semillas la vida en cápsulas de tiempo - Clh.es

formed by the diploid cells of the parent sporophyte. Genera-tion two is the nutritive tissue, which is providedeither by the haploid body cells of the female gametophyte (in gymnosperms) or by the triploid endosperm (inangiosperms). The third generation is the diploid embryo, which combines the genetic material of two differentindividuals, the mother sporophyte (providing the egg cells) and father sporophyte (providing the pollen). Thissophisticated combination of three genetically different generations of tissue in a single organ renders the seedthe most complex structure produced by a seed plant.... or sometimes just oneAlthough it is true that the vast majority of angiosperms produce their seeds sexually, there are a few notableexceptions. Some plants stopped exploiting the advantages of double fertilization and abstained – either partiallyor totally – from sexual reproduction. There are more than 400 species in over forty angiosperm families whichare able produce seeds asexually, by a process called apomixis. Curiously, although well represented amongMonocots and Dicots, apomixis appears to be entirely absent among the gymnosperms.Apomictic species are thought to have evolved independently from sexually repro-ducing ancestorsmultiple times. Some of them are only facultative apomicts and can still reproduce sexually, while others areobligate apomicts whose only way of reproduction is apomixis. Although the mechanisms leading to apomicticreproduction are diverse, the underlying principle is that meiotic division is by-passed, so that a diploid egg cellis produced and develops into an embryo without prior fertilization (parthenogenesis). For the development of theendosperm most apomicts still require fertilization of the central cell with its two polar nuclei. But someapomictic species have abandoned the fertilization of the central cell, which then initiates the development ofthe endosperm on its own.Seventy-five per cent of all known apomicts occur in just three families: the grasses (Poaceae), the rosefamily (Rosaceae) and the sunflower family (Asteraceae), which include such familiar examples as dandelion(Taraxacum officinalis), mouse-ear hawkweed (Hieracium pilosella) and cinquefoil (Potentilla spp.). As a result of thevarious apomictic mechanisms the seeds contain embryos that are genetically identical to the mother plant. Suchclonal embryos are useful in the propagation of apomictic crops such as Citrus species (Rutaceae), mangosteen(Garcinia mangostana, Clusiaceae) and blackberries (Rubus fruticosus, Rosaceae). Their apomic-tically producedseeds yield an exact copy of the mother plant, thus perpetuating the valuable traits of a particular race or hybridthrough successive seed generations.Homage to the gymnosperms – at least they look good on paperBeautiful flowers, clever pollination strategies, double fertilization, and more efficient seed production – theangiosperm way of life was, and still is, a great success story. The fossil record shows that angiosperms appearedquickly and suddenly between the end of the Jurassic and the beginning of the Cretaceous (c.140 million yearsago). By the end of the Cretaceous (c. 65 million years ago), they had literally exploded into a huge diversity ofspecies, taking over most terrestrial plant communities. Soon, they had largely displaced ferns and gymnosperms,the dominant vegetation during the Permian, Triassic and Jurassic.Nevertheless, ferns and gymnosperms still grow on Earth today. In fact, recent research suggests that themajority of living fern species (80 per cent) diversified only in the Cretaceous, after the angiosperms appeared,which is much later than one would expect of such a supposedly ancient group. The new and more complexhabitats created by the angiosperms, especially the forests they formed, offered new niches for other organisms.Ferns were probably among the opportunists taking advantage of this, which would explain their Cretaceouscomeback. With more than 10,000 extant species they outnumber the gymnosperms ten times. Despite theirdiminished diversity, gymnosperms still play a signi-ficant role today. Conifers in particular put up some seriouscompetition for angiosperms. Although angiosperms mature more quickly than gymnosperms and generallyproduce more seeds in the same time, conifers, at least, are better adapted to dry, cool habitats. This is why theydominate forests in northern latitudes, at high elevations and on sandy soils. In fact, coniferous forests cover about25 per cent of the land surface and provide most of the cellulose used in papermaking.However, with an estimated 422,000 species, it is true that angiosperms vastly outnumber the thousandspecies of gymnosperms that still exist today. Their incredible versatility and ability to adapt to almost all climatesand situations is unrivalled in the plant kingdom and has allowed them to become the unchallenged rulers of theEarth’s flora. The fantastic diversity of the angiosperms is displayed in nearly every aspect of their appearance butnowhere more than in their reproductive organs, flowers, seeds and, of course, the containers in which theyproduce their seeds.The seed-bearing organs of the angiospermsThe evolution of animal-pollinated flowers with carpels and more efficient seeds is only the first part of theangiosperm success story. The second part covers the magnificent spectrum of strategies and adaptationsdeveloped by angiosperms to disperse their seeds in order to ensure the survival of their species and to expandinto new territories. Enclosing the ovules within carpels has many advantages. As the seeds ripen they eventuallyhave to leave the mother plant, and for early angiosperms confinement in the carpel could have been a significantobstacle. Soon though, the initial difficulty of liberating the seeds from the carpels was overcome. One way tosolve the problem of angiospermy was to develop one-seededness so that the carpel was incorporated into theseed and both were dispersed together. This condition is still found in most indehiscent fruits such as nuts anddrupes. However, during the course of evolution angiosperms have perfected their seed-bearing organs, betterknown as “fruits”, and so turned this initial obstacle into another advantage. They developed a wide range ofspecific adaptations, which enabled them to exploit every possible means of trans-port for their seeds and fruits,including wind, water and, most importantly, animals. Once more, the astonishing ability of the angiosperms todiversify and adapt to every available niche contributed towards their evolutionary success. Each dispersal strategyis reflected in a complex syndrome of adaptations that appear after the ovules have been successfully fer-tilised.It all starts with the wilting of a pollinated flower.MetamorphosisAs soon as a flower has been pollinated and its ovules fertilised there is no need to attract further pollinators. Theshowy petals and the stamens usually wilt and wither away, while the ovary starts to swell and turn into a fruit.Inside the ovary, the ovules grow, the endosperm forms, and the embryo develops, while the soft integumentsturn into the hard seed coat.Whereas gymnosperm seeds take at least twelve to twenty-four months to ripen, most angiosperms producetheir seeds much faster, usually within a few weeks or months. Bananas, for example, take only two to three months,cherries are edible after three to four months, and mangos can be harvested four to five months (100-130 days) afterflowering. Coconuts take a whole year after pollination to mature and Brazil nuts need even longer (fifteen months).With seven to ten years from flower to fruit, the Seychelles nut palm (Lodoicea maldivica, Arecaceae) – in many waysan angiosperm extremist – is the slowest fruiter of all. In comparison, the fastest reproducing weeds such as thalecress (Arabidopsis thaliana, Brassicaceae) cycle from seed to seed in less than six weeks.Going to extremes – fruits smaller than a sperm, heavier than a millstoneAlthough we are sometimes unaware of it, every angiosperm produces some kind of fruit: herb, shrub and treeall produce their own characteristic fruits, and the variety is endless. The spectrum of sizes alone ranges from thealmost microscopic fruits of the smallest angiosperm, the aquatic watermeal (members of the genus Wolffia), tothe enormous fruits of Cucurbita maxima, better known as giant pumpkins. To put this into perspective: a fullgrownWolffia fruit is just 0.3mm long and smaller than a sperm of the cycad Zamia roezlii, whereas the world’slargest pumpkins can weigh an incredible 600kg. The angiosperms developed a tremendous diversity of shapesand models, from juicy berries and drupes, hard nuts, winged gliders and helicopters, to exploding capsules andferociously spiny pods. There are so many different types that botanists over the past two centuries, in a desperateattempt to classify them, generated more than 150 technical fruit names. Explaining the tantalising intricacies offruit classification is beyond the scope of this book. However, to give an impression of the agony botanists stillface, the main criteria that are used to bring some degree of order into the overwhelming diversity of fruit typeswill be discussed briefly.A brief introduction to the classification of fruitsDespite the difficulties botanists experience with increasing attention to structural details, the three basicprinciples of fruit classification are relatively simple: (1) the underlying ovary type (apocarpous or syncarpous);(2) the consistency of the fruit wall (soft or hard); and (3) whether or not a fruit opens up at maturity to releaseits seeds. Since every combination of characters is possible in angiosperms, all three criteria are appliedindependently.The three basic types of fruitsThe most important criterion of fruit classification is the structure of the ovary from which a fruit develops.Simple fruits like cherries, tomatoes, oranges and cucumbers develop from one flower with only one pistil, whichcan be either a single carpel or several united carpels. Multiple fruits develop from flowers with several separatepistils. Such flowers are typically found in primitive angiosperm families such as the Annonaceae, Magnoliaceae,and Winteraceae. The fruit of the tulip tree (Liriodendron tulipifera), a member of the Magnoliaceae family, formsa cone-like structure in which each of the numerous carpels develops into a flat, narrow-winged fruitlet. In theWinter’s bark tree, Drimys winteri (Winteraceae), the apocarpous gynoecium of each flower produces not just onebut a whole cluster of bean-shaped berries. A delicious exotic relative of the Winter’s bark tree is the custardapple (Annona cherimola, Annonaceae). In this case the carpels fuse together as they develop into what appears tobe a simple fruit. The only giveaway of the originally apo-carpous gynoecium is the distinct hexagonal patternon the skin of the fruit, marking the boundaries between the individual carpels. More familiar examples of274 Semillas – La vida en cápsulas de tiempo