OF VASCULAR PLANTS EVOLUTION AND DIVERSITY
Transcript of 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
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—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
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o
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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
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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
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.
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
78 CHAPTER 4 EVOLUTION AND DIVERSITY OF VASCULAR PLANTS
sieve plate sieve plate(compound) (simple)
UNIT II EVOLUTION AND DIVERSITY OF PLANTS 79
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
78 CHAPTER 4 EVOLUTION AND DIVERSITY OF VASCULAR PLANTS
sieve plate sieve plate(compound) (simple)
UNIT II EVOLUTION AND DIVERSITY OF PLANTS 79
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
UNIT II EVOLUTION AND DIVERSITY OF PLANTS 81
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.
80 CHAPTER 4 EVOLUTION AND DIVERSITY OF VASCULAR PLANTS
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).