ovul<strong>es</strong>. This is mainly the case where the ovary contains only a single erect ovule whose micropyle fac<strong>es</strong> or ev<strong>en</strong>touch<strong>es</strong> the trans-mitting tissue (e.g. in Jug<strong>la</strong>ndaceae, Piperaceae, Polygonaceae, Myricaceae, Urticaceae), makingit unnec<strong>es</strong>sary to turn the ovule round.Precocious pubertyEv<strong>en</strong>ts insi<strong>de</strong> the nucellus of the angiosperms are initially the same as those in the gymnosperms. Within thetissue of the nucellus, one cell, the megaspore mother cell, un<strong>de</strong>rgo<strong>es</strong> meiosis to produce four haploid megaspor<strong>es</strong>of which three (those clos<strong>es</strong>t to the micropyle) usually die. 10 The survivor is called the functional megaspore.From this point on, however, everything is differ<strong>en</strong>t in angiosperms and – most importantly – much faster.Instead of producing a female gametophyte that consists of thousands of cells and one or more archegonia, themegagametophyte of the angiosperms is simplified. After only three mitotic (normal) divisions of its nucleus, thefunctional megaspore giv<strong>es</strong> rise to eight free haploid nuclei, which are arranged in two groups of four at eitherpole of the megaspore. In the next stage, one nucleus from each group migrat<strong>es</strong> to the c<strong>en</strong>tre. As they come fromopposite pol<strong>es</strong>, they are appropriately called po<strong>la</strong>r nuclei. After the formation of cell walls, the mature gametophytetypically consists of eight nuclei distributed over just sev<strong>en</strong> cells, one of which functions as an egg cell. 11 Th<strong>es</strong><strong>es</strong>ev<strong>en</strong> cells of the megagametophyte, in angiosperms also called the embryo sac, are arranged in a particu<strong>la</strong>r pattern:there are three small cells at the micropy<strong>la</strong>r <strong>en</strong>d forming the egg apparatus, which consists of the egg cell and twoaccompanying synergids. The three cells of the egg apparatus face another set of three antipodal cells at the opposite<strong>en</strong>d of the embryo sac. Finally, in betwe<strong>en</strong> the egg apparatus and the antipodal cells is a <strong>la</strong>rge c<strong>en</strong>tral cell containingthe two po<strong>la</strong>r nuclei. Once the female gametophyte has reached this stage, it is ready for fertilization.The progr<strong>es</strong>sive reduction of the female (and male) gametophyt<strong>es</strong> observed in seed p<strong>la</strong>nts is an impr<strong>es</strong>siveexample of a recurring ph<strong>en</strong>om<strong>en</strong>on in the evolution of both p<strong>la</strong>nts and animals. It is called prog<strong>en</strong><strong>es</strong>is. Prog<strong>en</strong><strong>es</strong>isis the politically correct term for something very simi<strong>la</strong>r to “precocious puberty” and refers to an organism thatreach<strong>es</strong> sexual maturity while still in its juv<strong>en</strong>ile stage. Certain amphibians and insects are good exampl<strong>es</strong>.Wh<strong>en</strong> it com<strong>es</strong> to sex, angiosperms want it allThe evolution of the sev<strong>en</strong>-celled/eight-nucleate megagametophyte is new and radical, but it is only the forep<strong>la</strong>yto a most sophisticated act of sexual reproduction. To begin with, gymnosperms have nothing that wouldcompare to the antipodal cells and the po<strong>la</strong>r nuclei of the angiosperms. The antipodal cells usually have noparticu<strong>la</strong>r function and soon <strong>de</strong>g<strong>en</strong>erate but the po<strong>la</strong>r nuclei assume a unique role: they join the unification ofmale and female. But in what way and why are they allowed to join in this most private affair?On the male si<strong>de</strong>, the microspore starts off as a single cell. Soon, the young microspore un<strong>de</strong>rgo<strong>es</strong> a mitoticdivision insi<strong>de</strong> the poll<strong>en</strong> grain, r<strong>es</strong>ulting in a vegetative cell and a g<strong>en</strong>erative cell. Ev<strong>en</strong> before the poll<strong>en</strong> grainsare released from the poll<strong>en</strong> sacs, the g<strong>en</strong>erative cell oft<strong>en</strong> divi<strong>de</strong>s mitotically to produce the two sperm nuclei.The sperm nuclei are gamet<strong>es</strong> with only a small volume of cytop<strong>la</strong>sm around the nucleus and no f<strong>la</strong>gel<strong>la</strong>e. Wh<strong>en</strong>a poll<strong>en</strong> grain reach<strong>es</strong> the stigma of a flower, it germinat<strong>es</strong> with a poll<strong>en</strong> tube. Its growth directed by the nucleusof the vegetative cell, the poll<strong>en</strong> tube p<strong>en</strong>etrat<strong>es</strong> the stigma and travers<strong>es</strong> the style, finally <strong>en</strong>tering the cavity ofthe ovary and ev<strong>en</strong>tually the micropyle of the ovule.Upon <strong>en</strong>tering the micropyle, the poll<strong>en</strong> tube releas<strong>es</strong> the two sperm nuclei into one of the synergids nextto the egg cell. Shortly before this, the two haploid po<strong>la</strong>r nuclei in the c<strong>en</strong>tral cell have fused into one diploidnucleus and migrated to a position close to the egg apparatus. What happ<strong>en</strong>s next is unique in the p<strong>la</strong>nt kingdom.Once pollinated, twice fertilisedWhilst their old-fashioned gymnosperm brethr<strong>en</strong> use only one of the two sperm nuclei to fertilise an egg cel<strong>la</strong>nd allow the other one to either go to waste or fertilise a neighbouring archegonium, angiosperms want both– to achieve an extraordinary double fertilization: one of the sperm nuclei <strong>en</strong>ters the egg cell as usual, while theother one mov<strong>es</strong> down into the c<strong>en</strong>tral cell where it meets the diploid nucleus (formed by the two po<strong>la</strong>r nuclei)already waiting close by. Th<strong>en</strong>, in the same way as the first haploid sperm nucleus fus<strong>es</strong> with the haploid nucleusof the egg cell to form the diploid nucleus of the zygote, the second haploid sperm nucleus fus<strong>es</strong> with the diploidnucleus of the c<strong>en</strong>tral cell to form a triploid nucleus. Only wh<strong>en</strong> this double fertilization has be<strong>en</strong> succ<strong>es</strong>sful,will the zygote give rise to the embryo and the ovule start <strong>de</strong>veloping into the seed. Before taking a closer lookat the <strong>de</strong>velopm<strong>en</strong>t of the embryo, the fate of the <strong>en</strong>igmatic triploid c<strong>en</strong>tral cell should be explored.The triploid lunch pack of the angiospermsEv<strong>en</strong> before the zygote produc<strong>es</strong> a recognizable embryo, the triploid c<strong>en</strong>tral cell grows into a tissue whichconstitut<strong>es</strong> the <strong>de</strong>fining compon<strong>en</strong>t of the angiosperm seed, the <strong>en</strong>dosperm. 12 The <strong>en</strong>dosperm surrounds thegrowing embryo and nourish<strong>es</strong> it during its <strong>de</strong>velopm<strong>en</strong>t. As the seed matur<strong>es</strong>, the embryo grows <strong>de</strong>eper into the<strong>en</strong>dosperm. What the embryo has not consumed by the time the seed is mature persists in the seed and serv<strong>es</strong> asthe young sporophyte’s food r<strong>es</strong>erve during germination. Since it forms the main constitu<strong>en</strong>t of cereal grains,<strong>en</strong>dosperm is the reason that rice, wheat, maize, oat and millet provi<strong>de</strong> the staple food for billions of peopleworldwi<strong>de</strong>. It is this unique and unparalleled triploid seed storage tissue that more than anything else characteris<strong>es</strong>the angiosperms. But why do<strong>es</strong> it emerge from a second fertilization wh<strong>en</strong> it do<strong>es</strong> not produce an embryo?Is the <strong>en</strong>dosperm a sacrificial twin?In 1898-99 the Russian biologist Sergei Gavrilovich Navashin (1857-1930) and the Fr<strong>en</strong>ch botanist Jean-Louis-Léon Guignard (1852-1928) discovered – in<strong>de</strong>p<strong>en</strong>d<strong>en</strong>tly – the <strong>de</strong>velop-m<strong>en</strong>tal origin of <strong>en</strong>dosperm from doublefertilization. But since th<strong>en</strong>, the evolution of the specific ev<strong>en</strong>ts that led to the formation of a triploid <strong>en</strong>dospermhas remained a mystery.Rec<strong>en</strong>t r<strong>es</strong>earch has shown that members of the gymnospermous Gnetal<strong>es</strong> – believed by some to be theclos<strong>es</strong>t living re<strong>la</strong>tiv<strong>es</strong> of the angiosperms – also un<strong>de</strong>rgo a proc<strong>es</strong>s of double fertilization. In Gnetal<strong>es</strong>, however,double fertilization do<strong>es</strong> not lead to the formation of a zygote and an <strong>en</strong>dosperm but – as would be expected –to two diploid zygot<strong>es</strong>. One theory is that in the long extinct anc<strong>es</strong>tors of the angiosperms, the secondfertilization ev<strong>en</strong>t yiel<strong>de</strong>d a g<strong>en</strong>etically id<strong>en</strong>tical twin embryo but once the angiosperm stem lineage branchedoff, this twin embryo evolved into an embryo-nourishing structure, the <strong>en</strong>dosperm.However, it is still not prov<strong>en</strong> that the Gnetal<strong>es</strong> are the clos<strong>es</strong>t living re<strong>la</strong>tiv<strong>es</strong> of the angiosperms. Rec<strong>en</strong>tevid<strong>en</strong>ce sugg<strong>es</strong>ts that they may be more closely re<strong>la</strong>ted to pin<strong>es</strong>. Their method of sexual reproduction may not,therefore, be of immediate relevance to un<strong>de</strong>r-standing how double fertilization evolved in angiosperms.Whether sci<strong>en</strong>tists will ever be able to solve this riddle is uncertain. Because of incomplete fossil records,angiosperms seem to have appeared sudd<strong>en</strong>ly and with consi<strong>de</strong>rable diversity in the Earth’s history but withoutobvious anteced<strong>en</strong>ts. The evolutionary origin of the angiosperms – the most promin<strong>en</strong>t unr<strong>es</strong>olved issue in p<strong>la</strong>ntevolutionary biology – therefore remains Darwin’s “abominable mystery” of more than a c<strong>en</strong>tury ago.The advantag<strong>es</strong> of double fertilizationJust as the evolutionary origin of the <strong>en</strong>dosperm remains a mystery, the advantag<strong>es</strong> of double fertilization are notcompletely clear. By incorporating both maternal and paternal g<strong>en</strong><strong>es</strong>, the <strong>en</strong>dosperm tissue becom<strong>es</strong> g<strong>en</strong>eticallyid<strong>en</strong>tical to the embryo. This could <strong>en</strong>hance the <strong>de</strong>velopm<strong>en</strong>t of the embryo by reducing the risk of g<strong>en</strong>eticincompatibiliti<strong>es</strong> betwe<strong>en</strong> mother and offspring. This is also a possibility in gymnosperms, where the tissu<strong>es</strong>urrounding the embryo (the haploid megagametophyte) is <strong>en</strong>tirely maternal.From an economic point of view also, the evolution of <strong>en</strong>dosperm seems to be an improvem<strong>en</strong>t. Althoughgymnosperms such as ginkgos, cycads and conifers can be proud of their seeds, the way they “make” them is notparticu<strong>la</strong>rly effici<strong>en</strong>t. Their primeval sex life forc<strong>es</strong> them to produce their storage tissue (the massivemegagametophyte) in advance, before the egg cell is fertilised. Angiosperms put their <strong>en</strong>ergy into the formationof the exp<strong>en</strong>sive, <strong>en</strong>ergy-rich <strong>en</strong>dosperm only after succ<strong>es</strong>sful fertilization of the ovule. If the pollination of aflower fails, angiosperms have not wasted too much precious <strong>en</strong>ergy and materials. Conservation is always a greatadvantage in the evolutionary race.The embryo of the angiospermsAfter fertilization, or sometim<strong>es</strong> ev<strong>en</strong> before, the synergids and antipodal cells <strong>de</strong>g<strong>en</strong>erate while the zygote startsto divi<strong>de</strong> mitotically. In all angiosperms, the very first division of the zygote is asymmetrical. It divi<strong>de</strong>s the zygotetransversely into a <strong>la</strong>rge basal cell facing the micropyle, and a small apical cell facing in the opposite direction.This first division <strong>de</strong>ter-min<strong>es</strong> the po<strong>la</strong>rity of the embryo. The apical cell grows into the embryo proper whereasthe <strong>la</strong>rger basal cell produc<strong>es</strong> a stalk-like susp<strong>en</strong>sor, simi<strong>la</strong>r to the one already <strong>en</strong>countered in gymnosperms.Originally, the susp<strong>en</strong>sor was merely conceived as a means to anchor the embryo at the micropyle while itpushed the embryo <strong>de</strong>eper into the <strong>en</strong>dosperm. Today, we know that its function is much more complex. Th<strong>es</strong>usp<strong>en</strong>sor not only nourish<strong>es</strong> the young sporophyte with nutri<strong>en</strong>ts transferred from the mother p<strong>la</strong>nt but alsocontrols the early stag<strong>es</strong> of <strong>de</strong>velopm<strong>en</strong>t of the embryo by supplying it with hormon<strong>es</strong>. Unlike the embryo, th<strong>es</strong>usp<strong>en</strong>sor is short lived. In the mature seed it has long since disappeared, oft<strong>en</strong> without trace.Monocots and DicotsInitially the embryo proper is just a globu<strong>la</strong>r lump of cells but it soon starts to differ<strong>en</strong>-tiate into the embryonicaxis (hypocotyl), with the root (radicle) at one <strong>en</strong>d and the first leav<strong>es</strong> of the young sporophyte, the seed leav<strong>es</strong> orcotyledons, at the other. Since the <strong>de</strong>velopm<strong>en</strong>t of the embryo starts right un<strong>de</strong>rneath the micropyle, the tip ofthe radicle always marks the spot.The embryo of gymnosperms can have any number betwe<strong>en</strong> one and more than t<strong>en</strong> cotyledons, but theembryo of angiosperms has either two or just one. Botanists have long used this very conv<strong>en</strong>i<strong>en</strong>t, clear-cutdistinction to separate the angiosperms into two groups, the Dicotyledons with two seed leav<strong>es</strong> and theMonocotyledons with only one seed leaf. There are very few exceptions to this rule where the number ofcotyledons exceeds two. Some individuals of Magnolia grandiflora (Magnoliaceae), for example, may occasionally272 Semil<strong>la</strong>s – La <strong>vida</strong> <strong>en</strong> cápsu<strong>la</strong>s <strong>de</strong> <strong>tiempo</strong>
<strong>de</strong>velop three or more cotyledons, while other speci<strong>es</strong> regu<strong>la</strong>rly produce three (e.g. Deg<strong>en</strong>eria viti<strong>en</strong>sis,Deg<strong>en</strong>eriaceae) or up to eight cotyledons (Persoonia spp., Proteaceae). There are also some Dicotyledons in whichthe two cotyledons are unequal in size (anisocotyly). In extreme cas<strong>es</strong>, such as sowbread (Cyc<strong>la</strong>m<strong>en</strong> europaeum,Primu<strong>la</strong>ceae) and <strong>la</strong>rkspur (Corydalis spp., Ranuncu<strong>la</strong>ceae), the embryo <strong>de</strong>velops only one cotyledon, while th<strong>es</strong>econd one is suppr<strong>es</strong>sed. In Streptocarpus w<strong>en</strong>d<strong>la</strong>ndii (G<strong>es</strong>neriaceae) anisocotyly is not expr<strong>es</strong>sed until aftergermination. In the seed the cotyledons are morphologically id<strong>en</strong>tical, but soon after germination one cotyledondi<strong>es</strong> while the other remains the only leaf the p<strong>la</strong>nt will ever <strong>de</strong>velop, ev<strong>en</strong>tually 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 (Ranuncu<strong>la</strong>ceae).To complete the selection of exceptions, members of the mono-cotyledonous yam family (Dioscoreaceae)sometim<strong>es</strong> have two (unequal) cotyledons. The Dicotyledons are the <strong>la</strong>rger group. It is <strong>es</strong>timated that theycomprise some 261,000 speci<strong>es</strong> distributed over more than 443 famili<strong>es</strong>, compared with 53,000 speci<strong>es</strong> in 91famili<strong>es</strong> for Monocotyledons. However, in view of the <strong>la</strong>t<strong>es</strong>t <strong>es</strong>timate of a total of 422,000 speci<strong>es</strong> of angiospermsworldwi<strong>de</strong> th<strong>es</strong>e figur<strong>es</strong> are likely to be a gross un<strong>de</strong>rstatem<strong>en</strong>t.Monocots and Dicots, as botanists abbreviate them affectionately, are g<strong>en</strong>erally easy to distinguish, ev<strong>en</strong>without counting the leav<strong>es</strong> of their embryos. Monocots are mostly herbaceous p<strong>la</strong>nts with simple leav<strong>es</strong> that<strong>la</strong>ck a division into a stalk and b<strong>la</strong><strong>de</strong> and show parallel v<strong>en</strong>ation. All the grass<strong>es</strong>, bananas, bamboos, lili<strong>es</strong>, orchidsand palms, for example, are Monocots. Rec<strong>en</strong>t r<strong>es</strong>earch 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 re<strong>la</strong>tion to the amount of <strong>en</strong>dosperm at the timethe seed is released from the mother p<strong>la</strong>nt. This is true of members of the annona family (Annonaceae), hollyfamily (Aquifoliaceae), barberry family (Berberi-daceae), magnolia family (Magnoliaceae), poppy family(Papaveraceae), buttercup family (Ranuncu<strong>la</strong>ceae), and Winter’s bark family (Winteraceae). Trying to find theembryo in a seed of a magnolia, custard apple (Annona cherimo<strong>la</strong>), poppy, buttercup, Winter’s bark tree (Drimyswinteri), or the twinleaf (Jeffersonia diphyl<strong>la</strong>) can be very difficult and seem more like occupational therapy. Theembryos in th<strong>es</strong>e seeds are microscopically small with hardly discernible cotyledons and the amount of<strong>en</strong>dosperm is huge in re<strong>la</strong>tion to the embryo.A whole range of embryos of differ<strong>en</strong>t shap<strong>es</strong> and siz<strong>es</strong> can be found in seed p<strong>la</strong>nts from microscopicallysmall on<strong>es</strong> to those filling the <strong>en</strong>tire seed. In 1946, Alexan<strong>de</strong>r C. Martin published a paper <strong>en</strong>titled “Thecomparative internal morphology of seeds” based on an inv<strong>es</strong>tigation of the seeds of 1,287 p<strong>la</strong>nt g<strong>en</strong>era, includingboth gymnosperms and angiosperms. What Martin found was that the internal structure of seeds vari<strong>es</strong>trem<strong>en</strong>dously with r<strong>es</strong>pect to the re<strong>la</strong>tive size, shape and position of the embryo. His c<strong>la</strong>ssification system, whichis still wi<strong>de</strong>ly used today, distinguish<strong>es</strong> t<strong>en</strong> differ<strong>en</strong>t embryo typ<strong>es</strong> and two typ<strong>es</strong> of extremely small seeds. Th<strong>es</strong>etwelve typ<strong>es</strong> are divi<strong>de</strong>d into two divisions. Embryos of the peripheral division are r<strong>es</strong>tricted to the lower half ofthe seed or ext<strong>en</strong>d along its periphery. They predominantly belong to the Monocots. Because of the loss of onecotyledon, their asymmetrical embryos can have the most unusual shap<strong>es</strong> such as f<strong>la</strong>t discs (“broad type”) orhead-like structur<strong>es</strong> (“capitate type”). Tiny, <strong>la</strong>rgely undiffer<strong>en</strong>tiated disc-shaped embryos are found in re<strong>la</strong>tiv<strong>es</strong> ofthe grass family (Poaceae) such as the pipewort family (Eriocau<strong>la</strong>ceae), the rush family (Juncaceae), the r<strong>es</strong>tiofamily (R<strong>es</strong>tionaceae) and the yellow-eyed grass family (Xyridaceae). The rarer capitate embryos are found onlyin a few famili<strong>es</strong>, such as the sedge family (Cyperaceae), the yam family (Dioscoreaceae) and the spi<strong>de</strong>rwortfamily (Commelinaceae). “Lateral type” embryos, which are in the shape of a wedge tightly pr<strong>es</strong>sed against th<strong>es</strong>i<strong>de</strong> of the <strong>en</strong>dosperm, are unique to the grass family. The fourth and <strong>la</strong>st embryo type of the “peripheraldivision” is (somewhat redundantly) called “peripheral type”. It is found in only one or<strong>de</strong>r (a taxonomic rankabove the family level) of the Dicotyledons, the Caryophyl<strong>la</strong>l<strong>es</strong>, which inclu<strong>de</strong> the cactus family (Cactaceae), pinkfamily (Caryophyl<strong>la</strong>ceae), pokeweed family (Phyto<strong>la</strong>ccaceae), four o’clock family (Nyctaginaceae), stone p<strong>la</strong>ntfamily (Aizoaceae), purs<strong>la</strong>ne family (Portu<strong>la</strong>ccaceae), amaranth family (Amaranthaceae), and knotweed family(Polygonaceae). Here, the embryo occupi<strong>es</strong> a unique position in that it ext<strong>en</strong>ds along the periphery of the seedrather than following its longitudinal axis through the c<strong>en</strong>tre. The reason for the marginal position of the embryoli<strong>es</strong> in the peculiar nature of the seed storage tissue of the Caryophyl<strong>la</strong>l<strong>es</strong>. Rather than repr<strong>es</strong><strong>en</strong>ting triploid<strong>en</strong>dosperm, the c<strong>en</strong>tral starch-<strong>la</strong>d<strong>en</strong> tissue around which the embryo curv<strong>es</strong>, originat<strong>es</strong> from the diploid nucellus.Although rare, such a storage nucellus or perisperm is found elsewhere in the angiosperms. Most of what is insi<strong>de</strong>an 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 <strong>en</strong>dosperm 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 c<strong>en</strong>tre. “Linear type” embryos are cylindrical with the cotyledon(s) notsignificantly wi<strong>de</strong>r than the embryo axis. This rather unspecific embryo shape is found in both gymnosperms(conifers, cycads, Ginkgo) and angiosperms (both Mono- and Dicots). R<strong>es</strong>tricted to dicotyledonous angiospermsare embryos with broad and f<strong>la</strong>t (“spatu<strong>la</strong>te type”) or fol<strong>de</strong>d (“fol<strong>de</strong>d type”) cotyledons. The seeds of the castoroil p<strong>la</strong>nt (Ricinus communis, Euphorbiaceae) are a good example of the former, and the seeds of cotton (Gossypiumherbaceum, Malvaceae) of the <strong>la</strong>tter type. Two other typ<strong>es</strong> are limited to Dicots: spatu<strong>la</strong>te embryos b<strong>en</strong>t like a jackknife(“b<strong>en</strong>t type”), and straight spatu<strong>la</strong>te embryos in which the thick cotyledons over<strong>la</strong>p and <strong>en</strong>case the shortembryo axis (“inv<strong>es</strong>ting type”). Exampl<strong>es</strong> of both typ<strong>es</strong> can be found in the legume family (Fabaceae), where thepapilionoid subfamily (Papilionoi<strong>de</strong>ae) is characterised by the “b<strong>en</strong>t type” and the mimosoid and ca<strong>es</strong>alpinioidsubfamili<strong>es</strong> (Mimosoi<strong>de</strong>ae and Ca<strong>es</strong>alpinioi<strong>de</strong>ae) typically have “inv<strong>es</strong>ting type” embryos.Peripheral, b<strong>en</strong>t and sometim<strong>es</strong> fol<strong>de</strong>d embryos are the r<strong>es</strong>ult of a curvature of the longi-tudinal axis of th<strong>es</strong>eed. Seeds with a curved longitudinal axis, where micropyle and cha<strong>la</strong>za 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 chanc<strong>es</strong> wh<strong>en</strong> competing with otherseedlings for light.For very small seeds Martin created two categori<strong>es</strong> based solely on size: seeds that are 0.3 to 2mm longbelong to the “dwarf type”; seeds l<strong>es</strong>s than 0.2mm long are the “micro type”. Martin’s measurem<strong>en</strong>ts exclu<strong>de</strong> th<strong>es</strong>eed coat. Such minute seeds contain tiny, un<strong>de</strong>r<strong>de</strong>veloped embryos with either poorly <strong>de</strong>veloped cotyledons orno cotyledons at all. They are usually wind-dispersed and found in a variety of angiosperm famili<strong>es</strong>, mostfamously in orchids (Orchidaceae) but also in the broomrape family (Orobanchaceae), sun<strong>de</strong>w family(Droseraceae), bellflower family (Campanu<strong>la</strong>ceae) and g<strong>en</strong>tian family (G<strong>en</strong>tianaceae). Medium or <strong>la</strong>rger seedswith tiny embryos, such as those of the annona family (Annonaceae), magnolia family (Magnoliaceae), buttercupfamily (Ranuncu<strong>la</strong>ceae) and Winter’s bark family (Winteraceae), repr<strong>es</strong><strong>en</strong>t Martin’s “rudim<strong>en</strong>tary type”.Size do<strong>es</strong> matterThe size of the embryo in the seed is an important factor in the life of a p<strong>la</strong>nt. Seeds with very small embryosoft<strong>en</strong> need some lead time before they can germinate. During this lead time, which can <strong>la</strong>st for months, th<strong>en</strong>utri<strong>en</strong>ts stored in the <strong>en</strong>dosperm are first mobilised, th<strong>en</strong> absorbed by the embryo. Once the seeds of the ash(Fraxinus excelsior, Oleaceae) or those of magnolias have be<strong>en</strong> dispersed, the embryo has to mature and grow<strong>la</strong>rger before it is strong <strong>en</strong>ough 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 brasili<strong>en</strong>sis), the seed remains attached to the seedling until all the <strong>en</strong>dosperm has be<strong>en</strong> r<strong>es</strong>orbed, whichtak<strong>es</strong> 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 sudd<strong>en</strong> downpour in an area of low rainfall (for example,<strong>de</strong>serts and semi-<strong>de</strong>serts). The compelling advantag<strong>es</strong> of fast-germinating seeds were therefore a driving forcetowards the evolution of seeds with <strong>la</strong>rger and more <strong>de</strong>velop-ed embryos. In the most extreme case, the embryous<strong>es</strong> up all avai<strong>la</strong>ble <strong>en</strong>dosperm before the seed is mature. The nutri<strong>en</strong>ts provi<strong>de</strong>d by the mother p<strong>la</strong>nt are th<strong>en</strong>stored directly in the embryo’s own tissue, usually its leav<strong>es</strong>. Such storage embryos <strong>de</strong>velop thick and fl<strong>es</strong>hy orthin and fol<strong>de</strong>d cotyledons, filling the <strong>en</strong>tire cavity of the seed. With all the nutri<strong>en</strong>ts of the <strong>en</strong>dosperm alreadyabsorbed before germination, storage embryos are “ready to go” and thus able to take immediate advantage offavourable chang<strong>es</strong> in their <strong>en</strong>vironm<strong>en</strong>t.A member of the legume family holds the world record among storage embryos. The <strong>en</strong>dosperml<strong>es</strong>s seedsof Mora megistosperma (syn. Mora oleifera), a <strong>la</strong>rge tree from tropical America, can be up to 18cm long and 8cmwi<strong>de</strong> and weigh up to a kilogram, which mak<strong>es</strong> them the <strong>la</strong>rg<strong>es</strong>t dicot seeds on earth. The bulk of the seedconsists of the two thick<strong>en</strong>ed cotyledons as do the seeds of more familiar legum<strong>es</strong> such as beans, peas and l<strong>en</strong>tils.The only differ<strong>en</strong>ce is that the seeds of Mora megistosperma have an air-filled cavity betwe<strong>en</strong> the cotyledons, whichaffords the seeds buoyancy in seawater, an adaptation to their tidal marsh<strong>la</strong>nd habitat. Other pa<strong>la</strong>table storageembryos with <strong>la</strong>rge cotyledons inclu<strong>de</strong> sunflower kernels, cashew nuts, peanuts and walnuts. That th<strong>es</strong>e popu<strong>la</strong>r“nuts” are storage embryos exp<strong>la</strong>ins why they split so easily into two halv<strong>es</strong>, the cotyledons.The opposite extreme are the dust-like “micro type” seeds of orchids. All they contain insi<strong>de</strong> the wafer-thinloose seed coat is a spherical, un<strong>de</strong>r<strong>de</strong>veloped embryo without any <strong>en</strong>dosperm. With no significant food r<strong>es</strong>ervefor the embryo, orchids have to <strong>en</strong>ter into a symbiotic re<strong>la</strong>tionship with compatible mycorrhizal fungi as soon asthey germinate. The fungus provi<strong>de</strong>s the germinating embryo with precious carbohydrat<strong>es</strong> and minerals. For a few,<strong>es</strong>pecially the terr<strong>es</strong>trial orchids of the temperate zone, the re<strong>la</strong>tionship with their specific fungus remains vital forthe r<strong>es</strong>t of their liv<strong>es</strong>. Little is known about how specific orchids are in choosing their fungal partners. Someorchids at least are known to be able to <strong>es</strong>tablish a mycorrhizal re<strong>la</strong>tionship with several differ<strong>en</strong>t speci<strong>es</strong> of fungi.It tak<strong>es</strong> three g<strong>en</strong>erations to create one seedSeeds are composed of the tissu<strong>es</strong> of three differ<strong>en</strong>t g<strong>en</strong>erations; this is true of both gymno-sperms andangiosperms. G<strong>en</strong>eration one is the tough protective seed coat. It <strong>de</strong>velops from the integum<strong>en</strong>t, which itself isEnglish texts 273
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R O B K E S S E L E R Y W O L F G A
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S E M I L L A SL A V I DA E N C Á
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Erica cinerea (Ericaceae) - brezo;
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Thamnosma africanum (Rutaceae); rec
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INTRODUCCIÓNAntirrhinum coulterian
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LA VIDA EN CÁPSULAS DE TIEMPORO B
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Esta nueva pasión sentó las bases
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20 Semillas - La vida en cápsulas
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22 Semillas - La vida en cápsulas
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Semillas - La vida en cápsulas de
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nadar libremente hasta encontrar un
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Cuando los machos son micro y las h
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página anterior arriba: Archaeospe
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Semillas desnudasLos óvulos de las
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(Ginkgoaceae), propio orden (Ginkgo
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Cuando mega realmente significa meg
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página anterior: Pinus lambertiana
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página anterior: Drimys winteri (W
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vino dado por la combinación de mi
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las plantas en 1664 y en 1672 publi
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especialmente aquellas que oliendo
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página anterior: Angraecum sesquip
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La sorprendente vida sexual de las
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Melocactus zehntneri (Cactaceae) -
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el otro baja hacia la célula centr
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62 Semillas - La vida en cápsulas
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angiospermas en dos grupos, las dic
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Una gran variedad de embriones de d
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sus hojas. Estos embriones almacena
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70 Semillas - La vida en cápsulas
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los aspectos de su apariencia, pero
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abajo: Punica granatum (Lythraceae)
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Una breve introducción a la clasif
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abajo: secciones transversales de u
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página anterior: Scutellaria orien
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página anterior: Ochna natalitia (
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agutí logra perforar un agujero de
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La dispersión de frutos y semillas
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predestinadas a fracasar en su empe
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Centrolobium microchaete (Fabaceae)
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96 Semillas - La vida en cápsulas
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Semillas de espuela de caballero (R
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La dispersión de frutos y semillas
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página anterior: Darlingtonia cali
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página anterior: Clematis tangutic
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izquierda: Blepharis mitrata (Acant
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abajo: Arenaria franklinii (Caryoph
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116 Semillas - La vida en cápsulas
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118 Semillas - La vida en cápsulas
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página anterior: Cistanche tubulos
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La dispersión de frutos y semillas
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específicos requerimientos de germ
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La dispersión de frutos y semillas
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La dispersión de frutos y semillas
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Cephalophyllum loreum (Aizoaceae) -
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página anterior: Cerbera manghas (
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página anterior: : habas de mar -
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Frutos explosivos activosLos frutos
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142 Semillas - La vida en cápsulas
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ecta mientras la parte inferior est
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pero un sentido del olfato poco des
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La dispersión de frutos y semillas
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página anterior y arriba: Afzelia
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Polygala arenaria (Polygalaceae) -
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Dispersión por recolectores y alma
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La dispersión de frutos y semillas
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La dispersión de frutos y semillas
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página anterior: Uncarina spp. (Pe
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Semillas sin ninguna adaptación ob
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Viajar en el tiempo y el espacio 16
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Un año de semillas, siete años de
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página anterior: Nemesia versicolo
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Viajar en el tiempo y el espacio 17
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arriba: Strelitzia reginae (Strelit
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UN PROYECTO ARQUITECTÓNICORO B K E
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página anterior: Cleome sp. (Cappa
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página siguiente: Downland Gridshe
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página siguiente: El Proyecto Edé
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FITOPIARO B K E S S E L E RStellari
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La diferencia entre mirar y ver...
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Trichodesma africanum (Boraginaceae
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página anterior: Crassula pellucid
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Calandrinia eremaea (Portulacaceae)
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Euphorbia peplus (Euphorbiaceae) -
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Alcea pallida (Malvaceae) - malva p
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Codonocarpus cotinifolius (Gyrostem
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Fitopia 211
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Melocactus neryi (Cactaceae) - melo
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Lophophora williamsii (Cactaceae) -
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