OF VASCULAR PLANTS EVOLUTION AND DIVERSITY

72 CHAPTER 3 EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS

EXERCISES

1. Peruse the most recent literature on phylogenetic relationships of the “green algae” relative to the land plants. Are there any

differences relative to Figure 3.1?2. Peruse the recent literature on phylogenetic relationships of the hornworts, liverworts, and mosses. Do any show relation

ships different from that of Figure 3.6?

3. Peruse botanical journals and find a systematic article on a moss, liverwort, or homwort. What is the objective of the article

and what techniques were used to address it?

4. Collect and identify local liverworts, hornworts, and mosses. What features are used to distinguish among families, genera,

and species?

REFERENCES FOR FURTHER STUDY

EVOLUTION AND DIVERSITYOF VASCULAR PLANTS

Bremer, Kâre. 1985. Summary of green plant phylogeny and classification. Cladistics 1(4): 369—385.Cox, C. J., B. Goffinet, A. J. Shaw, and S. B. Boles. 2004. Phylogenetic relationships among the mosses based on heterogeneous Bayesian

analysis of multiple genes from multiple genomic compartments. Systematic Botany 29: 234—250.Crandall-Stotler B., and R. E. Stotler. 2000. Morphology and classification of the Marchantiophyta. In: A. J. Shaw, B. Goffinet, eds. Bryophyte

biology. Pp. 21—70. Cambridge University Press, Cambridge.Crum, H. 2001. Structural diversity of bryophytes. University of Michigan Herbarium, Ann Arbor.Duff, R. J., D. C. Cargill, 3. C. C. Villarreal, and K. S. Renzaglia. 2004. Phylogenetic relationships of the hornworts based on rbcL sequence

data: novel relationships and new insights. In: B. Goffinet, V. Hollowell, and R. Magill (eds.). Molecular systematics of bryophytes. Pp.41—58. Missouri Botanical Garden, St. Louis.

Forrest, L.L., E. C. Davis, D. G. Long, B. J. Crandall-Stotler, A. Clark, and M. L. Hollingsworth. 2006. Unraveling the evolutionary historyof the liverworts (Marchantiophyta): multiple taxa, genomes and analyses. Bryologist 109: 303—334.

Gensel, P. G., and D. Edwards (eds.). 2001. Plants invade the land: Evolutionary and environmental perspectives. Columbia University Press,New York.

Goffinet, B., and W. R. Buck. 2004. Systematics of the bryophyta (mosses): From molecules to a revised classification. In: B. Goffinet,V. Hollowell, and R. Magill (eds.) Molecular systematics of bryophytes. Missouri Botanical Garden, St. Louis.

Goremykin, V. V., and F. H. Hellwig. 2005. Evidence for the most basal split in land plants dividing bryophyte and tracheophyte lineages.Plant Systematics and Evolution 254: 93—103.

Graham, L. E. 1985. The origin of the life cycle of land plants. American Scientist 73: 178—1 86.Graham, L. E. 1993. Origin of land plants. Wiley, New York.He-Nygren, X., A. Juslen, I. Ahonen, D. Glenny, and S. Piippo. 2006. Illuminating the evolutionary history of liverworts (Marchantiophyta):

Towards a natural classification. Cladistics 22: 1—31.Heinrichs, I., S. R. Gradstein, R. Wilson, and H. Schneider. 2005. Towards a natural classification of liverworts (Marchantiophyta) based on

the chloroplast gene rbcL. Cryptogamie Bryologie 26: 131—150.Karol, K. G., R. M. McCourt, M. T. Cimino, and C. F. Delwiche. 2001. The closest living relatives of land plants. Science 294: 235 1—2353.Kelch, D. G., A. Driskell, and B. D. Mishler. 2004. Inferring phylogeny using genomic characters: A case study using land plant plastomes.

In: B. Goffinet, V. Hollowell, and R. Magill (eds.) Molecular systematics of bryophytes. Pp. 3—11. Missouri Botanical Garden Press, St.Louis.

Kenrick, P., and P. R. Crane. 1997. The origin and early diversification of land plants: a cladistic study. Smithsonian Institution Press,Washington, DC.

Mishler, B. D., and S. P. Churchill. 1984. A cladistic approach to the phylogeny of the “Bryophytes.” Brittonia 36(4): 406—424.Mishler, B. D., and S. P. Churchill. 1985. Transition to a land flora: Phylogenetic relationships of the green algae and bryophytes. Cladistics

1(4): 305—328.Mishler, B. D., L. A. Lewis, M. A. Buchlieim, K. S. Renzaglia, D. J. Garbary, C. F. Delwiche, F. W. Zechman, T. S. Kantz, and R. L. Chapman.

1994. Phylogenetic relationships of the “Green Algae” and “Bryophytes.” Annals of the Missouri Botanical Garden 81: 451—483.Nickrent, D. L., C. L. Parkinson, J. D. Palmer, and R. J. Duff. 2000. Multigene phylogeny of land plants with special reference to bryophytes

and the earliest land plants. Molecular Biology and Evolution 17: 1885—1895.Pickett-Heaps, J. D. 1975. Green algae: structure, reproduction and evolution in selected genera. Sinauer, Sunderland, MA.Qiu, Y-L., L. Li, B. Wang, Z. Chen, 0. Dombrovska, J. Lee, L. Kent, R. Li, R. W. Jobson, T. A. Hendry, D. W. Taylor, C. M. Testa, and M.

Ambros. 2007. A nonflowering land plant phylogeny inferred from nucleotide sequences of seven chloroplast, mitochondrial, and nucleargenes. International Journal of Plant Sciences 168: 691-708.

Renzaglia, K. S., R. J. Duff, D. L. Nickrent, and D. J. Garbary. 2000, Vegetative and reproductive innovations of early land plants: Implicationsfor a unified phylogeny. Philosophical Transactions: Biological Sciences 355: 769—793.

VASCULAR PLANT APOMORPHIES 73

Independent, 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 DIVERSITY 81

Rhyniophytes 81

Lycopodiophyta—Lycophytes 82

Lycopodiopsida 83

Lycopodiaceae 85

Isoetopsida 85

Isoetaceae 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

VASCULAR PLANT APOMORPHIES

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

73

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 ...................:................. 110

Schizaeales—Schizaeoid Ferns 110

Lygodiaceae ..................................... 110

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

REVIEW QESTIONS 123

EXERCISES 125

REFERENCES FOR FURTHER STUDY 125

WEB SITE 128

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.

© 2010 Elseyjer Inc. All rights reserved.doj; 10. 10161B978-0- 12-374380.0.00004-0

74 CHAPTER 4 EVOLUTION AND DIVERSITY OF VASCULAR PLANTS UNIT II EVOLUTION AND DIVERSITY OF PLANTS 75

C

I I I II I I II I II II I IIII I II I I II I I II I I II I I II I I

I I I II I Iliii L.

I I II I II I I

I ——

I I IL__

IL___

Psilotopsida

I

z

I

_______________________________________ ______________________________________

I —-

I —- sporangiophore

I — — leaves reduced,I whorled — —

I — — stems ribbedI with canals

L

Ct

—i a.:: 1.)

Ct

Ct >•

utL.

Ct.

•tu

.—.-

cCt

c —

30 kB chloroplast DNA inversion— shoot with euphylls— sporangia terminal on lateral branches,

longitudinally dehiscent— root protoxylem exarch• roots monopodial

LycopodiophytaLycophytes

Polysporangiomorpha/Pan-Tracheophyta —

TracheophytaTracheophytes (Vascular Plants)

Euphyllophyta1 Euphyllophyes

MonilophytaMonilophyes

IsoetopsidaCt

>-.

C— ?

o

Ct

©Ct

0 -=5.) .ziE :H

dominantbranchapical meristem

divides equally

pseudomonopodial

t leaf w/sterile &fertilesegments

• spores w/ elaters

wood

heterospory

leaves ligulate

shoot withlycophylls

F IGURE 4.2 Dichotomous (A) and pseudomonopodial (B) branching patterns in vascular plants.

synangium,w/bifidappendageleavesreducedroots lost

gametophytesubterranean,mycorrhizal

rootsunbranched,root hairs absent

— leptosporangium

— polycyclicsiphonostele

sporangiadorsiventral,transversely dehiscent

stem protoxylemexarch

root protoxylemendarch

— roots dichopodial

• siphonostele

stem protoxylemmesarch

apical meristem branches

divides equally equal

A dichotomous B

INDEPENDENT, LONG-LIVED SPOROPHYTELike all land plants, the vascular plants have a haplodiplontic

“alternation of generations,” with a haploid gametophyte anda diploid sporophyte. Unlike the liverworts, mosses, andhornworts, however, vascular plants have a dominant, free-living, photosynthetic, relatively persistent sporophyte generation (although, as discussed in Chapter 3, the hornwortshave a sporophyte that is photosynthetic and relatively long-persistent). In the vascular plants, the gametophyte generation is also (ancestrally) free-living and may be photosynthetic,but it is smaller (often much more so) and much shorter livedthan the sporophyte generation (although the gametophytemay be somewhat persistent). In all land plants, the sporophyteis initially attached to and nutritionally dependent upon thegametophyte. However, in the vascular plants, the sporophytesoon grows larger and becomes nutritionally independent, usually with the subsequent death of the gametophyte. (In seedplants the female gametophyte is attached to and nutritionallydependent upon the sporophyte; see Chapter 5.)

BRANCHED SPOROPHYTEThe sporophytic axes, or stems, of vascular plants are

different from those of liverworts, hornworts, and mosses inthat they are branched and bear multiple (not just one) sporangia. Extant vascular plants share this apomorphy with somefossil plants that are transitional between the “bryophytes” andthe tracheophytes. This more inclusive group, including fossiland extant taxa having branched sporophytic stems and multiple sporangia, has been called the Polysporangiomorpha(Kenrick and Crane 1997) or “polysporangiophytes.” The evenmore 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 wasdichotomous, in which the apical meristem splits into two,equal meristems, each of which grows independently more orless equally (Figure 4.2A). Later lineages evolved a modifiedgrowth pattern, called pseudomonopodial, which starts out

— roots

— endoderrnissieve elements (of phloem)

vascular tissuetracheary elements (of xylem)

— sclerenchyma— lignin, in lignified secondary cell walls— sporophyte branched, with multiple sporangia

— sporophyte independent, long-lived

t = extinct taxon

= extinct lineage

dichotomous, but then one branch becomes dominant andovertops the other, the latter appearing lateral (Figure 4.2B).Subsequent vascular plant lineages evolved monopodialgrowth. (See Euphyllophytes.)

The sporophytic stems of vascular plants function as supportive organs, bearing and usually elevating reproductiveorgans and leaves (see below). They also function as conductive organs, via vascular tissue, of water, minerals, and sugarsbetween roots, leaves, and reproductive organs. Structurally,stems can be distinguished from roots by several anatomicalfeatures (to be discussed).

LIGNIFIED SECONDARY CELL WALLSVascular plants possess a chemical known as lignin, which isa complex polymer of phenolic compounds. Lignin is incorporated into an additional cell wall layer, known as the secondary (2°) wall (Figure 4.3), which is found in certain,specialized cells of vascular plants. Secondary walls aresecreted to the outside of the plasma membrane (betweenthe plasma membrane and the primary cell wall) after theprimary wall has been secreted, which is also after the cellceases to elongate. Secondary cell walls are usually muchthicker than primary walls and, like primary walls, containcellulose. However, in secondary walls, lignin is secreted intothe space between the cellulose microfibrils, forming a sortof interbinding cement. Thus, lignin imparts significantstrength and rigidity to the cell wall.

In virtually all plant cells with secondary, lignified cell walls,there are holes in the secondary wall called pits (Figure 4.3).Pits commonly occur in pairs opposite the sites of numerousplasmodesmata in the primary cell wall. This group of plasmodesmata is called a primary pit field. Pits function in allowingchemical “communication” between cells, via the plasmodesmata of the primary pit field, during their development and differentiation. They may also have specialized functions inwater conducting cells (discussed later). Plant cells with secondary walls include sclerenchyma and tracheary elements

FIGURE 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.

(see later discussion).

74 CHAPTER 4 EVOLUTION AND DIVERSITy OF VASCULAR PLANTS UNIT II EVOLUTION AND DIVERSITY OF PLANTS 75

— roots dichopodial

— endodermjs— sieve elements (of phloem)

vascular tissue— tracheary elements (of xylem)scierenchymalignin, in lignified secondary cell wallssporophyte branched, with multiple sporangia

EuphyllophytaEuphyllophyes

1

INDEPENDENT, LONG-lIVED SPOROPHYTELike all land plants, the vascular plants have a haplodiplontic“alternation of generations,” with a haploid gametophyte anda diploid sporophyte. Unlike the liverworts, mosses, andhornworts, however, vascular plants have a dominant, free-living, photosynthetic, relatively persistent sporophyte generation (although, as discussed in Chapter 3, the hornwortshave a sporophyte that is photosynthetic and relatively long-persistent). In the vascular plants, the gametophyte generation is also (ancestrally) free-living and maybe photosynthetic,but it is smaller (often much more so) and much shorter livedthan the sporophyte generation (although the gametophytemay be somewhat persistent). In all land plants, the sporophyteis initially attached to and nutritionally dependent upon thegametophyte. However, in the vascular plants, the sporophytesoon grows larger and becomes nutritionally independent, usually with the subsequent death of the gametophyte. (In seedplants the female gametophyte is attached to and nutritionallydependent upon the sporophyte; see Chapter 5.)

BRANCHED SPOROPHYTEThe sporophytic axes, or stems, of vascular plants are

different from those of liverworts, homworts, and mosses inthat they are branched and bear multiple (not just one) sporangia. Extant vascular plants share this apomorphy with somefossil plants that are transitional between the “bryophytes” andthe tracheophytes. This more inclusive group, including fossiland extant taxa having branched sporophytic stems and multiple sporangia, has been called the Polysporangiomorpha(Kennck and Crane 1997) or “polysporangiophytes.” The evenmore 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 wasdichotomous, in which the apical meristem splits into two,equal meristems, each of which grows independently more orless equally (Figure 4.2A). Later lineages evolved a modifiedgrowth pattern, called pseudomonopodial, which starts out

dominantapical meristem branchdivides equally

B pseudomonopodial

dichotomous, but then one branch becomes dominant andovertops the other, the latter appearing lateral (Figure 4.2B).Subsequent vascular plant lineages evolved monopodialgrowth. (See Euphyllophytes.)

The sporophytic stems of vascular plants function as supportive organs, bearing and usually elevating reproductiveorgans and leaves (see below). They also function as conductive organs, via vascular tissue, of water, minerals, and sugarsbetween roots, leaves, and reproductive organs. Structurally,stems can be distinguished from roots by several anatomicalfeatures (to be discussed).

LIGNIFIED SECONDARY CELL WALLSVascular plants possess a chemical known as lignin, which isa complex polymer of phenolic compounds. Lignin is incorporated into an additional cell wall layer, known as the secondary (2°) wall (Figure 4.3), which is found in certain,specialized cells of vascular plants. Secondary walls aresecreted to the outside of the plasma membrane (betweenthe plasma membrane and the primary cell wall) after theprimary wall has been secreted, which is also after the cellceases to elongate. Secondary cell walls are usually muchthicker than primary walls and, like primary walls, containcellulose. However, in secondary walls, lignin is secreted intothe space between the cellulose microfibrils, forming a sortof interbinding cement. Thus, lignin imparts significantstrength and rigidity to the cell wall.

In 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 numerousplasmodesmata in the primary cell wall. This group of plasmodesmata is called a primary pit field. Pits function in allowingchemical “communication” between cells, via the plasmodesmata of the primary pit field, during their development and differentiation. They may also have specialized functions inwater conducting cells (discussed later). Plant cells with secondary walls include scierenchyma and tracheary elements(see later discussion).

LycopodiophytaLycophytes

Polysporangiomorphalpan.iracheophyta

TracheophytaTracheophytes (Vascular Plants)

Isoetopsida

.*—

branchesequal

iC,)

MonilophytaMonilophyes 1

Psilotopsida

IC,) 10=r,l.)

I•0‘)E Cd)ZC,)o, On©L0o_

© rJ)U ECo

apical meristemdivides equally

4’

A dichotomous

t leaf w/sterile &fertilesegments

spores w/ elaters

synangium,w/bifidappendageleavesreducedroots lost

FIGURE 4.2 Dichotomous (A) and pseudomonopodial (B) branching patterns in vascular plants.

Cd)

C,)

—

0a - 0

C,)

Ill

Ii IL

L__

L

L_Jwood

_ heterospory

leaves ligulate

— shoot withlycophylls

— sporangiadorsiventral,transversely dehiscent

- stem protoxylemexarch

• root protoxylemendarch

— leptosporangium

I — — — - gametophyte— - sporangiophore subterranean,

mycorrhizal— — leaves reduced,

I whorled — — rootsI — — stems ribbed unbranched,I with canals root hairs absent

L

— polycyclicsiphonostele

siphonostele

stem protoxylemmesarch

— roots

30 kB chloroplast DNA inversionshoot with euphyllssporangia terminal on lateral branches,longitudinally dehiscent

— root protoxylem exarch- roots monopodial

t = extinct taxon

= extinct lineage

— — sporophyte independent, long-lived

FIGURE 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.

r

FIGURE 4.3 Lignified secondary cell wall of specialized cells ofvascular plants. Note pit-pair adjacent to primary pit field.

SCLERENCHYMASclerenchyma (Gr. scieros, hard + enchy,na, infusion, in reference to the infusion of lignin in the secondary cell walls)consists of nonconductive cells that have a thick, lignifledsecondary cell wall, typically with pits, and that are dead atmaturity. There are two types of sclerenchyma (Figure 4.4):(1) fibers, which are long, very narrow cells with sharplytapering end walls; and (2) sclereids, which are isodiametricto irregular or branched in shape. Fibers function in mechanical support of various organs and tissues, sometimes makingup the bulk of the tissue. Fibers often occur in groups or bundles. They may be components of the xylem and/or phloemor may occur independently of vascular tissue. Sclereids mayalso function in structural support, but their role in some plantorgans is unclear; they may possibly help to deter herbivoryin some plants. The evolution of sclerenchyma, especiallyfibers, with lignified secondary cell walls, constitutes a majorplant adaptation enabling the structural support needed toattain greater stem height.

Another tissue type that functions in structural support iscollenchyina, consisting of live cells with unevenly thickened,pectic-rich, primary cell walls (see Chapter 10). Collenchymais found in many vascular plants, but is probably not anapomorphy for the group.

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 holesor perforations, with no intervening 1° or 2° wall betweenadjacent 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 compoundif 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).

SIEVE 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

76 CHAPTER 4 EVOLUTION AND DIVERSITY OF VASCULAR PLANTS

pit

UNIT II EVOLUTION AND DIVERSITY OF PLANTS 77

plasmamembrane

pit(pits of two adjacent

cells = pit-pair)

Cell #1

_____

p

A.middle lamella

primary cell wall(cellulosic)

secondary cell wall(lignified)

pnmary pit field

(collection

of severalplasmodesmata)

plasmodesmata

Cell#2 H:

perforation plate(compound)

I I

o

lignified o

cell wall

DC

tracheid vessels

FIGURE 4.5 Conductive cells of vascular plants: tracheary elements. A. Types of tracheary elements. B. Vessel.

pitB.

lignified secondarycell wall

B”

pit

A

_____

TFIACHEARY 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.) Vasculartissue was a major adaptive breakthrough in plant evolution;

more efficient conductivity allowed for the evolution of muchgreater plant height and diversity of form.

Tracheary elements are specialized cells that function inwater and mineral conduction. Tracheary elements are generally elongate cells, are dead at maturity, and have lignified2° cell walls (Figure 4.5A,B). They are joined end-to-end, forming a tube-like continuum. Tracheary elements are typically

associated with parenchyma and often some sclerenchyma ina common tissue known as xylem (Gr. xylo, wood, after thefact that wood is composed of secondary xylem). The function of tracheary elements is to conduct water and dissolved

essential mineral nutrients, generally from the roots to otherparts of the plant.

There are two types of tracheary elements: tracheidsand vessel members (Figure 4.5A). These differ with regardto the junction between adjacent end-to-end cells, whetherimpejforate or peiforate. Tracheids are imperforate, meaningthat water and mineral nutrients flow between adjacent cells

FIGURE 4.4 Sclerenchyma. A. Fiber cell. B. Sciereid cells.c.s cross-section.

r

FIGURE 4.3 Lignified secondary cell wall of specialized cells ofvascular plants. Note pit-pair adjacent to primary pit field.

SCLERENCHYMASclerenchyma (Gr. scieros, hard + enchy,na, infusion, in reference to the infusion of lignin in the secondary cell walls)consists of nonconductive cells that have a thick, lignifledsecondary cell wall, typically with pits, and that are dead atmaturity. There are two types of sclerenchyma (Figure 4.4):(1) fibers, which are long, very narrow cells with sharplytapering end walls; and (2) sclereids, which are isodiametricto irregular or branched in shape. Fibers function in mechanical support of various organs and tissues, sometimes makingup the bulk of the tissue. Fibers often occur in groups or bundles. They may be components of the xylem and/or phloemor may occur independently of vascular tissue. Sclereids mayalso function in structural support, but their role in some plantorgans is unclear; they may possibly help to deter herbivoryin some plants. The evolution of sclerenchyma, especiallyfibers, with lignified secondary cell walls, constitutes a majorplant adaptation enabling the structural support needed toattain greater stem height.

Another tissue type that functions in structural support iscollenchyina, consisting of live cells with unevenly thickened,pectic-rich, primary cell walls (see Chapter 10). Collenchymais found in many vascular plants, but is probably not anapomorphy for the group.

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 holesor perforations, with no intervening 1° or 2° wall betweenadjacent 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 compoundif 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).

SIEVE 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


pit


plasmamembrane

pit(pits of two adjacent

cells = pit-pair)

Cell #1

_____

p

A.middle lamella

primary cell wall(cellulosic)

secondary cell wall(lignified)

pnmary pit field

(collection

of severalplasmodesmata)

plasmodesmata

Cell#2 H:

perforation plate(compound)

I I

o

lignified o

cell wall

DC

tracheid vessels

FIGURE 4.5 Conductive cells of vascular plants: tracheary elements. A. Types of tracheary elements. B. Vessel.

pitB.

lignified secondarycell wall

B”

pit

A

_____

TFIACHEARY 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.) Vasculartissue was a major adaptive breakthrough in plant evolution;

more efficient conductivity allowed for the evolution of muchgreater plant height and diversity of form.

Tracheary elements are specialized cells that function inwater and mineral conduction. Tracheary elements are generally elongate cells, are dead at maturity, and have lignified2° cell walls (Figure 4.5A,B). They are joined end-to-end, forming a tube-like continuum. Tracheary elements are typically

associated with parenchyma and often some sclerenchyma ina common tissue known as xylem (Gr. xylo, wood, after thefact that wood is composed of secondary xylem). The function of tracheary elements is to conduct water and dissolved

essential mineral nutrients, generally from the roots to otherparts of the plant.

There are two types of tracheary elements: tracheidsand vessel members (Figure 4.5A). These differ with regardto the junction between adjacent end-to-end cells, whetherimpejforate or peiforate. Tracheids are imperforate, meaningthat water and mineral nutrients flow between adjacent cells

FIGURE 4.4 Sclerenchyma. A. Fiber cell. B. Sciereid cells.c.s cross-section.

F

parenchyma cells associated with them. Parenchyma cells associated with sieve cells are called albuininous cells; those associated with sieve tube members are called companion cells. Thetwo differ in that companion cells are derived from the sameparent cell as are sieve tube members, whereas albuminous cellsand sieve cells are usually derived from different parent cells.Both albuminous cells and companion cells function to load andunload sugars into the cavity of the sieve cells or sieve tubemembers. Sieve cells (and associated albuminous cells) are theancestral sugar-conducting cells and are found in all nonflowering vascular plants. Sieve tube members were derived from sievecells and are found only in flowering plants, the angiosperms(see Chapter 6).

Stems of the vascular plants typically have a consistent andcharacteristic spatial arrangement of xylem and phloem. Thisorganization of xylem and phloem in the stem is known as astele. In several groups of early vascular plant lineages, thestelar type is a protostele, with a central solid cylinder ofxylem and phloem (Figure 4.7). A modification of the protostele, in which xylem and phloem interdigitate, is called aplectostele (e.g., Figure 4.14A,B). The largely parenchymatous tissue between the epidermis and vascular tissue definesthe cortex. Protosteles, thought to be the most ancestral typeof stem vasculature, are found, e.g., in the rhyniophytes (seelater discussion).

epidermis

phloem

xylem

cortex

FIGURE 4.7 Example of a protostele, an ancestral vasculature ofvascular plants.

ENDODERMISAnother apparent apomorphy for the vascular plants is theoccurrence, 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 aCasparian strip, which is a band or ring of lignin andsuberin (chemically similar to lignin) that infiltrates the cellwall, oriented tangentially (along the two transverse walls) andaxially (vertically, along the two radial walls; Figure 4.8C).The Casparian strip acts as a water-impermeable material thatbinds to the plasma membrane of the endodermal cells.Because of the presence of the Casparian strip, absorbedwater and minerals that flow from the outside environment tothe central vascular tissue must flow through the plasmamembrane of the endodermal cells (as opposed to flowingthrough the intercellular spaces, i.e., between the cells or

through the cell wall). Because the plasma membrane maydifferentially control solute transfer, the endodermis (withCasparian strips) selectively controls which compounds areor are not absorbed by the plant; thus, toxic or unneededchemicals may be differentially excluded.

ROOTA major novelty in the evolution of vascular plants was thedifferentiation between stems and roots. Roots are specialized plant organs that function in anchorage and absorptionof water and minerals. Roots are found in all vascular plantsexcept for the Psilotales, Salviniales, and a few other specialized groups, all of which lost roots secondarily (see later discussion). Other fossil groups of vascular plants may havelacked roots; plants lacking roots generally have uniseriate(one cell thick), filamentous rhizoids (similar to those of “bryophytes”), which assume a similar absorptive function. Rootsconstituted a major adaptive advance in enabling much moreefficient water and mineral acquisition and conduction, permitting the evolution of plants in more extreme habitats.

Roots, like stems, develop by the formation of new cellswithin the actively growing apical meristem of the roottip, a region of continuous mitotic divisions (Figure 4.9B). Ata later growth stage and further up the root, these cell derivatives elongate significantly. This cell growth, which occursby considerable expansion both horizontally and vertically,pushes the apical meristem tissue downward. At an even laterstage and further up the root, the fully-grown cells differentiate into specialized cells. The ancestral apical meristem ofroots most likely consisted of a single, apical cell, a feature


sieve plate sieve plate(compound) (simple)


sieveareas

sieve plate(simple)

A.sieve cell L.._ sieve tube members .J

FIGURE 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 continuoushole in the 10 cell wall that is lined with a substance calledcallose, 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 otherorganelles but retain the endoplasmic reticulum, mitochondria, and plastids. Like tracheary elements, sieve elements areoriented end-to-end, forming a tubelike continuum. Sieveelements function by conducting dissolved sugars from asugar-rich “source” to a sugar-poor “sink” region of the plant.Source regions include the leaves, where sugars are synthesized during photosynthesis, or mature storage organs, wheresugars may be released by the hydrolysis of starch. Sinks caninclude actively dividing cells, developing storage organs, orreproductive organs such as flowers or fruits.

There are two types of sieve elements: sieve cells and sievetube members (Figure 4.6A). Sieve cells have only sieve areason both end and side walls. Sieve tube members have both sieve

areas and sieve plates (Figure 4.6B). Sieve plates consist of oneor more sieve areas at the end wall junction of two sieve tubemembers; the pores of a sieve plate, however, are significantlylarger than are those of sieve areas located on the side wall(Figure 4.6B,C). Both sieve cells and sieve tube members have

WAThR FLOW(outside to inside)

C endodermai cell(cross-section)

FIGURE 4.8 Endodermis of vascular plants. A,B. Equiserum rhizome. A. Rhizome cross-section, showing single layer of endodermalcells. B. Close-up of endodermal cells (in cross-section), showing Casparian strip thickenings. C. Diagram of Casparian strip, indicating

function.

F

parenchyma cells associated with them. Parenchyma cells associated with sieve cells are called albuininous cells; those associated with sieve tube members are called companion cells. Thetwo differ in that companion cells are derived from the sameparent cell as are sieve tube members, whereas albuminous cellsand sieve cells are usually derived from different parent cells.Both albuminous cells and companion cells function to load andunload sugars into the cavity of the sieve cells or sieve tubemembers. Sieve cells (and associated albuminous cells) are theancestral sugar-conducting cells and are found in all nonflowering vascular plants. Sieve tube members were derived from sievecells and are found only in flowering plants, the angiosperms(see Chapter 6).

Stems of the vascular plants typically have a consistent andcharacteristic spatial arrangement of xylem and phloem. Thisorganization of xylem and phloem in the stem is known as astele. In several groups of early vascular plant lineages, thestelar type is a protostele, with a central solid cylinder ofxylem and phloem (Figure 4.7). A modification of the protostele, in which xylem and phloem interdigitate, is called aplectostele (e.g., Figure 4.14A,B). The largely parenchymatous tissue between the epidermis and vascular tissue definesthe cortex. Protosteles, thought to be the most ancestral typeof stem vasculature, are found, e.g., in the rhyniophytes (seelater discussion).

epidermis

phloem

xylem

cortex

FIGURE 4.7 Example of a protostele, an ancestral vasculature ofvascular plants.

ENDODERMISAnother apparent apomorphy for the vascular plants is theoccurrence, 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 aCasparian strip, which is a band or ring of lignin andsuberin (chemically similar to lignin) that infiltrates the cellwall, oriented tangentially (along the two transverse walls) andaxially (vertically, along the two radial walls; Figure 4.8C).The Casparian strip acts as a water-impermeable material thatbinds to the plasma membrane of the endodermal cells.Because of the presence of the Casparian strip, absorbedwater and minerals that flow from the outside environment tothe central vascular tissue must flow through the plasmamembrane of the endodermal cells (as opposed to flowingthrough the intercellular spaces, i.e., between the cells or

through the cell wall). Because the plasma membrane maydifferentially control solute transfer, the endodermis (withCasparian strips) selectively controls which compounds areor are not absorbed by the plant; thus, toxic or unneededchemicals may be differentially excluded.

ROOTA major novelty in the evolution of vascular plants was thedifferentiation between stems and roots. Roots are specialized plant organs that function in anchorage and absorptionof water and minerals. Roots are found in all vascular plantsexcept for the Psilotales, Salviniales, and a few other specialized groups, all of which lost roots secondarily (see later discussion). Other fossil groups of vascular plants may havelacked roots; plants lacking roots generally have uniseriate(one cell thick), filamentous rhizoids (similar to those of “bryophytes”), which assume a similar absorptive function. Rootsconstituted a major adaptive advance in enabling much moreefficient water and mineral acquisition and conduction, permitting the evolution of plants in more extreme habitats.

Roots, like stems, develop by the formation of new cellswithin the actively growing apical meristem of the roottip, a region of continuous mitotic divisions (Figure 4.9B). Ata later growth stage and further up the root, these cell derivatives elongate significantly. This cell growth, which occursby considerable expansion both horizontally and vertically,pushes the apical meristem tissue downward. At an even laterstage and further up the root, the fully-grown cells differentiate into specialized cells. The ancestral apical meristem ofroots most likely consisted of a single, apical cell, a feature


sieve plate sieve plate(compound) (simple)


sieveareas

sieve plate(simple)

A.sieve cell L.._ sieve tube members .J

FIGURE 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 continuoushole in the 10 cell wall that is lined with a substance calledcallose, 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 otherorganelles but retain the endoplasmic reticulum, mitochondria, and plastids. Like tracheary elements, sieve elements areoriented end-to-end, forming a tubelike continuum. Sieveelements function by conducting dissolved sugars from asugar-rich “source” to a sugar-poor “sink” region of the plant.Source regions include the leaves, where sugars are synthesized during photosynthesis, or mature storage organs, wheresugars may be released by the hydrolysis of starch. Sinks caninclude actively dividing cells, developing storage organs, orreproductive organs such as flowers or fruits.

There are two types of sieve elements: sieve cells and sievetube members (Figure 4.6A). Sieve cells have only sieve areason both end and side walls. Sieve tube members have both sieve

areas and sieve plates (Figure 4.6B). Sieve plates consist of oneor more sieve areas at the end wall junction of two sieve tubemembers; the pores of a sieve plate, however, are significantlylarger than are those of sieve areas located on the side wall(Figure 4.6B,C). Both sieve cells and sieve tube members have

WAThR FLOW(outside to inside)

C endodermai cell(cross-section)

FIGURE 4.8 Endodermis of vascular plants. A,B. Equiserum rhizome. A. Rhizome cross-section, showing single layer of endodermalcells. B. Close-up of endodermal cells (in cross-section), showing Casparian strip thickenings. C. Diagram of Casparian strip, indicating

function.

found today in the Selaginellaceae of the lycophytes and allmonilophytes (discussed later). In the Lycopodiaceae,Isoetaceae, and seed plants (see Chapter 5), the apical meristem is complex, consisting of a group of continuously dividing cells.

Roots are characterized by several anatomical features.First, the apical meristem is covered on the outside by arootcap (also called a calyptra; Figure 4.9A,B); stems lacksuch a cell layer. The rootcap functions both to protect theroot apical meristem from mechanical damage as the rootgrows into the soil and to provide lubrication as the outer cellsslough off. Second, with the exception of the Psilotopsida(Psilotales and Ophioglossales), the epidermal cells away fromthe root tip develop hairlike extensions called root hairs(Figure 4.9A); these are absent from stems (although underground stems of the Psilotales bear rhizoids, which resembleroot hairs). Root hairs function to greatly increase the surfacearea available for water and mineral absorption. Third, rootsalways have a central vascular cylinder (Figure 4.9C,D). Asin stems, the mostly parenchymatous region between the vasculature and epidermis is called the cortex (Figure 4.9C); thecenter of the vascular cylinder, if vascular tissue is lacking, iscalled a pith. Fourth, the vascular cylinder of roots is surrounded by an endodermis with Casparian strips (Figure4.9D). As with some stems, the endodermis in roots selectively controls which chemicals are and are not absorbed bythe plant, functioning in selective absorption. (An undifferentiated layer internal to the endodermis, called the pericycle,is also typically present.) Fifth, roots generally have endogenous lateral roots (Figure 4.10), in which new lateral rootsoriginate by means of actively growing meristems, arising at

the pericycle or endodermis. Lateral roots penetrate the tissues of the cortex before exiting to the outside.

Numerous modifications of roots have evolved, most ofthese restricted to the flowering plants (see Chapter 9). Rootsof many, if not most, vascular plants have an interesting symbiotic interaction with various species of fungi; this association between the two is known as mycorrhizae. The fungalcomponent of mycorrhizae appears to aid the plant in bothincreasing overall surface area for water and mineral absorption and increasing the efficiency of selective mineral absorption, such as of phosphorus. The fungus benefits in obtainingphotosynthates (sugars and other nutrients) from the plant.

FIGURE 4.10 Root cross-section (LilIum sp.), showing endogenous lateral root, a characteristic of vascular plant roots.

LYCOPODIOPHYTALYCOPODIOPSIDA

Lycopodiaceae (5/300)ISOETOPSIDA

lsoetaceae (1/200)Selaginellaceae (1/700)

EUPHYLLOPHYTAMONILOPHYTA

EQUISETOPSIDAEquisetaceae (1/15)

PSILOTOPSIDAOphioglossaceae (4/55—80)Psilotaceae (2/17)

MARATTIOPSIDAMarattiaceae (6/80)

POLYPODIOPSIDAOsmundalesOsmundaceae (3/20)HymenophyllalesHymenophyllaceae (9/600)

VASCULAR PLANT DIVERSITY

A classification scheme of vascular plants, after Smith et al.

(2006) and Cantino et al. (2007), is seen in Table 4.1. Of the

tremendous diversity of vascular plants that have arisen since

their first appearance some 400 million years ago, only the

major lineages will be described here. These include the

rhyniophytes, known only from fossils, plus clades that have

modem-day descendants: the Lycopodiophyta (lycophytes)

and Euphyllophyta (euphyllophytes; Figure 4.1, Table 4.1).

See Bierhorst (1971) and Foster and Gifford (1974) for gen

eral information on vascular plant morphology.

Features that have been used to classify vascular plants

include sporophyte vegetative morphology (branching pat

tern, leaf type/shape/arrangementlvenation, stem and leaf

anatomy), life cycle and reproductive morphology (homo

spory/heterospory, sporophyll morphology, sporangium

shape/dehiscence/attachment, spore morphology), and game

tophyte morphology (whether green and photosynthetic or

nongreen and saprophytic or mycorrhizal). Spore morphol

ogy in particular has been useful in the classification of vas

cular plant groups. (See Chapter 12.) Features include spore

size, shape (e.g., reniform, tetrahedral, globose), sculpturing

patterns, and whether green (photosynthetic) or not. One major

spore feature is related to the laesura (plural laesurae), the

differentially thickened wall region corresponding to the tetrad

attachment scar on each of the four immature spores following

meiosis. Three basic spore types are recognized: 1) trilete


GleichenialesDipteridaceae (2/11)Gleicheniaceae (6/125)Matoniaceae (2/4)

SchizaealesAnemiaceae (1/100+)Lygodiaceae (1/25)Schizaeaceae (2/30)

SalvinialesMarsileaceae (3/75)Salviniaceae (2/16)

CyathealesCibotiaceae (1/11)Culcitaceae (1/2)Cyatheaceae (4/600÷)Dicksoniaceae (3/30)Loxomataceae (2/2)Metaxyaceae (1/2)Plagiogyriaceae (1/15)Thyrsopteridaceae (1/1)

spores, with a 3-branched laesura (Figure 4.1 1A); 2) monolete

spores, with a laesura that is linear and unbranched (Figure

4.1 1B); and 3) alete, lacking any evidence of a laesura.

RHYNIOPHYTES

Rhyniophytes are a paraphyletic assemblage that included

the first land plants with branched sporophytic axes, some of

which (but not all) also had vascular tissue. Rhyniophytes

include the genus Rhynia (Figure 4. l2A,B), a well-known

vascular plant from the early Devonian, ca. 416—369 million

years ago. Rhyniophyte sporophytes consisted of dichoto

mously branching axes bearing terminal sporangia that

dehisced longitudinally.


epidermis cortex

-

central vascular cylinder

FIGURE 4.9 Anatomy of the root, an apomorphy of the vascular plants. A. Root whole mount. B. Root longitudinal-section. C. Wholeroot cross-section. D. Close-up of central vascular cylinder, showing tissues.

xylem phloem

PolypodialesAspleniaceae (1—10/700+)Blechnaceae (9/200)Davalliaceae (4—5/65)Dennstaedtiaceae (11/170)Dryopteridaceae (40—45/1700)Lindsaeaceae (8/200)Lomariopsidaceae (4/70)Oleandraceae (1/40)Onocleaceae (4/5)Polypodiaceae (56/1200)Pteridaceae (50/950)Saccolomataceae (1/12)Tectariaceae (3—15/230)Thelypteridaceae (5—30/950)Woodsiaceae (15/700)

SPERMATOPHYTA (See Chapter 5)

TABLE 4.1 Taxonomic groups of Tracheophyta, vascular plants (minus those of Spermatophyta, seed plants). Classes, orders, and family

names after Smith et al. (2006). Higher groups (traditionally treated as phyla) after Cantino et al. (2007). Families in bold are described in

detail. Number of genera and species (often approximate), respectively, are indicated in parentheses, separated by slash mark.

FIGURE 4.11 MONILOPHYTA. Spore morphology. A. Spore

with trilete scar (Pentagramma triangularis, Pteridaceae). B. Spore

with monolete scar (Asplenium nidus, Aspleniaceae).

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OF VASCULAR PLANTS EVOLUTION AND DIVERSITY