Apex of the Expanded Base, Most Sporophyllous. Sporangia Are

I II EVOLUTION 88 CHAPTER 4 EVOLUTION AND DIVERSITY OF VASCULAR PLANTS UNIT AND DIVERSITY OF PLANTS 89

apex of the expanded base, most sporophyllous. Sporangia microsporophylls and megasporophylls, respectively),grouped are heterosporous, and are located on the adaxial side of leaf together in terminal strobili, the sporophylls in four rows, not (sporophyll) bases; megasporangia occur on outer leaves of much differentiated from vegetative leaves. Gametophytes a flush of growth, the megaspores large (50—300per sporan are endosporic. gium), trilete, spore sculpturing used in species identification; The Selaginellaceae are mostly distributed in tropical and microsporangia occur on inner leaves (or in alternating cycle warm regions, worldwide. Economic importance includes with megasporangia), the microspores small, monolete, very cultivated ornamentals and local medicinal plants. See Jermy numerous (up to I million per sporangium). Both sporangia (1990b) for general infonnation and Korall and Kenrick (2002, are marginally covered by a membrane, the “velum”, and are 2004) for phylogenetic analyses of the family. internally traversed by sterile strands (“trabeculae”); sporan The Selaginellaceae are distinctive in being erect to pros gia lack a precise dehiscence mechanism and open by tissue trate herbs, with dichotomously branched stems, sometimes degradation. Gametophytes are endosporic. Plants have CAM forming planar branch systems, the leaves microphyllous, photosynthesis. Air chambers occur in roots and leaves. spiral, either homomorphic or dimorphic and four-rowed The Isoetaceae have a worldwide distribution. Economic (with two upper rows of leaves smaller than the two lower importance is limited to some cultivated ornamentals. See rows), sporangia heterosporous, microsporangia and me Jermy (1990a) for general information and Rydin and Wik gasporangia borne in axils of ligulate sporophylls of terminal strom (2002) and Hoot et al. (2006) for phylogenetic and strobili; gametophytes endosporic. biogeographic studies of the family. The Isoetaceae are distinctive in being cormose to rhi EUPHYLLOPHITA—EUPHYLLOPHYTE S zo,natous plants with a basal rosette of microphyllous, ligu The sister group of the lycophytes are the euphyllophytes, late leaves, the leaves basally sheathing, apically linear to including all the other extant vascular plants (Figure 4.1). acicular, heterosporous, bearing adaxial megasporangia or Several major apomorphies that unite the euphyllophytes are microsporangia within sheathing leaf base. mentioned here. First, in contrast to the lycophytes, the roots [Note that Isoetes and Isoetaceae can be spelled Isoëtes and of euphyllophytes are monopodial, meaning that they do not Isoëtaceae, respectively, the umlaut indicating that the “e” is dichtomously branch at the apical meristem. Lateral roots a separate vowel and should be pronounced, not part of the arise endogenously from either the endodermis (in monilo diphthong “oe.” See Botanical Names, in Chapter 16.] phytes) or the pericycle (in spermatophytes, Chapter 5). Second, the roots of euphyllophytes have an exarch protoxy Selaginellaceae—Spike-Moss family (Latin Selago, a moss- 1cm, in which the protoxylem is positioned outer to the like plant of the Scrophulariaceae + ella, diminutive). 1 genus metaxylem (Figure 4.20A,B); lycophyte roots have an endarch (Selaginella)Ica. 700 species. (Figure 4.19) protoxylem. Third, the ancestral sporangia in euphyllophytes The Selaginellaceae consist of perennial herbs, rarely tree- were terminal in position with longitudinal deshiscence like, some species xeric-adapted “resurrection plants” (e.g., S. (although these features have undergone considerable modi lepidophylla). The roots are adventitious and dichotomously fication in some groups). Fourth, extant euphyllophytes have branching (dichopodial), in some taxa arising from branch a molecular apomorphy, a 30-kilobase inversion located in junctions and growing downward (formerly interpreted as leaf the large single-copy region of chloroplast DNA (Figure less stems, termed “rhizophores”). The stems are generally di 4.20C; see Figure 14.4 of Chapter 14). Fifth, the leaves of chotomouslybranching, with erect, cespitose,prostrate/repand, euphyllophytes, termed euphylls, are distinctive. (Note that or climbing habit; the stems may be pseudomonopodial or euphyll is essentially synonymous with megaphyll, a more sympodial, forming a very flattened, “fern-like” branch system traditional term.) Euphylls, like lycophylls, are generally in some species, somewith aerial tubers; the stem vasculatureis dorsiventral organs, functioning as the primary organ of pho a protostele (exarchor mesarch). The leaves are simple, sessile, tosynthesis. Euphylls are different in being associated with a spiral, with a single midrib (microphyllous), adaxially ligulate, leaf gap, a region of nonvascular, parenchyma tissue inter - blades generally small, either homomorphic (“isophyllous”) rupting the vasculature of the stem, and in (usually) having or, in some prostrate taxa, dimorphic (“anisophyllous”) and in more than one vein per leaf (Figure 4.20F). Euphylls gener FIGURE 4.19 LYCOPODIOPHYTA—ISOETOPSIDA. Selaginellaceae. A. Selaginella bigelovii, with isomorphic leaves. B—K. four rows, leaves of two upper (dorsal) rows smaller, those of ally have a highly branched system of veins, between which Selagineila spp. B. Shoot with dimorphic leaves. C. Close-up of vegetative shoot, showing 2 rows of large and 2 rows of small leaves. the other two lower (ventral or lateral) rows larger. Sporangia is the mesophyll, the chloroplast-containing tissue. (Note D. Close-up of ligule, adaxial side of leaf base. E. Cone (strobilus), an axis bearing microsporophylls and megasporophylls. F. Close-up of megasporangium (sporophylls removed). G. Adaxial view of microsporophyll and megasporophyll with axillary are heterosporous; microsporangia (bearing numerous, small, that in a few euphyllous taxa, the veins have become second microsporangium and microsporangiumand megasporangium,respectively.H. Strobilus longitudinal-section,showing sporophylls, megasporangia,and microspo trilete microspores) and megasporangia (bearing usually four arily reduced to a single mid-vein, an evolutionary reversal.) rangia. I. Close-up of microsporangium, containing numerous microspores. J. Close-up of megasporangium, containing 4 megaspores. [numerous], large, trilete. gen. ornamented megaspores) oc In addition, euphylls, in contrast to lycophylls, grow by means K. Dispersed microspores and megaspores, the latter showing trilete mark. Note great size difference. cur on short stalks in the axils of ligulate sporophylls (termed of either marginal or apical meristems. I II EVOLUTION 88 CHAPTER 4 EVOLUTION AND DIVERSITY OF VASCULAR PLANTS UNIT AND DIVERSITY OF PLANTS 89

FIGURE

al.

(1975),

ycJ2

insert

4.20

CHAPTER -

psbM).

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permission

system

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EVOLUTION

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the

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evolution

outermost

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the

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were

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DIVERSITY

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Chapter

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2001a)

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DIVERSITY

the

composed

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the

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one

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the

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they

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morphological

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into

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4.2

EVOLUTION

lower region

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Inversion

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PLANTS

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Root

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Institution,

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DIVERSITY

Research

and

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AND

reconstruction,

Euphyllophyta.

planation

Close-up

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Paleontological

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from

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EVOLUTION

Apomorphies

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Psilophyton

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4.20

CHAPTER

insert

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FIGURE

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cylinder; et vascular section. FIGURE

E—G.

plant

CHAPTER

4.21

leaf

P1ectranthjs

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Shoot

[Coleus

EVOLUTION

longitudinaI-secjo0

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DIVERSITY

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Siphonosteles

phyt, 4.22A—D).

of A—C.

and

monilophyte

4.22B,D; surrounded

apomorphy phloem

inside.

plant) Siphonostele

close-up

modified

this

restricted

elements

mesarch

protoxylem

Figure

Siphonosteles

(amphiphloic

have

of in

Dictyostele,

dissected,

(L.

vasculature of

reference

types. first

monilo,

4.22A)

the

some central,

position

the

mature

outer

monilophytes

(unlike

lobes

called

Ectophloic

monilophytes.

necklace showing

dissected layer

parenchymatous siphonostele

have

(Figure near this

by a

that

dictyostele,

anatomy

of the

the

UNIT

evidently an

mesarch

siphonostele,

of or

phloem

amphiphloic

4.22E),

middle

xylem.

outer

string

some

that

protoxylem,

EVOLUTION

with

The tophytes, EQJISETOPSIDA—HORSETAILS siphonostele. As group Carboniferous

4.23), were molecular

monilophytes

in (Figure Rothwell

four

The

phloem

particular,

Equisetopsida

with

large

that

another

equisetophytes

4.1). apomorphy which

sphenophytes,

and

diverged systematic

woody

the

outside

AND

However,

Nixon

but period,

fossil Adiantuin

are

contributor

lycophytes, phloem

xylem

trees. [Equisetales],

for cited

early

DIVERSITY

2006). leaf

various

data

studies xylem.

approximately the

are or incorporation Among

rhizome,

here

gap

monilophytes in

sphenopsids,

may

united to the

positions,

place some

(Figure

Amphiphloic

coal

evolution commonly

these

yield

by the

amphiphloic

equisetophytes

deposits.

300

was

4.1):

of (ferns).

PLANTS

different

several equisetophytes

none

are

morphological

of million

Calamites

called

(1) siphonostele,

vascular

well-supported

monophyletic

Most

ridged

apomorphies,

siphonostele.

results

the

years

equise

(Figure

current

plants.

stems

with

ago,

and,

(see

the

the the 93 (see with and, ago, pith in stems plants. of current equise (Figure years the siphonostele. results apomorphies, Most ridged monophyletic well-supported vascular a siphonostele, called (1) Calamites of million PLANTS morphological are none equisetophytes several (ferns). different of was 4.1): 300 OF equisetophytes deposits. amphiphloic the by an yield commonly these evolution Amphiphloic coal some (Figure place B. positions, the to united gap sphenopsids, may monilophytes

in C rhizome, here or Among incorporation are leaf DIVERSITY the xylem. approximately studies data various 2006). of early for cited xylem [Equisetales], in phloem trees. lycophytes, AND contributor Adiantum are fossil period, but cortex Nixon However, outside D. to the woody diverged systematic

and / sphenophytes, epidermis apomorphy 4.1). which equisetophytes another that an large of with Equisetopsida phloem particular, The EVOLUTION siphonostele. four (Figure monilophytes Rothwell in were EQJBSETOPSWA—HORSETMLS molecular As Carboniferous group 4.23), The tophytes, with II protoxylem, A in or that some outer string is xylem. middle amphiphloic 4.22E), phloem UNIT siphonostele, of or mesarch an evidently of the the anatOmy dictyostele, that by a of gap (Figure near

this B siphonostele have dissected parenchymatous layer xylem or showing monilophytes. necklace leaf a to Ectophloic called lobes phloem (unlike A. monilophytes pith outer mature central, position the some the 4.22A) an a monilo, in types. to first vasculature in of reference epidermis cortex by (L. Dictyostele, dissected, of in have (amphiphloic if Siphonosteles C. Figure is protoxylem mesarch elements restricted close-up modified this is Siphonostele inside. plant) is phloem apomorphy surrounded 4.22B,D; monilophyte and of is A—C. 4.22A—D). of Siphonosteles phyt, xylem; taxa) rhizome, tracheary siphonostele, Figure of Gr. layer outside 4.22 xylem secondarily that + protoxylem anatomical fossil to 4.22C). of plants. (Figure patch inner derivation gap stem a beads Polypodium

ring A xylem phloem leaf FIGURE fossil of E. second The of the meaning phloem become related (ectophloic pith a solenostele, Figure and a of vein longitudinal Cross-section shoot xylem phloem of H. Diagram D. vasculature. cell. and PLANTS mestem pmordia, bud apical and VASCULAR single, leaf OF showing meristem, apical DIVERSITY Equjsetum A—C. AND

complex :fr Note shoot. EVOLUTION

longitudjna1cj 00 I 4 [Coleus] Shoot i.leaIprimordmrnU JI

(Ligustru, 1’ A—G. crran(hj -IAPTER f