is mostly the seeds themselves that represent the air-borne dispersal units, less often indehiscent(and then usually single-seeded) fruits. Wind dispersal or anemochory has several advantages. Strong air currents ora storm can carry a fruit or seed far away, sometimes many kilometres. Travelling on the wind is also cheap sincethe energy-rich rewards needed to attract animal dispersers are unneces-sary. However, a significant disadvantageof wind dispersal is that the distribution of the diaspores depends on the direction and strength of the wind. Winddispersal is therefore haphazard and hence wasteful. Most wind-dispersed seeds are doomed because they fail toreach a suitable place where they can grow into a new plant. And so part of the energy that is saved on rewardsfor more reliable animal dispersers has to be invested in the production of a larger number of seeds. A singlecapsule of an orchid, for example, can contain up to four million dust-like seeds.Wind-dispersed diaspores display some distinct functional adaptations. Their structure and shape areadapted to catch as much wind as possible and to increase their buoyancy in the air. This can be achieved throughappendages like wings and hairs, by an ultra-light seed coat or pericarp, by the inclusion of air chambers, or bya combination of these adaptations. Whichever organs are involved, the tissues from which these structures areformed usually consist of dead, air-filled cells with thin walls in order to reduce weight and achieve minimumoverall density of the diaspore.Winged diasporesWind-dispersed fruits and seeds with wings are common among the gymnosperms and angiosperms. Dependingon whether the diaspore is a seed or a fruit, the wing can be formed by the seed coat, the ovary wall, by theenlarged sepals of the flower or subtending leaves (bracts). Wings can be expressed as a single unilateral structure,a pair of opposite blades, a continuous ring surrounding the circumference of the diaspore, or multiple wings.The shape and arrangement of the wings determines the flight characteristics of a diaspore.Diaspores with a single lateral wingIn maple “seeds” (which are in reality fruitlets of a schizocarpic fruit) and the samaras shed by ash trees the single,one-sided wing permits a helicopter-like flight. During the flight, the diaspore rotates around its centre of gravity,276 Semillas – La vida en cápsulas de tiempo