EVOLUTION AND DIVERSITY OF VASCULAR PLANTS

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1 EVOLUTON AND DVERSTY OF VASCULAR PLANTS VASCULAR PLANT APOMORPHES 73 ndependent, Long-Lived Sporophyte 75 Branched Sporophyte 75 Lignified Secondary Cell Walls 75 Scierenchyma 76 Tracheary Elements (of Xylem) 77 Sieve Elements (of Phloem) 77 Endodermis 79 Root 79 VASCULAR PLANT DVERSTY 81 Rhyniophytes 81 Lycopodiophyta Lycophytes 82 Lycopodiopsida 83 Lycopodiaceae 85 soetopsida 85 soetaceae 86 Selaginellaceae 88 Euphyllophyta Euphyllophytes 88 Monilophyta Monilophytes, Ferns 91 Equisetopsida Horsetails 93 Equisetaceae 94 Psilotopsida 94 Ophioglossales Ophioglossoid Ferns 96 Ophioglossaceae 96 Psilotales Whisk Ferns 96 Psilotaceae 98 Marattiopsida Marattioid Ferns 98 Marattiaceae 98 Polypodiopsida Leptosporangiate Ferns 100 Osmundales Osmundaceous Ferns 105 Osmundaceae 105 Hymenophyllales Filmy Ferns 106 Hymenophyllaceae 106 Gleicheniales Gleichenioid Ferns 110 Gleicheniaceae...: Schizaeales Schizaeoid Ferns 110 Lygodiaceae Salviniales Aquatic/Heterosporous Ferns 110 Marsileaceae 113 Salviniaceae 113 Cyatheales Tree Ferns 116 Cyatheaceae 116 Polypodiales Polypod Ferns 116 Aspleniaceae 116 Dryopteridaceae 116 Polypodiaceae 120 Pteridaceae 120 REVEW QESTONS 123 EXERCSES 125 REFERENCES FOR FURTHER STUDY 125 WEB STE 128 VASCULAR PLANT APOMORPHES The vascular plants, or Tracheophyta (also called tracheo phytes), are a monophyletic subgroup of the land plants. The major lineages of tracheophytes (excluding many fossil groups) are seen in Figure 4.1 (after Pryer et al. 2001a, 2004a,b, and Qiu et al. 2006, 2007; but see Rothwell and Nixon 2006 for alternative relationships). Vascular plants together share a number of apomorphies, including (1) an independent, long-lived sporophyte; (2) a branched sporo phyte; (3) lignified secondary walls, with pits, in certain specialized cells; (4) sclerenchyma, specialized cells that function in structural support; (5) tracheary elements, cells of xylem tissue, involved in water transport; (6) sieve elements, cells of phloem tissue, involved in sugar transport (the xylem and phloem comprising the vascular tissue); (7) an endodermis, involved in selective transfer of compounds; and (8) roots, functioning in anchorage and absorption of water and nutrients. See Kenrick and Crane (1997) and Pryer et al. (2004b) for detailed information Elseyjer nc. All rights reserved. doj; B

2 74 CHAPTER 4 EVOLUTON AND DVERSTY OF VASCULAR PLANTS C >-. C? o Lycopodiophyta Lycophytes soetopsida Polysporangiomorpha/Pan-Tracheophyta Tracheophyta Tracheophytes (Vascular Plants) Euphyllophyta 1 Euphyllophyes 0 -= 5.).zi E : H Monilophyta Monilophyes Psilotopsida z. t u..- c i a u tl. c.:: 1.) > liii L. L L shoot with lycophylls wood heterospory leaves ligulate sporangia dorsiventral, transversely dehiscent stem protoxylem exarch root protoxylem endarch roots dichopodial roots - - sporangiophore leaves reduced, whorled stems ribbed with canals L endoderrnis sieve elements (of phloem) tracheary elements (of xylem) sclerenchyma spores w/ elaters vascular tissue lignin, in lignified secondary cell walls sporophyte branched, with multiple sporangia sporophyte independent, long-lived tleaf w/ sterile & fertile segments gametophyte subterranean, mycorrhizal synangium, w/bifid appendage leaves reduced roots lost roots unbranched, root hairs absent siphonostele stem protoxylem mesarch t polycyclic siphonostele lepto sporangium 30 kb chloroplast DNA inversion shoot with euphylls sporangia terminal on lateral branches, longitudinally dehiscent root protoxylem exarch roots monopodial = extinct taxon = extinct lineage FGURE 4.1 Phylogeny of the tracheophytes, the vascular plants, modified from Pryer et al. (2001a, 2004a,b) and Qiu et al. (2006, 2007), with selected apomorphies.

3 UNT EVOLUTON AND DVERSTY OF PLANTS 75 apical meristem divides equally 4 branches equal apical meristem divides equally dominant branch A dichotomous B pseudomonopodial FGURE 4.2 Dichotomous (A) and pseudomonopodial (B) branching patterns in vascular plants. NDEPENDENT, LONG-lVED SPOROPHYTE Like all land plants, the vascular plants have a haplodiplontic alternation of generations, with a haploid gametophyte and a diploid sporophyte. Unlike the liverworts, mosses, and hornworts, however, vascular plants have a dominant, freeliving, photosynthetic, relatively persistent sporophyte gen eration (although, as discussed in Chapter 3, the hornworts have a sporophyte that is photosynthetic and relatively longpersistent). n the vascular plants, the gametophyte genera tion is also (ancestrally) free-living and maybe photosynthetic, but it is smaller (often much more so) and much shorter lived than the sporophyte generation (although the gametophyte may be somewhat persistent). n all land plants, the sporophyte is initially attached to and nutritionally dependent upon the gametophyte. However, in the vascular plants, the sporophyte soon grows larger and becomes nutritionally independent, usu ally with the subsequent death of the gametophyte. (n seed plants the female gametophyte is attached to and nutritionally dependent upon the sporophyte; see Chapter 5.) BRANCHED SPOROPHYTE The sporophytic axes, or stems, of vascular plants are different from those of liverworts, homworts, and mosses in that they are branched and bear multiple (not just one) sporan gia. Extant vascular plants share this apomorphy with some fossil plants that are transitional between the bryophytes and the tracheophytes. This more inclusive group, including fossil and extant taxa having branched sporophytic stems and multi ple sporangia, has been called the Polysporangiomorpha (Kennck and Crane 1997) or polysporangiophytes. The even more inclusive Pan-Tracheophyta (Cantino et al. 2007) encompasses all descendents exclusive of the liverworts, mosses, and hornworts. The earliest vascular plant stems had branching that was dichotomous, in which the apical meristem splits into two, equal meristems, each of which grows independently more or less equally (Figure 4.2A). Later lineages evolved a modified growth pattern, called pseudomonopodial, which starts out dichotomous, but then one branch becomes dominant and overtops the other, the latter appearing lateral (Figure 4.2B). Subsequent vascular plant lineages evolved monopodial growth. (See Euphyllophytes.) The sporophytic stems of vascular plants function as sup portive organs, bearing and usually elevating reproductive organs and leaves (see below). They also function as conduc tive organs, via vascular tissue, of water, minerals, and sugars between roots, leaves, and reproductive organs. Structurally, stems can be distinguished from roots by several anatomical features (to be discussed). LGNFED SECONDARY CELL WALLS Vascular plants possess a chemical known as lignin, which is a complex polymer of phenolic compounds. Lignin is incorpo rated into an additional cell wall layer, known as the second ary (2 ) wall (Figure 4.3), which is found in certain, specialized cells of vascular plants. Secondary walls are secreted to the outside of the plasma membrane (between the plasma membrane and the primary cell wall) after the primary wall has been secreted, which is also after the cell ceases to elongate. Secondary cell walls are usually much thicker than primary walls and, like primary walls, contain cellulose. However, in secondary walls, lignin is secreted into the space between the cellulose microfibrils, forming a sort of interbinding cement. Thus, lignin imparts significant strength and rigidity to the cell wall. n virtually all plant cells with secondary, lignifled cell walls, there are holes in the secondary wall called pits (Figure 4.3). Pits commonly occur in pairs opposite the sites of numerous plasmodesmata in the primary cell wall. This group of plasmod esmata is called a primary pit field. Pits function in allowing chemical communication between cells, via the plasmodes mata of the primary pit field, during their development and dif ferentiation. They may also have specialized functions in water conducting cells (discussed later). Plant cells with sec ondary walls include scierenchyma and tracheary elements (see later discussion).

4 76 CHAPTER 4 EVOLUTON AND DVERSTY OF VASCULAR PLANTS p pit plasma membrane middle lamella primary cell wall (cellulosic) secondary cell wall (lignified) A. pit (pits of two adjacent cells = pit-pair) pnmary pit field of several plasmodesmata) plasmodesmata Cell #1 Cell#2 H: FGURE 4.3 Lignified secondary cell wall of specialized cells of vascular plants. Note pit-pair adjacent to primary pit field. B. pit SCLERENCHYMA Sclerenchyma (Gr. scieros, hard + enchy,na, infusion, in ref erence to the infusion of lignin in the secondary cell walls) consists of nonconductive cells that have a thick, lignifled secondary cell wall, typically with pits, and that are dead at maturity. There are two types of sclerenchyma (Figure 4.4): (1) fibers, which are long, very narrow cells with sharply tapering end walls; and (2) sclereids, which are isodiametric to irregular or branched in shape. Fibers function in mechan ical support of various organs and tissues, sometimes making up the bulk of the tissue. Fibers often occur in groups or bun dles. They may be components of the xylem and/or phloem or may occur independently of vascular tissue. Sclereids may also function in structural support, but their role in some plant organs is unclear; they may possibly help to deter herbivory in some plants. The evolution of sclerenchyma, especially fibers, with lignified secondary cell walls, constitutes a major plant adaptation enabling the structural support needed to attain greater stem height. Another tissue type that functions in structural support is collenchyina, consisting of live cells with unevenly thickened, pectic-rich, primary cell walls (see Chapter 10). Collenchyma is found in many vascular plants, but is probably not an apomorphy for the group. lignified secondary cell wall FGURE 4.4 Sclerenchyma. A. Fiber cell. B. Sciereid cells. c.s cross-section. pit

5 UNT EVOLUTON AND DVERSTY OF PLANTS 77 o perforation plate (compound) lignified cell wall o DC A tracheid vessels B FGURE 4.5 Conductive cells of vascular plants: tracheary elements. A. Types of tracheary elements. B. Vessel. TFACHEARY ELEMENTS (OF XYLEM) The vascular plants, as the name states, have true vascular tissue, consisting of cells that have become highly special ized for conduction of fluids. (A tissue consists of two or more cell types that have a common function and often a common developmental history; see Chapter 10.) Vascular tissue was a major adaptive breakthrough in plant evolution; more efficient conductivity allowed for the evolution of much greater plant height and diversity of form. Tracheary elements are specialized cells that function in water and mineral conduction. Tracheary elements are gener ally elongate cells, are dead at maturity, and have lignified 2 cell walls (Figure 4.5A,B). They are joined end-to-end, form ing a tube-like continuum. Tracheary elements are typically associated with parenchyma and often some sclerenchyma in a common tissue known as xylem (Gr. xylo, wood, after the fact that wood is composed of secondary xylem). The func tion of tracheary elements is to conduct water and dissolved essential mineral nutrients, generally from the roots to other parts of the plant. There are two types of tracheary elements: tracheids and vessel members (Figure 4.5A). These differ with regard to the junction between adjacent end-to-end cells, whether impejforate or peiforate. Tracheids are imperforate, meaning that water and mineral nutrients flow between adjacent cells through the primary cell walls at pit-pairs, which are adjacent holes in the lignified 2 cell wall. Vessel members are perfo rate, meaning that there are one or more continuous holes or perforations, with no intervening 1 or 2 wall between adjacent cells through which water and minerals may pass. The contact area of two adjacent vessel members is called the perforation plate. The perforation plate may be compound if composed of several perforations, or simple if composed of a single opening (see Chapter 10). Vessels may differ consid erably in length, width, angle of the end walls, and degree of perforation. Tracheids are the primitive type of tracheary element. Vessels are thought to have evolved from preexisting trache ids independently in several different groups, including a few species of Equisetum, a few leptosporangiate ferns, all Gnetales (Chapter 5), and almost all angiosperms (Chapter 6). SEVE ELEMENTS (Of PHLOEM) Sieve elements are specialized cells that function in the con duction of sugars. They are typically associated with paren chyma and often some scierenchyma in a common tissue known as phloem (Gr. phloe, bark, after the location of secondary phloem in the inner bark). Sieve elements are elongate cells having only a primary (1 ) wall with no ligni fled 2 cell wall. This primary wall has specialized pores

78 CHAPTER 4 EVOLUTON AND DVERSTY OF VASCULAR PLANTS sieve plate (compound) sieve plate (simple) sieve areas sieve plate (simple) A ṡieve cell L.._ sieve tube members.j FGURE 4.

6 78 CHAPTER 4 EVOLUTON AND DVERSTY OF VASCULAR PLANTS sieve plate (compound) sieve plate (simple) sieve areas sieve plate (simple) A ṡieve cell L.._ sieve tube members.j FGURE 4.6 Conductive cells of vascular plants: sieve elements. A. Types of sieve elements. B,C. Sieve tube members. (Figure 4.6C), which are aggregated together into sieve areas (Figure 4.6A). Each pore of the sieve area is a continuous hole in the 10 cell wall that is lined with a substance called callose, a polysaccharide composed of 13-1,3-glucose units. (Note the difference in chemical linkage from cellulose, which is a polymer of J3-1,4-glucose.) Sieve elements are semi-alive at maturity. They lose their nucleus and other organelles but retain the endoplasmic reticulum, mitochon dria, and plastids. Like tracheary elements, sieve elements are oriented end-to-end, forming a tubelike continuum. Sieve elements function by conducting dissolved sugars from a sugar-rich source to a sugar-poor sink region of the plant. Source regions include the leaves, where sugars are synthe sized during photosynthesis, or mature storage organs, where sugars may be released by the hydrolysis of starch. Sinks can include actively dividing cells, developing storage organs, or reproductive organs such as flowers or fruits. There are two types of sieve elements: sieve cells and sieve tube members (Figure 4.6A). Sieve cells have only sieve areas on both end and side walls. Sieve tube members have both sieve areas and sieve plates (Figure 4.6B). Sieve plates consist of one or more sieve areas at the end wall junction of two sieve tube members; the pores of a sieve plate, however, are significantly larger than are those of sieve areas located on the side wall (Figure 4.6B,C). Both sieve cells and sieve tube members have parenchyma cells associated with them. Parenchyma cells asso ciated with sieve cells are called albuininous cells; those associ ated with sieve tube members are called companion cells. The two differ in that companion cells are derived from the same parent cell as are sieve tube members, whereas albuminous cells and sieve cells are usually derived from different parent cells. Both albuminous cells and companion cells function to load and unload sugars into the cavity of the sieve cells or sieve tube members. Sieve cells (and associated albuminous cells) are the ancestral sugar-conducting cells and are found in all nonflower ing vascular plants. Sieve tube members were derived from sieve cells and are found only in flowering plants, the angiosperms (see Chapter 6). Stems of the vascular plants typically have a consistent and characteristic spatial arrangement of xylem and phloem. This organization of xylem and phloem in the stem is known as a stele. n several groups of early vascular plant lineages, the stelar type is a protostele, with a central solid cylinder of xylem and phloem (Figure 4.7). A modification of the pro tostele, in which xylem and phloem interdigitate, is called a plectostele (e.g., Figure 4.14A,B). The largely parenchyma tous tissue between the epidermis and vascular tissue defines the cortex. Protosteles, thought to be the most ancestral type of stem vasculature, are found, e.g., in the rhyniophytes (see later discussion).

7 UNT EVOLUTON AND DVERSTY OF PLANTS 79 phloem epidermis cortex through the cell wall). Because the plasma membrane may differentially control solute transfer, the endodermis (with Casparian strips) selectively controls which compounds are or are not absorbed by the plant; thus, toxic or unneeded chemicals may be differentially excluded. xylem FGURE 4.7 Example of a protostele, an ancestral vasculature of vascular plants. ENDODERMS Another apparent apomorphy for the vascular plants is the occurrence, in some (especially underground) stems and all roots, of a special cylinder of cells known as the endodermis (Figure 4.8). Each cell of the endodermis possesses a Casparian strip, which is a band or ring of lignin and suberin (chemically similar to lignin) that infiltrates the cell wall, oriented tangentially (along the two transverse walls) and axially (vertically, along the two radial walls; Figure 4.8C). The Casparian strip acts as a water-impermeable material that binds to the plasma membrane of the endodermal cells. Because of the presence of the Casparian strip, absorbed water and minerals that flow from the outside environment to the central vascular tissue must flow through the plasma membrane of the endodermal cells (as opposed to flowing through the intercellular spaces, i.e., between the cells or ROOT A major novelty in the evolution of vascular plants was the differentiation between stems and roots. Roots are special ized plant organs that function in anchorage and absorption of water and minerals. Roots are found in all vascular plants except for the Psilotales, Salviniales, and a few other special ized groups, all of which lost roots secondarily (see later dis cussion). Other fossil groups of vascular plants may have lacked roots; plants lacking roots generally have uniseriate (one cell thick), filamentous rhizoids (similar to those of bry ophytes ), which assume a similar absorptive function. Roots constituted a major adaptive advance in enabling much more efficient water and mineral acquisition and conduction, per mitting the evolution of plants in more extreme habitats. Roots, like stems, develop by the formation of new cells within the actively growing apical meristem of the root tip, a region of continuous mitotic divisions (Figure 4.9B). At a later growth stage and further up the root, these cell deriva tives elongate significantly. This cell growth, which occurs by considerable expansion both horizontally and vertically, pushes the apical meristem tissue downward. At an even later stage and further up the root, the fully-grown cells differenti ate into specialized cells. The ancestral apical meristem of roots most likely consisted of a single, apical cell, a feature C WAThR FLOW (outside to inside) endodermai cell (cross-section) FGURE 4.8 Endodermis of vascular plants. A,B. Equiserum rhizome. A. Rhizome cross-section, showing single layer of endodermal cells. B. Close-up of endodermal cells (in cross-section), showing Casparian strip thickenings. C. Diagram of Casparian strip, indicating function.

80 CHAPTER 4 EVOLUTON AND DVERSTY OF VASCULAR PLANTS epidermis cortex - central vascular cylinder xylem

Root longitudinal-section. C. Whole root cross-section. D.

found today in the Selaginellaceae of the lycophytes and all monilophytes (discussed later).

consisting of a group of continuously divid ing cells.

First, the apical meristem is covered on the outside by a rootcap (also called a calyptra; Figure 4.

The rootcap functions both to protect the root apical meristem from mechanical damage as the root grows

Second, with the exception of the Psilotopsida (Psilotales and Ophioglossales), the epidermal cells away

9A); these are absent from stems (although under ground stems of the Psilotales bear rhizoids, which

Root hairs function to greatly increase the surface area available for water and mineral absorption.

As in stems, the mostly parenchymatous region between the vas culature and epidermis is called the cortex

Fourth, the vascular cylinder of roots is sur rounded by an endodermis with Casparian strips (Figure 4.9D).

by the plant, functioning in selective absorption.

) Fifth, roots generally have endog enous lateral roots (Figure 4.

or endodermis. Lateral roots penetrate the tis sues of the cortex before exiting to the outside.

8 80 CHAPTER 4 EVOLUTON AND DVERSTY OF VASCULAR PLANTS epidermis cortex - central vascular cylinder xylem phloem FGURE 4.9 Anatomy of the root, an apomorphy of the vascular plants. A. Root whole mount. B. Root longitudinal-section. C. Whole root cross-section. D. Close-up of central vascular cylinder, showing tissues. found today in the Selaginellaceae of the lycophytes and all monilophytes (discussed later). n the Lycopodiaceae, soetaceae, and seed plants (see Chapter 5), the apical mer istem is complex, consisting of a group of continuously divid ing cells. Roots are characterized by several anatomical features. First, the apical meristem is covered on the outside by a rootcap (also called a calyptra; Figure 4.9A,B); stems lack such a cell layer. The rootcap functions both to protect the root apical meristem from mechanical damage as the root grows into the soil and to provide lubrication as the outer cells slough off. Second, with the exception of the Psilotopsida (Psilotales and Ophioglossales), the epidermal cells away from the root tip develop hairlike extensions called root hairs (Figure 4.9A); these are absent from stems (although under ground stems of the Psilotales bear rhizoids, which resemble root hairs). Root hairs function to greatly increase the surface area available for water and mineral absorption. Third, roots always have a central vascular cylinder (Figure 4.9C,D). As in stems, the mostly parenchymatous region between the vas culature and epidermis is called the cortex (Figure 4.9C); the center of the vascular cylinder, if vascular tissue is lacking, is called a pith. Fourth, the vascular cylinder of roots is sur rounded by an endodermis with Casparian strips (Figure 4.9D). As with some stems, the endodermis in roots selec tively controls which chemicals are and are not absorbed by the plant, functioning in selective absorption. (An undifferen tiated layer internal to the endodermis, called the pericycle, is also typically present.) Fifth, roots generally have endog enous lateral roots (Figure 4.10), in which new lateral roots originate by means of actively growing meristems, arising at the pericycle or endodermis. Lateral roots penetrate the tis sues of the cortex before exiting to the outside. Numerous modifications of roots have evolved, most of these restricted to the flowering plants (see Chapter 9). Roots of many, if not most, vascular plants have an interesting sym biotic interaction with various species of fungi; this associa tion between the two is known as mycorrhizae. The fungal component of mycorrhizae appears to aid the plant in both increasing overall surface area for water and mineral absorp tion and increasing the efficiency of selective mineral absorp tion, such as of phosphorus. The fungus benefits in obtaining photosynthates (sugars and other nutrients) from the plant. FGURE 4.10 Root cross-section (Lilum sp.), showing endoge nous lateral root, a characteristic of vascular plant roots.

Topic 2: Plant Structure & Growth Ch. 35 Angiosperms are the most complex plants. They are composed of cells, tissues, organs and organ systems.

Topic 2: Plant Structure & Growth Ch. 35 Angiosperms are the most complex plants. They are composed of cells, tissues, organs and organ systems. Fig. 35.8 Plant Cells pp.798-802 Types of plant cells Include: