semillas la vida en cápsulas de tiempo - Clh.es

margins fuse to form a bag with the ovules inside. The result is what botanists call a carpel. However, the ovuleslocked away inside the carpel face a small hurdle. How will the pollen, or at least the male sperm, reach them?This prob-lem was solved by the creation of a pollen-capturing zone, the stigma (Greek for spot or scar). Thestigma is a wet, receptive tissue on the surface of the carpel, initially along the line where the margins of thesporophyll fused but later reduced to a small platform at the tip of the carpel. It provides pollen grains with idealconditions for germination. Pushing their way into the stigma, the pollen tubes soon reach a special canal ortransmitting tissue that supports their growth and guides them to the ovules down inside the carpel, the wombof the flower. With the development of the stigma, the initial handicap created by the closed carpel has – quiteelegantly – been turned into yet another advantage: whilst the naked ovules of the wind-pollinated gymnospermshave to be pollinated individually, the stigma of the angiosperms has created a single entry point for all incomingpollen. A sole pollina-tion event delivers enough pollen for the fertilization of all the ovules in the carpel.Furthermore, the germination of the “wrong” pollen can easily be inhibited or even prevented by chemicalsignals produced by the stigma surface.The carpel was without doubt the revolutionary innovation of the angiosperms and would pave the wayfor their almost total domination of the plant world. But why is it such a great advantage for ovules to be enclosedwithin carpels rather than “naked” along the leaf margin or on the surface of the cone scales? It is true that theyare better protected from predators, but a cone can do more or less the same job and it is much easier for thepollen to get to the ovules if they are not locked away. In order to fully comprehend the signifi-cance of theevolution of the carpel, it is necessary to look at the bigger picture.Gone with the windEarly seed plants relied on the wind to transport and disperse their pollen in the same way that mosses and fernsentrusted their spores to passing air currents. Wind pollination is still the preferred method among gymnospermsbut it is quite an expensive way of getting the pollen to the ovules. There is only a minute chance that the windwill carry a pollen grain straight to an ovule of the same species. Hence, the wind is not a particularly reliablecourier. Cycads and conifers make up for this by producing huge amounts of pollen: clouds of yellow pollen canbe seen coming from the small male cones of a pine tree in spring when the wind blows. Despite the strategicplacement of the larger female cones at the tips of the branches, it takes an enormous number of pollen grains toensure their successful pollination. When it comes to pollinating flowers, insects are much more reliable and targetedthan the wind. Insect pollinators such as bees move from flower to flower seeking rewards, typically in the form ofpollen or nectar, and thereby ensure the relatively precise movement of pollen. Insect-pollinated flowers thereforeneed to produce fewer pollen grains to ensure pollination, a clear reproductive benefit over wind-pollinated flowers.This rather intimate relationship between seed plants and animals took some time to become established.“Louis, I think this is the beginning of a beautiful friendship” 6Animals, birds and insects occasionally visited the flowers of wind-pollinated early gymnosperms. Among theseearly visitors were insects with strong mandibles (mainly beetles), which were able to chew through the toughsporopollenin wall to gorge on the nutritious contents of the pollen. Thirsty after their meal, they sometimesalso visited the female flowers to take a sip from the sugary pollination drop at the tip of the ovule and thus,unintentionally, deposited some pollen. This rather casual relationship gradually developed into something moreserious.Modern-day cycads, for example, are dioecious, which means that they bear their male and female cones(flowers) on separate individuals. When the time for pollination arrives, both male and female cones emit heatand a strong odour that attracts insects (e.g. weevils), as their scales (sporophylls) begin to loosen and separate.This is also the strategy of the cardboard palm 7 (Zamia furfuracea), a cycad of the coontie family (Zamiaceae).When mature, the male cones attract swarms of tiny weevils by offering them shelter, food (nutritious pollen)and even a breeding place; but the female cones are poisonous in order to protect the precious ovules. Theywould therefore have nothing to tempt potential visitors if they did not cleverly trick them by mimicking theappearance and smell of the male cones. This is already quite smart for a supposedly primitive gymnosperm, butit is the much more advanced angiosperms that have become the true masters of animal seduction. The need forless pollen to achieve successful pollination was a great advantage since it meant substantial savings in energy andmaterials. Moreover, with their ovules safely stowed away in carpels, sufficient safeguard against hungry animalvisitors was also in place. Angio-sperms therefore very quickly discovered the enormous advantages of a closefriendship with insects and other animals, and since this niche was still largely unoccupied, they were able toexploit it relentlessly. How? By means of a beauty contest.The secrets of attractionTheir newly developed friendship with animals gave rise to stiff competition between the angiosperms for theattention of potential pollinators. In order to become more attractive to catch the eye of passers-by and to makepollination more efficient, angiosperms developed the conspicuous, often colourful structures that are thoughtof as “true” flowers.One of the secrets of a proper flower is a successful advertising strategy to lure potential pollinatingcustomers. To make their flowers more conspicuous, angiosperms added colour-ful leaves to the shoot bearingthe sporophylls and often an enticing fragrance. Take the rose. The queen of flowers owes its wondrous beautyentirely to its showy petals, which consist of modified leaves around the reproductive organs in the centre of theflower. Its exquisite scent complements the positive experience, enhancing the attraction of the flower just as anenchanting perfume adds to the allure of a beautiful woman. Another major step forward in the evolution of theangiosperm flower was the combination of microsporophylls and megasporophylls in a single flower, somethingonly very few gymnosperms (most of which are extinct today) ever managed. Such bisexual or hermaphrodite 8flowers avoid the dupli-cation of effort required by separate male and female flowers, both of which would haveto be equipped with attractants and rewards for pollinators. Since the microsporophylls (stamens) andmegasporophylls (carpels) are in the same flower pollen can be received from visiting insects and at the same timesome of the flower’s own pollen may get attached to the visitors. Bisexual flowers are simply a one-stop shop forreceiving and dispatching pollen, as well as for rewarding the dispatcher with food (pollen) and drink (nectar).Floral architectureA typical angiosperm flower consists of four or five whorls of specialised leaves. The outer whirl is the calyx, acup-shaped structure formed by three to five small green leaves, called sepals. Within the calyx is the large, oftenbrightly coloured corolla, usually made up of three to five petals. Sepals and petals together form the perianth of aflower. Between or opposite the petals, one or two whorls of microsporophylls or stamens are inserted. A stamenconsists of a slender stalk, the filament, carrying the anther at the top. The anther is the fertile part of the stamenand bears four microsporangia, the pollen sacs. The stamens themselves encircle the central whorl, the female partof the flower, the carpels. The number of carpels in each flower depends on the species. They can be numerous,as in buttercups (Ranunculus spp. Ranunculaceae), the marsh marigold (Caltha palustris, Ranunculaceae) and moreexotic examples, such as the Winter’s bark tree (Drimys winteri, Winteraceae) and its relatives, the magnolias(Magnolia spp., Magnoliaceae) Other species, such as members of the legume family (Fabaceae), which includesbeans and peas, have only one carpel per flower.Why everything has two namesScientists have given the sexual organs of seed plants different names although their direct equivalents werealready present and properly named in the life cycles of mosses and ferns. In seed plants, microsporophylls andmegasporophylls are called stamens and carpels, the microsporangium and megasporangium are the pollen sacand nucellus, and the microspores are pollen grains. It may be true that some scientists love to create new termsfrom the vocabulary of our Greek and Latin ancestors, but in the case of seed plants at least, it is for strictlyhistoric reasons.Before Hofmeister’s revolutionary discovery in 1851, scientists had already created a whole set of differenttechnical terms for the reproductive organs of seed plants. It was the English naturalist and physician NehemiahGrew (1641-1712) who invented much of the terminology used today for the different parts of a flower. Grewbegan his observations on the anatomy of plants in 1664 and in 1672 he published his first important essayAnatomy of Vegetables begun, which was followed in 1673 by Idea of a Phytological History. His most importantpublication on the Anatomy of Plants appeared in 1682. It included a chapter on the “Anatomy of Leaves, Flowers,Fruits and Seeds” in which he analysed the function of flowers and for the first time identified stamens andcarpels as male and female sex organs. By the time Hofmeister was able to prove that the sex organs ofcryptogams (mosses, ferns, etc.) and seed plants have a fundamental evolutionary similarity, Grew’s terminologyhad long been established. Both sets of terms have been retained and are used in parallel.Fusion technologyDuring the course of evolution, angiosperms developed a tendency towards fusing parts of their flowers,especially the carpels. In more advanced families, the carpels are usually united to form a single ovary or pistil, forwhich the scientific term is a syncarpous gynoecium. 9 In a syncarpous gynoecium the carpels share a single stigma,which can be raised above the swollen ovule-bearing part by a slender extension of the carpels, the style. Theshared stigma helped further rationalise pollination since a single pollination event could now achieve thefertilization of all ovules of not only one but several carpels. This fusion of carpels is easily visible when the ovaryof a tulip, for example, is cut in two. Even though they are fused together, the three carpels of a tulip flower retaintheir walls and divide the ovary into three clearly discernible chambers with ovules inside.Other parts of the angiosperm flower with a strong evolutionary tendency to amal-gamate are the petals.Good examples of this phenomenon are the bellflowers (campanulas) where the five petals form the single, bellshapedcorolla that gave the plants their name. Many evolutionarily advanced families such as the figwort family270 Semillas – La vida en cápsulas de tiempo