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‘OLUTION AND DIVERSITY OF VASCULAR PLANTS . 5 2005. Phylogeny of cryptogrannnoid ferns and related taxa based on rbcL sequences Nordic Journal of ‘tp://amerfernsoc.org lists many resources, including pubiications, references, local, national, and interna ites, commerjcal fern sites, and fern databases. I EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS LlGNOPHYTESWOODY PLANTS 129 SPERMATOPHYTES—SEED PLANTS Seed Evolution Pollination Droplet Pollen Grains Pollen Tube Ovule and Seed Development Seed Adaptations Eustele 131 131 135 135 136 136 139 139 DIVERSITY OF WOODYAND SEED PLANTS 139 Archeopteris “Pteridosperms”—”Seed Ferns” Gymnospermae—Gymnosperms Cycadophyta—Cycads Cycadaceae Zamiaceae 139 139 140 140 141 142 LIGNOPHYTIES—WOODY PLANTS The lignophytes, or woody plants (also called Lignophyta), are a monophyletic lineage of euphyllous vascular plants that share the derived features of a vascular cambium, which gives rise to wood, and a cork cambium, which produces cork (Figures 5.1, 5.2). Growth of the vascular and cork cambia is called secondary growth because it initiates after the vertical extension of stems and roots due to cell expansion (primary growth). A vascular cambium is a sheath, or hollow cylinder, of cells that develops within the stems and roots as a continuous layer, between the xylem and phloem in extant, eustelic spermatophytes (see later discussion). The cells of the vascular cambium divide mostly tangentially (parallel to a tangential plane), resulting initially in two concentric layers of cells (Figure 5.3A). One of these layers remains as the vas cular cambium and continues to divide indefinitely; the other layer eventually differentiates into either secondary xylem wood, if produced to the inside of the cambium, or secondary phloem, if produced to the outside (Figure 5.3A,B). Because Ginkgophyta Ginkgoaceae Coniferae—Conifers Pinopsida Pinaceae Cupressopsida Araucariaceae Cupressaceae Podocarpaceae Taxaceae Gnetales Ephedraceae REVIEW QJESTIONS 160 EXERCISES 160 REFERENCES FOR FURTHER STUDY 161 WEB SITES 162 layers of cells are produced both to the inside and outside of a continuously generated cambium, this type of growth is termed bifacial. Generally, much more secondary xylem is produced than secondary phloem. [Note that a secondary cambium independently evolved in fossil lineages within the lycophytes (e.g., Lepidodendron) and equisetophytes (e.g., Calamites), but this cambium was unifacial, producing sec ondary xylem (wood) to the inside but no outer secondary phloem, likely limiting in terms of an adaptive feature.] Secondary growth results in an increase of the width or girth of stems and roots (Figures 5.3B, 5.4). This occurs both by expansion of the new cells generated by the cambium and by accompanying radial divisions, increasing the number of cells within a given growth ring. Many woody plants have regular growth periods, e.g., forming annual rings of wood (Figure 5.4). A cork cambium is similar to a vascular cambium, only it differentiates near the periphery of the stem or root axis. The cork cambium and its derivatives constitute the periderm (referred to as the outer bark). The outermost layer of the periderm is cork (Figure 5.3B). Cork cells contain a waxy 129 © 2010 Elsevier Inc. All rights reserved. doi: 10.101 61B978-0- 12-374380-000005.2 144 145 145 148 148 151 151 151 154 154 156 157 ____I __ ______ 130 CHAPTER 5 UNIT II EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS - EVOLUTION AND DIVERSITY OF PLANTS classification and identification of woody plants. Wood ana tomical features may also be used to study the past, a spe cialty known as dendrochronolOgy (see Chapter 10). Another feature of lignophytes is that they possess ances shoot trally monopodial growth, in which a single main (see develops branches from lateral (usually axillary) buds to presumed is growth monopodial Chapters 4, 9). Although it monilophyte—lignophyte split, have arisen prior to the forming of enabled woody plants in particular the capability extensive (sometimes massive) woody branching systems, permitting them to survive and reproduce more effectively. Lignophyta (Woody Plants) Spermatophyta (Seed Plants) Gymnospermae (Gymnosperms) Coniferae (Conifers) Cupressopsida t C a a) 4., a C: a) S o a) a) S o 0 o S a) C is .sç C) 4. a) a) S S — 0 a) a) S t :t a) a) oO — (/, a) —, I 0-I. C a) r Gnetales S a) C o — -5c C - a) S Ct Ct 0 rj •0 . ‘1 5 0 (/D + epimatium SPERMATOPHYTESSEED PLANTS F receptacle porose - iiiiifiiiii 1 ovule/scale / - pollen tube—sperm nonmotile (siphonogamy) iiiiiiiifiiii leaves simple I c V -, 1g., W I - :14 — s eustele — — pollen tube—sperm motile (zooidogamy) — — endosporic, male gametophyte = pollen grain — — pollination droplet — — integument with micropyle — — retention of megaspore within megasporangium — — reduction to 1 megaspore per megasporangium — — endosporic female gametophyte =extincttaxon = extinct lineage — — heterospory — — cork cambium (periderm) — vascular cambium (secondary vascular tissue, SEED (embryo SEED EVOI.,UTION The The evolution of the seed involved several steps. or more exact sequence of these is not certain, and two concomitantly occurred “steps” in seed evolution may have in seed and be functionally correlated. The probable steps evolution are as follows (Figure 5.6): a,, - + nutntive tissue + integuments) lIst. r pa-. •‘ ,41 t wood) .— -‘- . -a - - - FIGURE 5.1 Cladogram of the woody and seed plants. Major apomorphies are indicated beside a thick hash mark. Families in bold are described in detail. Modified from Bowe et al. (2000); Chaw et al. (2000); Frohlich et al. (2000); and Samigullin et al. (1999). polymer called suberin (similar to cutin) that is quite resistant to water loss (see Chapter 10). The vascular cambium and cork cambium constituted major evolutionary novelties. Secondary xylem, or wood, functions in structural support, enabling the plant to grow tall and acquire massive systems of lateral branches. Thus, the vascular cambium was a precursor to the formation of intricately branched shrubs or trees with tall overstory canopies (e.g., Figure 5.2), a significant ecological adaptation. Cork produced by the cork cambium functions as a thick layer of cells that protects the delicate vascular cambium and secondary phloem from mechanical damage, predation, and desiccation. Wood anatomy can be quite complex. The details of cellular structure are important characters used in the — or The spermatophyta, commonly called spermatophytes lignophytes the within seed plants, are a monophyletic lineage this (Figure 5.1). The major evolutionary novelty that unites is an group is the seed. A seed is defined as an embryo, which sur immature diploid sporophyte developing from the zygote, coat seed a by enveloped rounded by nutritive tissue and immature (Figure 5.5). The embryo generally consists of an called the root called the radicle, a shoot apical meristem cotyledons; epicotyl, and one or more young seed leaves, the called the is stem the transition region between root and prior to hypocotyl (Figures 5.5, 5.10). An immature seed, fertilization, is known as an ovule. 1. !%: md. %H •4L I’ t 131 Et :rLct Composite massive, nonclonal giant sequoia, a woody conifer that is the most FIGURE 5.2 organism on Earth, and among the tallest of trees. HeterospOry. Heterospory is the formation of two types large, of haploid spores within two types of sporangia: meiosis via fewer-numbered megaspores, which develop numerous in the megasporangium, and small, more microsporafl the in meiosis microsporeS, the products of which a gium (Figures 5.6, 5.7). The ancestral condition, in Each single spore type forms, is called “homosporyY megaspore develops into a female gametophyte that bears game only archegonia; a microspore develops into a male heterospory Although tophyte, bearing only antheridia. plants was prerequisite to seed evolution, there are fossil among that were heterosporous but had not evolved seeds, 5.1 3A; 5.1, (Figures these being species of Archeopteris evolved has see later discussion). Note that heterospory extant independently in other, nonseed plants, e.g., in the ferns lycophytes Selaginella and Isoetes and in the water (Chapter 4). AND DIVERSITY OF WOODY AND SEED PLANTS — Lignophyta (Woody Plants) — Conjferae (Conifers) 1 1 r Gnetales -, a: a.) C.) a: a: — a: a.) C.) a: a: C.? . — 1 — — Cupressopsida a.) a.) a.) a.) — a: a: a C — EVOLUTION AND DIVERSITY OF PLANTS 131 classification and identification of woody plants. Wood ana tomical features may also be used to study the past, a spe cialty known as dendrochronology (see Chapter 10). Another feature of lignophytes is that they possess ances trally monopodial growth, in which a single main shoot develops branches from lateral (usually axillary) buds (see Chapters 4, 9). Although monopodial growth is presumed to have arisen prior to the monilophyte—lignophyte split, it enabled woody plants in particular the capability of forming extensive (sometimes massive) woody branching systems, permitting them to survive and reproduce more effectively. — — Spermatophyta (Seed Plants) Gymnospermae (Gymnosperms) - UNIT II a: C -a: C.) a.,) a: c_) SPERMATOPHYTES—SEED PLANTS aril The Spermatophyta, commonly called spermatophytes or seed plants, are a monophyletic lineage within the lignophytes (Figure 5.1). The major evolutionary novelty that unites this group is the seed. A seed is defined as an embryo, which is an immature diploid sporophyte developing from the zygote, sur rounded by nutritive tissue and enveloped by a seed coat (Figure 5.5). The embryo generally consists of an immature root called the radicle, a shoot apical meristem called the epicotyl, and one or more young seed leaves, the cotyledons; the transition region between root and stem is called the hypocotyl (Figures 5.5, 5.10). An immature seed, prior to fertilization, is known as an ovule. S pollen tube—sperm nonmotjie (siphonogamy) — — — I eustele SEED EVOLUTION The evolution of the seed involved several steps. The exact sequence of these is not certain, and two or more “steps” in seed evolution may have occurred concomitantly and be functionally correlated. The probable steps in seed evolution are as follows (Figure 5.6): pollen tube—sperm motile (zooidogamy) endosporic, male gametophyte pollen grain pollination droplet integument with micropyle retention of megaspore within megasporangjum reduction to 1 megaspore per megasporangjum endosporic female gametophyte heterospory 1. SEED (embryo + nutritive tissue + integuments) cork cambjum (periderm) vascular cambjum (secondary vascular tissue, mci. wood) plants. Major apomorphies are indicated beside a thick hash mark. Families in bold are 10); Chaw et al. (2000); Frohljch et al. (2000); and Samigullin et al. (1999). is quite resist flstituted major ‘ood, functions tall and acquire vascular cam ately branched shrubs or trees with tall overstory canopies (e.g., Figure 5.2), a significant ecological adaptation. Cork produced by the cork cambium functions as a thick layer of cells that protects the delicate vascular cambium and secondary phloem from mechanical damage, predation, and desiccation. Wood anatomy can be quite complex. The details of cellular structure are important characters used in the FIGURI 5.2 Composite photograph ofSequoiadendrongiganteum, giant sequoia, a woody conifer that is the most massive, nonclonal organism on Earth, and among the tallest of trees. Heterospory. Heterospory is the formation of two types of haploid spores within two types of sporangia: large, fewer-numbered megaspores, which develop via meiosis in the megasporangium, and small, more numerous microspores, the products of meiosis in the microsporan glum (Figures 5.6, 5.7). The ancestral condition, in which a single spore type forms, is called “homospory.” Each megaspore develops into a female gametophyte that bears only archegonia; a microspore develops into a male game tophyte, bearing only antheridia. Although heterospory was prerequisite to seed evolution, there are fossil plants that were heterosporous but had not evolved seeds, among these being species of Archeopteris (Figures 5.1, 5.13A; see later discussion). Note that heterospory has evolved independently in other, nonseed plants, e.g., in the extant lycophytes Selaginella and Isoetes and in the water ferns (Chapter 4). 132 CHAPTER 5 UNIT II EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS NTS EVOLUTION AND DIVERSITY OF PLA 133 I I I A vascular cambjum 2 xylem FIGURE 5.4 year’s growth. B. Four years’ growth. Woody stem cross-section, Pinus. sp. A. One the complete development of, 2. Endospory. Endospory is hyte within the original in this case, the female gametop condition, in which spore wall (Figure 5.6). The ancestral an external gameto the spore germinates and grows as ution of endosporic phyte, is called exospory. The evol with that of female gametophytes was correlated grains); see later endosporic male gametophytes (pollen discussion. ber to one. Reduction of 3. Reduction of megaspore num . First, the number megaspore number occurred in two ways undergo meiosis of cells within the megasporangium that aspore mother (each termed a megasporocyte or meg (Figure 5.6). This cell) was reduced, from several to one to four haploid single diploid megasporocyte gives rise aspores pro meg oid hapl megaspores. Second, of the four ng only leavi t, duced by meiosis, three consistently abor megaspore also one functional megaspore. This single lated with the undergoes a great increase in size, corre 2 phloem radicle periderrn embryo fluthtive tissue B (female gametophyle or endosperm) cotyledons cork (epidermis sloughed off to outside) FIGURE 5.3 eusteljc stem. A. Development of the vascular cambium. B. Development of secondary vascular tissue in the stem, illustrated here for a FIGURE 5.5 trated here. Morphology of a seed. Pinus sp. illus resources in the increased availability of space and megasporangium. ad of the megaspore 4. Retention of the megaspore. Inste ancestral condi being released from the sporangium (the eed plants), tion, as occurs in all homosporous nons megasporangium in seed plants it is retained within the a reduction in by (Figure 5.6). This was accompanied thickness of the megaspore wall. opyle. The final 5. Evolution of the integument & micr lopment of the event in seed evolution was the enve d the integu calle e, megasporangium by a layer of tissu s from the base of ment (Figure 5.6). The integument grow called a nucellus the megasporangium (which is often envelopes it, when surrounded by an integument) and ests that the sugg except at the distal end. Fossil evidence lobes derived integument likely evolved from separate surrounded the from telomes (ancestral branches) that ovules prior to megasporangium. These “preovules”, i.e., rim or ring of a the evolution of integuments, possessed the lagenos tissue at the apex of the megasporangium, n grains to a pol tome, which functioned to funnel polle Rothwell 1993 and art lination chamber. (See, e.g., Stew occurred with for details.) The epitome of seed evolution es to form the the evolutionary “fusion” of the telom completely sur integument, a continuous sheath that all extant seed of nt rounds the nucellus. The integume called the microplants has a small pore at the distal end stral lagenostome pyle. The micropyle replaced the ance in angiosperms, of as the site of entry of pollen grains (or functions in the also pollen tubes). The micropyle ation and resorp mechanics of pollination droplet form ument represents tion (see below). Note that a single integ s; in angiosperms the ancestral condition of spermatophyte (Chapter 6). later a second integument layer evolved AND DIVERSITY OF WOODYAND SEED PLANTS UNIT II EVOLUTION AND DIVERSITY OF PLANTS 133 1 FIGURE 5.4 2’ xylem vascular cambium vascular cambium 2 phloem l’phloem 2’ xylem Woody stem cross-section, Pinus. sp. A. One year’s growth. B. Four years’ growth. 2. Endospory. Endospory is the complete development of, in this case, the female gametophyte within the original spore wall (Figure 5.6). The ancestral condition, in which the spore germinates and grows as an external gameto phyte, is called exospory. The evolution of endosporic female gametophytes was correlated with that of endosporic male gametophytes (pollen grains); see later discussion. 3. Reduction of megaspore number to one. Reduction of megaspore number occurred in two ways. First, the number of cells within the megasporangium that undergo meiosis (each termed a megasporocyte or megaspore mother cell) was reduced, from several to one (Figure 5.6). This single diploid megasporocyte gives rise to four haploid megaspores. Second, of the four haploid megaspores pro duced by meiosis, three consistently abort, leaving only one functional megaspore. This single megaspore also undergoes a great increase in size, correlated with the 1’ xylem 2’ phloem seed coat radicle periderm cortex embryo nutritive tissue (female gametophyte or endosperm) epicotyl cotyledons cork (epidermis sloughed off to outside) ambium. B. Development of second vascular tissue in the stem, illustrated here for a FIGURE 5.5 Morphology of a seed. Pinus sp. illustrated here. increased availability of space and resources in the megasporangium. 4. Retention of the megaspore. Instead of the megaspore being released from the sporangium (the ancestral condi tion, as occurs in all homosporous nonseed plants), in seed plants it is retained within the megasporangium (Figure 5.6). This was accompanied by a reduction in thickness of the megaspore wall. 5. Evolution of the integument & micropyle. The final event in seed evolution was the envelopment of the megasporangium by a layer of tissue, called the integu ment (Figure 5.6). The integument grows from the base of the megasporangium (which is often called a nucellus when surrounded by an integument) and envelopes it, except at the distal end. Fossil evidence suggests that the integument likely evolved from separate lobes derived from telomes (ancestral branches) that surrounded the megasporangium. These “preovules”, i.e., ovules prior to the evolution of integuments, possessed a rim or ring of tissue at the apex of the megasporangium, the lagenos tome, which functioned to funnel pollen grains to a pol lination chamber. (See, e.g., Stewart and Rothwell 1993 for details.) The epitome of seed evolution occurred with the evolutionary “fusion” of the telomes to form the integument, a continuous sheath that completely sur rounds the nucellus. The integument of all extant seed plants has a small pore at the distal end called the micropyle. The micropyle replaced the ancestral lagenostome as the site of entry of pollen grains (or in angiosperms, of pollen tubes). The micropyle also functions in the mechanics of pollination droplet formation and resorp tion (see below). Note that a single integument represents the ancestral condition of spermatophytes; in angiosperms a second integument layer evolved later (Chapter 6). 134 CHAPTER 5 UNIT II EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS ,.._-‘ antheridia EVOLUTION AND DIVERSITY OF PLANTS 135 Sporophyte Body (2n) mitosis, growth, & differentiation mitosis, growth, & differentiation 7 Embryo (2n) / / male gametophyte (n) microsporangium (2N) t rneiosis ——fertilization /‘ I) ‘jN Egg (n) SpOraflgium female gametophyte (lost in the Angiosperms & some Gnetales) { k 1. Sperm (sperm nonflagellate in (n) j Conifers (mci. Gnetales) and Angiosperms) 3’ 0 megasporangium megasporangiurn 2. Endospory 3. Reduction to 1 megaspore megaspore gametophyte megasporangium 4. Retention of megaspore FIGURE 5.6 meioszs Microspores (n) ( —— Jn Megaspores..) (n) 1/ mitosis, growth, & differentiation Male Gametophyte (n) archegonia female gametophyte (contained in megaspore) —— mitosis, growth, & derentiaiion megasporangium wall © © Archegonium Antheridium 1 (reduced to absent in (n) (n) extant seed plants) Female Gametophyte (n) archegonia / Megasporocyte (2n) GAMETOPHYTE GENERATION (N) ) 1. Heterospory ‘\ Microsporocyte (2n) SPOROPHYTE GENERATION Zygote (2n) \ \ mitosis, growth, & d(fferentianon mitosis, growth, & differentiation gametophyte (n) Microsporangium Megasporangium (2n) (2n) 5. Evolution of Integument & Micropyle Ovule and seed evolution in the spermatophytes (hypothetical, for purpose of illustration). FIGURE 5.7 Life cycle of heterosporous seed plants. POLLINATION DROPLET One possible evolutionary novelty associated with seed evo lution is the pollination droplet. This is a droplet of liquid that is secreted by the young ovule through the micropyle (Figures 5.1OA, 5.171). This droplet is mostly water plus some sugars or amino acids and is formed by the breakdown of cells at the distal end of the megasporangium (nucellus). The cavity formed by this breakdown of cells is called the pollination chamber (Figure 5. bA). The pollination drop let functions in transporting pollen grains through the micropyle. This occurs by resorption of the droplet, which “pulls” pollen grains that have contacted the droplet into the pollina tion chamber. It is unknown whether a pollination droplet was present in the earliest seed plants. However, the presence of a pollination droplet in many nonflowering seed plants suggests that its occurrence may be apomorphic for at least the extant seed plant lineages. Note that the ovules of angiosperms lack pollination droplets or pollination cham bers, as flowering plants have evolved a different mechanism of pollen grain transfer (see Chapter 6). POLLEN GRAINS Concomitant with the evolution of the seed was the evolution of pollen grains (Figure 5.8). A pollen grain is, technically, an immature, endosporic male gametophyte. Endospory in pollen grain evolution was similar to the same process in seed evolu tion, involving the development of the male gametophyte within the original spore wall. Pollen grains of seed plants are extremely reduced male gametophytes, consisting of only a few cells. They are termed “immature” male gametophytes because, at the time of their release, they have not fully differentiated. After being released from the microsporangium, pollen must be transported to the micropyle of the ovule (or, in angio sperms, to the stigmatic tissue of the carpel; see Chapter 6) in order to ultimately effect fertilization. Wind dispersal, in com bination with an ovule pollination droplet (see later discus sion), was probably the ancestral means of pollen transport. After being transported to the ovule (or stigmatic tissue), the male gametophyte completes development by undergoing additional mitotic divisions and differentiation. The male gametophyte grows an exosporic pollen tube, which functions 134 CHAPTER 5 UNIT II EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS ,.._-‘ antheridia EVOLUTION AND DIVERSITY OF PLANTS 135 Sporophyte Body (2n) mitosis, growth, & differentiation mitosis, growth, & differentiation 7 Embryo (2n) / / male gametophyte (n) microsporangium (2N) t rneiosis ——fertilization /‘ I) ‘jN Egg (n) SpOraflgium female gametophyte (lost in the Angiosperms & some Gnetales) { k 1. Sperm (sperm nonflagellate in (n) j Conifers (mci. Gnetales) and Angiosperms) 3’ 0 megasporangium megasporangiurn 2. Endospory 3. Reduction to 1 megaspore megaspore gametophyte megasporangium 4. Retention of megaspore FIGURE 5.6 meioszs Microspores (n) ( —— Jn Megaspores..) (n) 1/ mitosis, growth, & differentiation Male Gametophyte (n) archegonia female gametophyte (contained in megaspore) —— mitosis, growth, & derentiaiion megasporangium wall © © Archegonium Antheridium 1 (reduced to absent in (n) (n) extant seed plants) Female Gametophyte (n) archegonia / Megasporocyte (2n) GAMETOPHYTE GENERATION (N) ) 1. Heterospory ‘\ Microsporocyte (2n) SPOROPHYTE GENERATION Zygote (2n) \ \ mitosis, growth, & d(fferentianon mitosis, growth, & differentiation gametophyte (n) Microsporangium Megasporangium (2n) (2n) 5. Evolution of Integument & Micropyle Ovule and seed evolution in the spermatophytes (hypothetical, for purpose of illustration). FIGURE 5.7 Life cycle of heterosporous seed plants. POLLINATION DROPLET One possible evolutionary novelty associated with seed evo lution is the pollination droplet. This is a droplet of liquid that is secreted by the young ovule through the micropyle (Figures 5.1OA, 5.171). This droplet is mostly water plus some sugars or amino acids and is formed by the breakdown of cells at the distal end of the megasporangium (nucellus). The cavity formed by this breakdown of cells is called the pollination chamber (Figure 5. bA). The pollination drop let functions in transporting pollen grains through the micropyle. This occurs by resorption of the droplet, which “pulls” pollen grains that have contacted the droplet into the pollina tion chamber. It is unknown whether a pollination droplet was present in the earliest seed plants. However, the presence of a pollination droplet in many nonflowering seed plants suggests that its occurrence may be apomorphic for at least the extant seed plant lineages. Note that the ovules of angiosperms lack pollination droplets or pollination cham bers, as flowering plants have evolved a different mechanism of pollen grain transfer (see Chapter 6). POLLEN GRAINS Concomitant with the evolution of the seed was the evolution of pollen grains (Figure 5.8). A pollen grain is, technically, an immature, endosporic male gametophyte. Endospory in pollen grain evolution was similar to the same process in seed evolu tion, involving the development of the male gametophyte within the original spore wall. Pollen grains of seed plants are extremely reduced male gametophytes, consisting of only a few cells. They are termed “immature” male gametophytes because, at the time of their release, they have not fully differentiated. After being released from the microsporangium, pollen must be transported to the micropyle of the ovule (or, in angio sperms, to the stigmatic tissue of the carpel; see Chapter 6) in order to ultimately effect fertilization. Wind dispersal, in com bination with an ovule pollination droplet (see later discus sion), was probably the ancestral means of pollen transport. After being transported to the ovule (or stigmatic tissue), the male gametophyte completes development by undergoing additional mitotic divisions and differentiation. The male gametophyte grows an exosporic pollen tube, which functions 136 CHAPTER 5 EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS UNIT II 137 EVOLUTION AND DIVERSITY OF PLANTS micropyle integument (2n) pollen grains pollen archegonial chamber pollination droplet integument - S A mitosiS and differentiation ICL FIGURE 5.8 Pollen grains—immature male gametophytes of seed plants. A. Zamia sp., a cycad. B. Ginkgo biloba. C. Pinus sp., a conifer. as a haustorial organ, obtaining nutrition by absorption from the surrounding sporophytic tissue (Figure 5.9; see Pollen Tabe). POLLEN TUBE The male gametophytes of all extant seed plants form a pollen tube (Figure 5.9) soon after the pollen grains make contact with the megasporangial (nucellar) tissue of the ovule. In extant seed plants the ancestral type of pollen type (found in cycads and ginkgophytes) was haustorial, in which the male gametophyte feeds (like a parasite) off the tissues of the nucellus. Motile sperm is delivered from this male gameto phyte into a fertilization chamber, where the sperm swims to the archegonium containing the egg, a process known as zooidogamy (zoom, animal + gamos, marriage). In the coni fers (including Gnetales), pollen tubes are also haustorial, but deliver nonmotile sperm cells to the archegonium or egg, a process known as siphonogamy (siphono, tube + gamos, megasporangium (nucellus) (2n) marriage). A type of siphonogamy evolved independently in the angiosperms. In angiosperms, however, the pollen tubes grow through stylar tissue prior to delivering the sperm to the egg of a female gametophyte (see Chapter 6). seed coat’ radicle mitosis and egg) embryo (new 2n) epicotyl (shoot apex) cotyledons female gametophyte (n) female gametophyte (n) B megasporangium megaspOrangiUm (degenerate) A. Ovule development in the nonflowering spermatophytes. B. Seed development. these male gametophytes may live in the megasporangial tissue for some time, generally several months to a year. The functional megaspore greatly expands, accompanied by numerous mitotic divisions, to form the endosporic female gametophyte (Figures 5.1OA, 5.11B,C). In the seeds of gymnosperms, archegonia differentiate at the apex of the female gametophyte (Figure 5.11C,D). As in the nonseed land plants, each archegonium has a large egg cell and a short line of neck cells (plus typically a ventral canal cell or nucleus). Eventually, the male gametophytes either release motile sperm cells (in cycads and Ginkgo) into a cavity between the megasporangium and female gametophyte (known as the archegonial chamber; Figure 5.1 DA), or the pollen tube of the male gametophyte delivers sperm cells directly into the archegonial neck (in conifers). (Note that germination & sperm mature male gametophytes, each with pollen tube hypocotyl megaSporangiUm (nucellus) (2n) pollen tube (haustorial) pollen grain (immature endosporic male gametophyte) female gametophyte (n) functional megaspore (n) micropyle FIGURE 5.10 differentiation (2n) A OVULE AND SEED DEVELOPMENT After pollination, the megasporocyte develops within the megasporangium of the ovule (Figures 5.1OA, 5.11A). The megasporocyte is a single cell that undergoes meiosis, producing a tetrad of four haploid megaspores, which in most extant seed plants are arranged in a straight line, or linearly (Figure 5.IOA). The three megaspores that are distal (away from the ovule base) abort; only the proximal megaspore (near the ovule base) con tinues to develop. In the pollination chamber, the resorbed pollen grains (Figures 5. iDA, 5.1 1A) develop into mature male gametophytes and form pollen tubes, which grow into the tissue of the megasporangium (Figures 5. iDA, 5.1 1B). In gymnosperms micropyle archegonium (with egg) pollination chamber motile sperm cell FIGURE 5.9 Male gametophyte morphology and development in the nonflowering spermatophytes; Cycas sp., illustrated. (Reproduced and modified from Swamy, B. G. L. 1948. American Journal of Botany 35: 77—88, by permission.) 1 the ovules of some Gnetales and all angiosperms lack arche gonia.) The end result is that a sperm cell from the male gametophyte fertilizes the egg of the female gametophyte. A long period of time (perhaps a year or more) may ensue between pollination, which is delivery of the pollen grains to the ovu)e, and fertilization, actual union of sperm and egg. Note: This is not true for the flowering plants, in which fertilization generally occurs very soon after pollination (see Chapter 6). The resulting diploid zygote, once formed, undergoes considerable mitotic divisions and differentiation, eventually maturing into the embryo, the immature sporophyte (Figures 5.1DB, ShE). The tissue of the female gametophyte contin ues to surround the embryo (Figure 5.11E) and serves as nutritive tissue for the embryo upon seed germination (except 136 CHAPTER 5 EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS UNIT II 137 EVOLUTION AND DIVERSITY OF PLANTS micropyle integument (2n) pollen grains pollen archegonial chamber pollination droplet integument - S A mitosiS and differentiation ICL FIGURE 5.8 Pollen grains—immature male gametophytes of seed plants. A. Zamia sp., a cycad. B. Ginkgo biloba. C. Pinus sp., a conifer. as a haustorial organ, obtaining nutrition by absorption from the surrounding sporophytic tissue (Figure 5.9; see Pollen Tabe). POLLEN TUBE The male gametophytes of all extant seed plants form a pollen tube (Figure 5.9) soon after the pollen grains make contact with the megasporangial (nucellar) tissue of the ovule. In extant seed plants the ancestral type of pollen type (found in cycads and ginkgophytes) was haustorial, in which the male gametophyte feeds (like a parasite) off the tissues of the nucellus. Motile sperm is delivered from this male gameto phyte into a fertilization chamber, where the sperm swims to the archegonium containing the egg, a process known as zooidogamy (zoom, animal + gamos, marriage). In the coni fers (including Gnetales), pollen tubes are also haustorial, but deliver nonmotile sperm cells to the archegonium or egg, a process known as siphonogamy (siphono, tube + gamos, megasporangium (nucellus) (2n) marriage). A type of siphonogamy evolved independently in the angiosperms. In angiosperms, however, the pollen tubes grow through stylar tissue prior to delivering the sperm to the egg of a female gametophyte (see Chapter 6). seed coat’ radicle mitosis and egg) embryo (new 2n) epicotyl (shoot apex) cotyledons female gametophyte (n) female gametophyte (n) B megasporangium megaspOrangiUm (degenerate) A. Ovule development in the nonflowering spermatophytes. B. Seed development. these male gametophytes may live in the megasporangial tissue for some time, generally several months to a year. The functional megaspore greatly expands, accompanied by numerous mitotic divisions, to form the endosporic female gametophyte (Figures 5.1OA, 5.11B,C). In the seeds of gymnosperms, archegonia differentiate at the apex of the female gametophyte (Figure 5.11C,D). As in the nonseed land plants, each archegonium has a large egg cell and a short line of neck cells (plus typically a ventral canal cell or nucleus). Eventually, the male gametophytes either release motile sperm cells (in cycads and Ginkgo) into a cavity between the megasporangium and female gametophyte (known as the archegonial chamber; Figure 5.1 DA), or the pollen tube of the male gametophyte delivers sperm cells directly into the archegonial neck (in conifers). (Note that germination & sperm mature male gametophytes, each with pollen tube hypocotyl megaSporangiUm (nucellus) (2n) pollen tube (haustorial) pollen grain (immature endosporic male gametophyte) female gametophyte (n) functional megaspore (n) micropyle FIGURE 5.10 differentiation (2n) A OVULE AND SEED DEVELOPMENT After pollination, the megasporocyte develops within the megasporangium of the ovule (Figures 5.1OA, 5.11A). The megasporocyte is a single cell that undergoes meiosis, producing a tetrad of four haploid megaspores, which in most extant seed plants are arranged in a straight line, or linearly (Figure 5.IOA). The three megaspores that are distal (away from the ovule base) abort; only the proximal megaspore (near the ovule base) con tinues to develop. In the pollination chamber, the resorbed pollen grains (Figures 5. iDA, 5.1 1A) develop into mature male gametophytes and form pollen tubes, which grow into the tissue of the megasporangium (Figures 5. iDA, 5.1 1B). In gymnosperms micropyle archegonium (with egg) pollination chamber motile sperm cell FIGURE 5.9 Male gametophyte morphology and development in the nonflowering spermatophytes; Cycas sp., illustrated. (Reproduced and modified from Swamy, B. G. L. 1948. American Journal of Botany 35: 77—88, by permission.) 1 the ovules of some Gnetales and all angiosperms lack arche gonia.) The end result is that a sperm cell from the male gametophyte fertilizes the egg of the female gametophyte. A long period of time (perhaps a year or more) may ensue between pollination, which is delivery of the pollen grains to the ovu)e, and fertilization, actual union of sperm and egg. Note: This is not true for the flowering plants, in which fertilization generally occurs very soon after pollination (see Chapter 6). The resulting diploid zygote, once formed, undergoes considerable mitotic divisions and differentiation, eventually maturing into the embryo, the immature sporophyte (Figures 5.1DB, ShE). The tissue of the female gametophyte contin ues to surround the embryo (Figure 5.11E) and serves as nutritive tissue for the embryo upon seed germination (except 138 CHAPTER 5 UNIT II EVOLUTiON AND DIVERSITY OF WOODYAND SEED PLANTS integument integument I I’, EVOLUTION AND DIVERSITY OF PLANTS 139 rchegonia 1, phloem female gametophyte 1xylern gametophyte . V / 4 cortex 7 a pith female gametophyte A B C vascular bundle outside. B. Helianthus stem FIGURE 5.12 Eustele. A. Diagram of eustele. Note single ring of vascular bundles, with xylem inside, phloem associated fibers. cross-section, an example of a eustele. C. Close-up of vascular bundle, showing xylem, phloem, and /-- megasporocyte female gametophyte r. 11 embryo / U-. nuêleus -1 - ••• ‘ . ‘\sIrile ‘.:;. ... • in the flowering plants; see Chapter 6). The megasporangium (nucellus) eventually degenerates. The integument matures into a peripheral seed coat, which may differentiate into various hard andlor fleshy layers. cells E FIGURE 5.11 Ovule and seed development, illustrated by Pinus sp. A. Young ovule, longitudinal-section, at time of pollination. Pollen grains are pulled into micropyle by resorption of pollination droplet. Meiosis of the megasporocyte has yet to occur. B. Post-pollination, showing development of the female gametophyte and haustorial pollen tube growth of the male gametophytes within tissue of megasporangium (nucellus). C. Mature ovule, showing two functional archegonia within female gametophyte. D. Close-up of archegonia, each containing a large egg cell with a surrounding layer of sterile cells and apical neck. E. Seed longitudinal-section, seed coat removed, showing embryo and surrounding nutritive layer of female gametophytic tissue. SEED ADAPTATIONS The adaptive significance of the seed is unquestioned. First, seeds provide protection, mostly by means of the seed coat, from mechanical damage, desiccation, and often predation. Second, seeds function as the dispersal unit of sexual repro duction. In many plants the seed has become specially modi fied for dispersal. For example, a fleshy outer seed coat layer may function to aid in animal dispersal. In fact, in some plants the seeds are eaten by animals, the outer fleshy layer is digested, and the remainder of the seed (including the embryo protected by an inner, hard seed coat layer) passes harmlessly through the gut of the animal, ready to germinate with a built in supply of fertilizer. In other plants, differentiation of the seed coat into one or more wings functions in seed dispersal by wind. Third, the seed coat may function in dormancy mechanisms that ensure germination of the seed only under ideal conditions of temperature, sunlight, or moisture. Fourth, upon germination, the nutritive tissue surrounding the embryo provides energy for the young seedling, aiding in successful establishment. Interestingly, in seed plants the female gametophyte (which develops within the megaspore) remains attached to and nutritionally dependent upon the sporophyte. This is exactly the reverse condition as is found in the liverworts, homworts, and mosses (Chapter 3). EUSTELE In addition to the seed, an apomorphy for spermatophytes is the eustele (Figure 5.12). A eustele is a primary stem vascu lature (“primary” meaning prior to any secondary growth) that consists of a single ring of discrete vascular bundles. Each vascular bundle contains an internal strand of xylem and an external strand of phloem that are radially oriented, i.e., positioned along a radius (Figure 5.12). The protoxylem of the vascular bundles of a eustele is endarch in position, i.e., toward the center of the stem. This is distinct from the exarch protoxylem of the lycophytes and the mesarch protoxylem of most monilophytes (Chapter 4) and of some fossil relatives that diverged prior to the seed plants. DIVERSITY OF WOODY AND SEED PLANTS ARCHEOPTEPJS A well-known lignophyte that lacked seeds was the fossil plant Archeopteris (not to be confused with the very famous fossil, reptilian bird Archeopteryx). Archeopteris was a large tree, with wood like a conifer but leaves like a fern (Figure 5.13A,B). Sporangia, producing spores, were born on fertile branch systems. Some species of Archeopteris were heterosporous. “PTERIDOSPERMS”—”SEED FERNS” The “pteridosperms,” or “seed ferns:’ are almost certainly a paraphyletic group of fossil plants that had femlike foliage, yet bore seeds. Medullosa is a well-known example of a seed fern 138 CHAPTER 5 UNIT II EVOLUTiON AND DIVERSITY OF WOODYAND SEED PLANTS integument integument I I’, EVOLUTION AND DIVERSITY OF PLANTS 139 rchegonia 1, phloem female gametophyte 1xylern gametophyte . V / 4 cortex 7 a pith female gametophyte A B C vascular bundle outside. B. Helianthus stem FIGURE 5.12 Eustele. A. Diagram of eustele. Note single ring of vascular bundles, with xylem inside, phloem associated fibers. cross-section, an example of a eustele. C. Close-up of vascular bundle, showing xylem, phloem, and /-- megasporocyte female gametophyte r. 11 embryo / U-. nuêleus -1 - ••• ‘ . ‘\sIrile ‘.:;. ... • in the flowering plants; see Chapter 6). The megasporangium (nucellus) eventually degenerates. The integument matures into a peripheral seed coat, which may differentiate into various hard andlor fleshy layers. cells E FIGURE 5.11 Ovule and seed development, illustrated by Pinus sp. A. Young ovule, longitudinal-section, at time of pollination. Pollen grains are pulled into micropyle by resorption of pollination droplet. Meiosis of the megasporocyte has yet to occur. B. Post-pollination, showing development of the female gametophyte and haustorial pollen tube growth of the male gametophytes within tissue of megasporangium (nucellus). C. Mature ovule, showing two functional archegonia within female gametophyte. D. Close-up of archegonia, each containing a large egg cell with a surrounding layer of sterile cells and apical neck. E. Seed longitudinal-section, seed coat removed, showing embryo and surrounding nutritive layer of female gametophytic tissue. SEED ADAPTATIONS The adaptive significance of the seed is unquestioned. First, seeds provide protection, mostly by means of the seed coat, from mechanical damage, desiccation, and often predation. Second, seeds function as the dispersal unit of sexual repro duction. In many plants the seed has become specially modi fied for dispersal. For example, a fleshy outer seed coat layer may function to aid in animal dispersal. In fact, in some plants the seeds are eaten by animals, the outer fleshy layer is digested, and the remainder of the seed (including the embryo protected by an inner, hard seed coat layer) passes harmlessly through the gut of the animal, ready to germinate with a built in supply of fertilizer. In other plants, differentiation of the seed coat into one or more wings functions in seed dispersal by wind. Third, the seed coat may function in dormancy mechanisms that ensure germination of the seed only under ideal conditions of temperature, sunlight, or moisture. Fourth, upon germination, the nutritive tissue surrounding the embryo provides energy for the young seedling, aiding in successful establishment. Interestingly, in seed plants the female gametophyte (which develops within the megaspore) remains attached to and nutritionally dependent upon the sporophyte. This is exactly the reverse condition as is found in the liverworts, homworts, and mosses (Chapter 3). EUSTELE In addition to the seed, an apomorphy for spermatophytes is the eustele (Figure 5.12). A eustele is a primary stem vascu lature (“primary” meaning prior to any secondary growth) that consists of a single ring of discrete vascular bundles. Each vascular bundle contains an internal strand of xylem and an external strand of phloem that are radially oriented, i.e., positioned along a radius (Figure 5.12). The protoxylem of the vascular bundles of a eustele is endarch in position, i.e., toward the center of the stem. This is distinct from the exarch protoxylem of the lycophytes and the mesarch protoxylem of most monilophytes (Chapter 4) and of some fossil relatives that diverged prior to the seed plants. DIVERSITY OF WOODY AND SEED PLANTS ARCHEOPTEPJS A well-known lignophyte that lacked seeds was the fossil plant Archeopteris (not to be confused with the very famous fossil, reptilian bird Archeopteryx). Archeopteris was a large tree, with wood like a conifer but leaves like a fern (Figure 5.13A,B). Sporangia, producing spores, were born on fertile branch systems. Some species of Archeopteris were heterosporous. “PTERIDOSPERMS”—”SEED FERNS” The “pteridosperms,” or “seed ferns:’ are almost certainly a paraphyletic group of fossil plants that had femlike foliage, yet bore seeds. Medullosa is a well-known example of a seed fern