semillas la vida en cápsulas de tiempo - Clh.es

develop three or more cotyledons, while other species regularly produce three (e.g. Degeneria vitiensis,Degeneriaceae) or up to eight cotyledons (Persoonia spp., Proteaceae). There are also some Dicotyledons in whichthe two cotyledons are unequal in size (anisocotyly). In extreme cases, such as sowbread (Cyclamen europaeum,Primulaceae) and larkspur (Corydalis spp., Ranunculaceae), the embryo develops only one cotyledon, while thesecond one is suppressed. In Streptocarpus wendlandii (Gesneriaceae) anisocotyly is not expressed until aftergermination. In the seed the cotyledons are morphologically identical, but soon after germination one cotyledondies while the other remains the only leaf the plant will ever develop, eventually becoming as long as 70cm ormore. In other Dicots the two cotyledons are fused into one in the mature embryo. Such “pseudomonocotyledonous”Dicotyledons are found in the carrot family (Apiaceae) and the buttercup family (Ranunculaceae).To complete the selection of exceptions, members of the mono-cotyledonous yam family (Dioscoreaceae)sometimes have two (unequal) cotyledons. The Dicotyledons are the larger group. It is estimated that theycomprise some 261,000 species distributed over more than 443 families, compared with 53,000 species in 91families for Monocotyledons. However, in view of the latest estimate of a total of 422,000 species of angiospermsworldwide these figures are likely to be a gross understatement.Monocots and Dicots, as botanists abbreviate them affectionately, are generally easy to distinguish, evenwithout counting the leaves of their embryos. Monocots are mostly herbaceous plants with simple leaves thatlack a division into a stalk and blade and show parallel venation. All the grasses, bananas, bamboos, lilies, orchidsand palms, for example, are Monocots. Recent research has shown that during the evolution of the angiosperms,Monocots branched off as a lineage from within the primitive Dicots. Somewhere on the way they must havelost one of their cotyledons.Embryo diversityIn many primitive angiosperms, the embryo is very small in relation to the amount of endosperm at the timethe seed is released from the mother plant. This is true of members of the annona family (Annonaceae), hollyfamily (Aquifoliaceae), barberry family (Berberi-daceae), magnolia family (Magnoliaceae), poppy family(Papaveraceae), buttercup family (Ranunculaceae), and Winter’s bark family (Winteraceae). Trying to find theembryo in a seed of a magnolia, custard apple (Annona cherimola), poppy, buttercup, Winter’s bark tree (Drimyswinteri), or the twinleaf (Jeffersonia diphylla) can be very difficult and seem more like occupational therapy. Theembryos in these seeds are microscopically small with hardly discernible cotyledons and the amount ofendosperm is huge in relation to the embryo.A whole range of embryos of different shapes and sizes can be found in seed plants from microscopicallysmall ones to those filling the entire seed. In 1946, Alexander C. Martin published a paper entitled “Thecomparative internal morphology of seeds” based on an investigation of the seeds of 1,287 plant genera, includingboth gymnosperms and angiosperms. What Martin found was that the internal structure of seeds variestremendously with respect to the relative size, shape and position of the embryo. His classification system, whichis still widely used today, distinguishes ten different embryo types and two types of extremely small seeds. Thesetwelve types are divided into two divisions. Embryos of the peripheral division are restricted to the lower half ofthe seed or extend along its periphery. They predominantly belong to the Monocots. Because of the loss of onecotyledon, their asymmetrical embryos can have the most unusual shapes such as flat discs (“broad type”) orhead-like structures (“capitate type”). Tiny, largely undifferentiated disc-shaped embryos are found in relatives ofthe grass family (Poaceae) such as the pipewort family (Eriocaulaceae), the rush family (Juncaceae), the restiofamily (Restionaceae) and the yellow-eyed grass family (Xyridaceae). The rarer capitate embryos are found onlyin a few families, such as the sedge family (Cyperaceae), the yam family (Dioscoreaceae) and the spiderwortfamily (Commelinaceae). “Lateral type” embryos, which are in the shape of a wedge tightly pressed against theside of the endosperm, are unique to the grass family. The fourth and last embryo type of the “peripheraldivision” is (somewhat redundantly) called “peripheral type”. It is found in only one order (a taxonomic rankabove the family level) of the Dicotyledons, the Caryophyllales, which include the cactus family (Cactaceae), pinkfamily (Caryophyllaceae), pokeweed family (Phytolaccaceae), four o’clock family (Nyctaginaceae), stone plantfamily (Aizoaceae), purslane family (Portulaccaceae), amaranth family (Amaranthaceae), and knotweed family(Polygonaceae). Here, the embryo occupies a unique position in that it extends along the periphery of the seedrather than following its longitudinal axis through the centre. The reason for the marginal position of the embryolies in the peculiar nature of the seed storage tissue of the Caryophyllales. Rather than representing triploidendosperm, the central starch-laden tissue around which the embryo curves, originates from the diploid nucellus.Although rare, such a storage nucellus or perisperm is found elsewhere in the angiosperms. Most of what is insidean ordinary peppercorn (Piper nigrum, Piperaceae) or the seed of a waterlily (Nymphaea spp., Nymphaeaceae)consists of perisperm. In the complicated seeds of the ginger family (Zingiberaceae), the storage tissue consistsof a significant amount of both endosperm and perisperm.In seeds belonging to Martin’s “axial division” (originally called “axile” by Martin), the embryo follows thelongitudinal axis of the seed through its centre. “Linear type” embryos are cylindrical with the cotyledon(s) notsignificantly wider than the embryo axis. This rather unspecific embryo shape is found in both gymnosperms(conifers, cycads, Ginkgo) and angiosperms (both Mono- and Dicots). Restricted to dicotyledonous angiospermsare embryos with broad and flat (“spatulate type”) or folded (“folded type”) cotyledons. The seeds of the castoroil plant (Ricinus communis, Euphorbiaceae) are a good example of the former, and the seeds of cotton (Gossypiumherbaceum, Malvaceae) of the latter type. Two other types are limited to Dicots: spatulate embryos bent like a jackknife(“bent type”), and straight spatulate embryos in which the thick cotyledons overlap and encase the shortembryo axis (“investing type”). Examples of both types can be found in the legume family (Fabaceae), where thepapilionoid subfamily (Papilionoideae) is characterised by the “bent type” and the mimosoid and caesalpinioidsubfamilies (Mimosoideae and Caesalpinioideae) typically have “investing type” embryos.Peripheral, bent and sometimes folded embryos are the result of a curvature of the longi-tudinal axis of theseed. Seeds with a curved longitudinal axis, where micropyle and chalaza are not opposite each other, are calledcampylotropous. The advantage of campylotropous seeds is that they allow the embryo to become much longerthan the actual seed and thus give rise to a taller seedling with improved chances when competing with otherseedlings for light.For very small seeds Martin created two categories based solely on size: seeds that are 0.3 to 2mm longbelong to the “dwarf type”; seeds less than 0.2mm long are the “micro type”. Martin’s measurements exclude theseed coat. Such minute seeds contain tiny, underdeveloped embryos with either poorly developed cotyledons orno cotyledons at all. They are usually wind-dispersed and found in a variety of angiosperm families, mostfamously in orchids (Orchidaceae) but also in the broomrape family (Orobanchaceae), sundew family(Droseraceae), bellflower family (Campanulaceae) and gentian family (Gentianaceae). Medium or larger seedswith tiny embryos, such as those of the annona family (Annonaceae), magnolia family (Magnoliaceae), buttercupfamily (Ranunculaceae) and Winter’s bark family (Winteraceae), represent Martin’s “rudimentary type”.Size does matterThe size of the embryo in the seed is an important factor in the life of a plant. Seeds with very small embryosoften need some lead time before they can germinate. During this lead time, which can last for months, thenutrients stored in the endosperm are first mobilised, then absorbed by the embryo. Once the seeds of the ash(Fraxinus excelsior, Oleaceae) or those of magnolias have been dispersed, the embryo has to mature and growlarger before it is strong enough to leave the shelter of the seed coat. If seeds with small embryos germinate faster,like those of some palms such as the Mexican fan palm (Washingtonia robusta) or the Brazilian needle-palm(Trithrinax brasiliensis), the seed remains attached to the seedling until all the endosperm has been resorbed, whichtakes a long time. The handicap of seeds that are not capable of germinating quickly is that they are unable toreact rapidly to small windows of opportunity, such as a sudden downpour in an area of low rainfall (for example,deserts and semi-deserts). The compelling advantages of fast-germinating seeds were therefore a driving forcetowards the evolution of seeds with larger and more develop-ed embryos. In the most extreme case, the embryouses up all available endosperm before the seed is mature. The nutrients provided by the mother plant are thenstored directly in the embryo’s own tissue, usually its leaves. Such storage embryos develop thick and fleshy orthin and folded cotyledons, filling the entire cavity of the seed. With all the nutrients of the endosperm alreadyabsorbed before germination, storage embryos are “ready to go” and thus able to take immediate advantage offavourable changes in their environment.A member of the legume family holds the world record among storage embryos. The endospermless seedsof Mora megistosperma (syn. Mora oleifera), a large tree from tropical America, can be up to 18cm long and 8cmwide and weigh up to a kilogram, which makes them the largest dicot seeds on earth. The bulk of the seedconsists of the two thickened cotyledons as do the seeds of more familiar legumes such as beans, peas and lentils.The only difference is that the seeds of Mora megistosperma have an air-filled cavity between the cotyledons, whichaffords the seeds buoyancy in seawater, an adaptation to their tidal marshland habitat. Other palatable storageembryos with large cotyledons include sunflower kernels, cashew nuts, peanuts and walnuts. That these popular“nuts” are storage embryos explains why they split so easily into two halves, the cotyledons.The opposite extreme are the dust-like “micro type” seeds of orchids. All they contain inside the wafer-thinloose seed coat is a spherical, underdeveloped embryo without any endosperm. With no significant food reservefor the embryo, orchids have to enter into a symbiotic relationship with compatible mycorrhizal fungi as soon asthey germinate. The fungus provides the germinating embryo with precious carbohydrates and minerals. For a few,especially the terrestrial orchids of the temperate zone, the relationship with their specific fungus remains vital forthe rest of their lives. Little is known about how specific orchids are in choosing their fungal partners. Someorchids at least are known to be able to establish a mycorrhizal relationship with several different species of fungi.It takes three generations to create one seedSeeds are composed of the tissues of three different generations; this is true of both gymno-sperms andangiosperms. Generation one is the tough protective seed coat. It develops from the integument, which itself isEnglish texts 273