EVOLUTION and Diversity of GREEN and LAND PLANTS UNIT II EVOLUTION and DIVERSITY of PLANTS 59

Home , Antheridium, Cell wall, Chlorophyceae, Viridiplantae

THE algae” isms The proteins, chromosomes Like Chiorobionta, membrane-bound land and tubules, plants tions, is beta-l,4 but One in (Figure Cellulose meshwork. intertwined doi: 02010 scopic THE DIVERSITY EMBRYOPHYTA-

EVOLUTION important chemical Several 10. Mosses Liverworts one golgi 1016/B978-0- plant Elsevier possible that green GREEN all or actually plus fiber-like GREEN position in 3.2A). that mitochondria, apomorphies, includes bodies. is eukaryotes, Inc. Viridiplantae apomorphies which

GREEN The bond plants, secreted the to are into OF 12-374380-0.00003-9 All novelty PLANTS composed arose are realize rights Cellulose, sorted function land NONVASCULAR (=3-1,4-g1ucopyranoside). position Although units organelles, larger the what a LAND reserved. monophyletic PLANTS formally before outside plants for glucose during that that the an have called unite will fibril of of this results PLANTS endoplasmic like some the we plants cellulose Viridiplantae linear the not traditionally or cell including the group called sugar microfibrils interrelationships normally units, starch, embryophytes plasma be in of division Viridiplantae group LAND colonized chains a the covered is units very the is forming a a

AND evolutionary is membrane of to reticulum, nucleus cellulosic PLANTS associate by been different Viridiplantae are of This a eukaryotic have impart mitosis), in that the polysaccharide, DNA bonded called detail (Figure slight a (Figure land. (containing of cells

are AND supportive rigidity molecule. with as bound cell the vesicles, innova here, “green micro change further organ micro in non— 3.1). with 3.1). wall land

3 the 55 65 62 62 59 or to to it

LAND 55 tion the REVIEW evolution REFERENCES EXERCISES cell self-supporting groups; apomorphy alone, teristics Chapter specialized tioned. conversion nificance light phyll from “algae,” like membranes, glucopyranoside). units green chemically a Hornworts Polysporangiophytes/Pan-Tracheophyta storage Perhaps cells, aggregations of wall capture; those

DIVERSITY (= a, as “algae” Chloroplasts a in the in polysaccharide) of 1, QjESTIONS acting cellulosic it of as product constitutes any chloroplasts the (1) of - traditionally type of may former for that more bonded (2) organelles most FOR containing case, primary light in shoot as having of the (see are have a complex other true Thus, a FURTHER chloroplast of in

PLANTS cell its energy stacked pond sort Viridiplantae Figure the systems. which an in evolved adaptive thylakoids, apomorphy are starch, defined organisms, wall functioning in of chlorophyll Viridiplantae, all apomorphy the or one which 3.2B,C); cellular types to green acts into was tide STUDY much of chemical It (Figure alpha-1,4 a significance “plants”; as is grana, the polymer of a the pooi the plus plants, such for exoskeleton. not preamble an and growth, in earlier, b major glucose chlorophyll-containing for the the accessory 3.2). in to as clear one (3) photosynthesis, which energy, their addition green the the Viridiplantae from position of giant manufacturing defining seems As constituting or particularly if glucose red to molecules Viridiplantae adaptive are discussed OF plants, a more filamentous The the sequoia is pigment and cellulosic to clear. pancake- unques charac (ci-l,4- further chloro evolu brown differ other sugar sig is 69 72 71 70 72 the are an of in as or in a chloroplasts. reinhardtii,

FIGURE

and

FIGURE

Mishler

CHAPTER

3.2

3.1 Chiorophytes

unicellular

“Green

al.

Diagram

A. Cladogram

(1994).

cellulose

Elodea,

- IE

“green

Important chlorophyll

Algae”

in chloroplast

whole

EVOLUTION

cell

the

alga,”

thylakoids

green

wall

leaf

apomorphies

Viridiplantae

showing

true

structure

(may

plants

(chlorophyll

paraphyletic

face starch

stacked

have

AND

(Viridiplantae

granum

view,

discussed

storage

evolved

green

DIVERSITY

grana

showing

plants,

ancestral)

chioroplast.

compound

[Chiorobionta]

earlier

the

group)

Chlorobionta),

apomorphies

showing

text

thus

(Photo

are

thylakoids

listed

not

GREEN

Streptophytes

Unique

courtesy

modified

sporophyte/embryo

synapomorphy beside

the —

and

Green

AND

green

Viridiplantae:

thick

cuticle

from

grana.

Rick

Charophytes

plant

hash

LAND

Bremer

Bizzoco.)

Plants

for

parenchyma

.thylakoid

marks.

Electron

Chlorobionta chloroplast

(alternation

(1985),

PLANTS

cellulosic

antheridium

micrograph

Mishler

Embryophytes

archegonium

Land

of cell

features granum alone)

generations)

wall

and -i’

of ‘i•:

Plants

Churchill

Chlamydomonas

and

-7:-’

green

(1985), —

plant

: ..- -

Rick Eucalyptus spiral

the FIGURE data cellular were

groups: or are plant quotation Viridiplantae, variety motile (Figure 3.3B),

Spores

The

Streptophyceae (n)

intracellular a

Bizzoco.)

imply

chloroplasts. paraphyletic

modified

streptophytes.

motile

filaments Viridiplantae

prokaryote

3.3A)

chiorophytes, /

mitosis

3.3

marks)

morphological

that

trees

Jneiosis

//Z

and B.

consisting

with

Examples

from

chioroplasts

cohabitation

Z3ote

Ulva,

(Figure

have Below:

nonmotile

group

and (Figure

inside

those

as “Green

without

a HAPLONTIC

this

FIGURE

Multicelled are

thalloid

HAPLOID reproductive

(which 3.3D).

Isogamy Chlorophyceae,

whole

forms. 3.1).

same

eukaryotic that

defined all

non—land

(2n)

colonies found

algae”

flagella,

chiorophytes

form.

The

Many

evolved

3.4 is

type

Stage

are

These

(n)

why

independently

conjugation

traditional

C. occur

classified

plant

(Figure

Haplontic

the

have

cell

thalloid

the

Volvox,

fertilization

via

include chioroplast.

green

and

(see

primarily

name Viridiplantae.

flagellated

and in

endosymbiosis,

3.3C),

Gamete

streptophytes,

forms “green a

stage, life Chapter

•;

the

plants (n)

colonial

tremendous

single

living,

two

cycles

placed non—land

and

UNIT

showing

Recent (Figure aquatic

Gamete

algae”

motile

today •;

sister

(n)

1).

cells

non-

uni form.

some

Chlansydomonas

II +

and

cells

habitats. algae” Spiro soil

Fertilization seems gametes is ing haploid spores, EVOLUTION ing plants, (Figure —

the

The

free-living,

Within

Spores

mating

what

(n)

gyra,

(or

green

inhabit

primitive

diploid

each

3.4A).

individual,

even

that

have is

strains

the

\ C

plants.

few

reinhardtii, filamentous

termed

least

occurs

of nze,oszs

are

streptophyte

fresh then

been

(2n)

which

AND

innovations

and

type

one

“isomorphic,”

snow!)

divides

which

Jsogamy.

and zygote

nonmotile

the

haplontic

may

form.

phase a

DIVERSITY union

HAPLONTIC

production marine

Multicelled

unicellular

HAPLOJD

produces

green

Oogamy lineage germinate

(Figure

Above:

Zygote

evolved

of zygotes.

(2n)

meiosis

Oogamy.

waters

(or

two

other

that

their

plant

Stage

vegetative

3.4A).

that “haplobiontic”)

(n) of

form.

is,

and

flagellate, that and

these

terrestrial

life

gave

sexual

that

PLANTS

fertilization

develop

gametes, (Photo The

form

some

history.

may

gametes,

form,

rise

look

zygote,

reproduction

four .

live

haploid

courtesy

to have

but

into

with

Egg

identical.

(n)

life

complet

the

“Green

haploid

result

which

moist

large, Sperm or

cycle

been

land

new

(n)

ii chloroplasts. reinhardtjj,

FIGURE

and

FIGURE

Mishler -

3.2

3.1 .

CHAPTER

Chiorophytes

unicellular

al.

“Green

Diagram

Cladogram

(1994).

cellulose

Elodea,

“green

Important

chlorophyll

Algae”

chloroplast

whole

EVOLUTION

cell the

alga,”

thylakoids

—j

green

wall

leaf

apomorphies

showing

Viridiplantae

true

structure

(may

plants

(chlorophyll

face

starch paraphyletic

stacked

have

granum

(Viridiplantae

AND

view,

discussed

storage

green

evolved

showing grana

of DIVERSITY

plants,

chioroplast.

ancestral)

compound

[Chiorobionta]

earlier

the

J::

group) Chlorobionta),

apomorphies

showing

text

thus

(Photo

are

thylakoids

listed

not

GREEN

Unique

Streptophytes

courtesy

modified

beside

synapomorphy sporophyte/embryo

the —

and

Viridiplantae:

green

Green

AND

thick

from cuticle grana.

Rick

Charophytes

hash plant

LAND

Bremer

Bizzoco.)

Plants for

parenchyma

marks.

Electron

hyIakoid_),

Chiorobionta chioroplast

(alternation

(1985), cellulosic

PLANTS

antheridjum

micrograph

Mishler -

Embryophytes

archegonium cell

Land

granum

features

alone)

.: -.

generations)

wall

- and

of -

‘b

Plants

Chlamydornonas

Churchill

and .,.“

green

(1985), _-

plant rd4 ..,

Rick

Eucalyptus FIGURE spiral data the were cellular groups:

or quotation plant are Viridiplantae,

variety motile 3.3B), (Figure

Spores

The

Streptophyceae

intracellular

Bizzoco.)

imply

chloroplasts.

paraphyletic

modified

©©

streptophytes.

motile

filaments

Viridiplantae

prokaryote

3.3A)

chiorophytes,

3.3 mitosis

marks)

morphological

that

trees

meiosis

B. and

consisting

with

Examples

from

chioroplasts

cohabitation

Ulva,

(Figure have Below:

nonmotile

group

and (Figure

inside

those

“Green

without a

HAPLONTIC

this

FIGURE

Multicelled are

thalloid

reproductive

HAPLOH) a

of of

(which 3.3D).

Chlorophyceae, Isogamy

whole

forms. 3.1).

same

that

eukaryotic Zygote defined all

(Adult)

non—land

(2n) found colonies

algae”

flagella,

chiorophytes

form.

The

Many evolved

3.4

type

Stage

are

These

(n)

why

independently

as conjugation

traditional

occur

classified

plant

(Figure

Haplontic

the

have

cell

thalloid the

the

Volvox,

fertilization via

include chloroplast.

green

and

(see

primarily

Viridiplantae.

name

flagellated

and in

endosymbiosis,

3.3C),

Gamete

streptophytes,

a “green forms

stage,

Chapter

life

the

plants

(n)

colonial

tremendous

is single

living,

two

cycles

placed

non—land

and

UNIT

showing

Recent

aquatic

(Figure

Gamete

algae”

motile

today

sister

(n)

1).

cells

non- VI

form. uni

some

Chiarnydornonas

and

cells

Spirogyra, algae” habitats. soil Fertilization gametes seems spores, ing EVOLUTIONANDDIVERSITYOFPLANT5 haploid is

ing

plants, (Figure —

the

The

free-living, Within

Spores

mating

what

(n)

(or

green

inhabit

primitive

diploid

each

3.4A).

individual,

even

that

have

strains

the

plants.

few

reinhardtii, filamentous

termed

least

occurs

meiosis

are

streptophyte

fresh then

been

(2n)

which

innovations

and

type

one

“isomorphic,”

snow!)

divides

which

and Isogamy.

zygote nonmotile

the

haplontic

may

form.

phase

union

HAPLONTIC

production marine

Multicelled

unicellular

HAPLOID

produces

green

germinate

Oogamy lineage

(Figure

Above:

Zygote

evolved

zygotes. of

(2n)

meiosis

Oogamy.

waters

(or

two

other

that

their

plant

Stage

vegetative

that “haplobiontic”) 3.4A).

(n)

form.

of more

is,

and

flagellate, and

that

these

terrestrial

life

gave

sexual

that

develop

fertilization

The (Photo

gametes,

form

some

history.

may

gametes,

form,

rise

look

zygote,

// reproduction

four

live

haploid

courtesy

have

but

into

with

Egg

identical.

(n) life

complet

the

“Green

haploid

result

which

moist

large,

Sperm

cycle

been

land

new

(n)

of 58 CHAPTER 3 EVOLUTION AND DIVERSiTY OF GREEN AND LAND PLANTS UNIT II EVOLUTION AND DIVERSITY OF PLANTS 59

“preadaptations” to survival on land. First of these was the efficient or rapid transport of solutes, including regulatory Embryophyta — land plants evolution of oogamy, a type of sexual reproduction in which and growth-mediating compounds, such as hormones. 1 Pan-Tracheophytal one gamete, the egg, becomes larger and nonflagellate; the Members of the Charales, such as the genera Chara and 0 Polysporangiophytes - other gamete is, by default, called a sperm cell (Figure 3.4B). Nitella, are perhaps the closest living relatives to the land Oogamy is found in all land plants but independently evolved plants. These fresh water, aquatic organisms have a haplontic Tracheophytes — vascular plants in many other groups, including many other “algae” and in life cycle, and consist of a central axis bearing whorls of lat t t the animals. eral branches (Figure 3.5D) or (if small) “leaves” on the hap gametophytic pseudo-elaters Several other apomorphies of and within the Viridiplantae bid body. Some Charales are capable of precipitating calcium leaves in sporangium include ultrastructural specializations of flagella and some carbonate as an outer layer of the plant body (accounting for gametophyte columella in features of biochemistry. Although these have been valuable the common names “brittleworts” or “stoneworts”). Members leafy (in some) sporangium sporophyte branched in elucidating phylogenetic relationships, their adaptive of the Charales grow by means of a single apical cell, similar with multiple sporangia significance is unclear, and they will not be considered to that of some land plants and representing a possible elaters in syna sporangium further here. pomorphy with them. However, the Charales differ from An apomorphy for the charophytes, a dade within the land plants in lacking true parenchyma (see later discussion). oil bodies sporophyte photosynthetic, nutritionally independent streptophytes that includes Coleochaete (Figure 3.5B), The Charales have specialized male and female gametangia, Charales (Figure 3.5C—E),and the land plants (Figure 3.1), termed antheridia and oogonia (Figure 3.5C,D). The oogonia sporophyte axis are plasmodesmata. Plasmodesmata are essentially pores in are distinctive in having a spirally arranged group of outer aerial the primary (10) cell wall through which membranes traverse “tube” cells (Figure 3.5D); fossilized casts of oogonia retain stomates between cells, allowing for transfer of compounds between the outline of these tube cells (Figure 3.5E). Oogonia and archegonium t = extinct cells (Figure 3.5A). Plasmodesmata may function in more antheridia of the Charales resemble the archegonia and antheridium parenchyma cuticle sporophyte/embryo (alternation of generations)

FIGURE 3.6 One hypothesis of relationships of the land plants(Embryophyta), withmajorapomorphiesindicated.AfterQiuet al. (2007), some apomorphies after Bremer (1985); Mishler and Churchill (1985); Mishler et al. (1994).

antheridia of land plants (see later discussion) in having an green plants to survive and reproduce in the absence of a sur outer layer of sterile cells, but the gametangia of the two rounding water medium. groups are generally thought not to be directly homologous One major innovation of land plants was the evolution of because of major differences in structure and development. the embryo and sporophyte (Figure 3.6). The sporophyte is However, members of the Charales retain the egg and zygote a separate diploid (2n) phase in the life cycle of all land (although the latter only briefly) on the plant body. This plants. The corresponding haploid, gamete-producing part of retention of egg and zygote on the haploid body may repre the life cycle is the gametophyte. The life cycle of land sent a transition to their permanent retention on the gameto plants, having both a haploid gametophyte and a diploid phyte of land plants (see later discussion). sporophyte, is an example of a haplodiplontic (also called “diplobiontic”) life cycle, commonly called alternation of generations (Figure 3.7). Note that alternation of generations EMBRYOPHYTA- LAND PLANTS does not necessarily mean that the two phases occur at differ ent points in time; at any given time, both phases may occur The Embryophyta, or embryophytes (commonly known as in a population. land plants), are a monophyletic assemblage within the green The sporophyte can be viewed as forming from the zygote plants (Figures 3.1, 3.6). The first colonization of plants on by the delay of meiosis and spore production. Instead of mei land during the Silurian period, ca. 400 million years ago, osis, the zygote undergoes numerous mitotic divisions, which FIGURE 3.5 A. Diagram of plasmodesmatain cellulosiccell wall, an apomorphy some of green plants, including the land plants. was concomitant with the evolution of several important fea result in the development of a separate entity. The embryo is B. Coleochaete sp., a close relative to the embryophytes. (Photo courtesy of Linda Graham.) C—E.Charales. C. Nitella sp., oogonia and tures. These shared, evolutionary novelties (Figure 3.6) con defined as an immature sporophyte that is attached to or sur antheridia.D. Chara sp., oogonium and antheridium. Note spiral tube cells of oogonia. E. Tectochara helicteres, a fossil oogonium from the adaptations that enabled formerly aquatic rounded by the gametophyte. In many land plants, such as the Eocene, showing remnants of spiral tube cells. stituted major 58 CHAPTER 3 EVOLUTION AND DIVERSiTY OF GREEN AND LAND PLANTS UNIT II EVOLUTION AND DIVERSITY OF PLANTS 59

antheridia of land plants (see later discussion) in having an green plants to survive and reproduce in the absence of a sur outer layer of sterile cells, but the gametangia of the two rounding water medium. groups are generally thought not to be directly homologous One major innovation of land plants was the evolution of because of major differences in structure and development. the embryo and sporophyte (Figure 3.6). The sporophyte is However, members of the Charales retain the egg and zygote a separate diploid (2n) phase in the life cycle of all land (although the latter only briefly) on the plant body. This plants. The corresponding haploid, gamete-producing part of retention of egg and zygote on the haploid body may repre the life cycle is the gametophyte. The life cycle of land sent a transition to their permanent retention on the gameto plants, having both a haploid gametophyte and a diploid phyte of land plants (see later discussion). sporophyte, is an example of a haplodiplontic (also called “diplobiontic”) life cycle, commonly called alternation of generations (Figure 3.7). Note that alternation of generations EMBRYOPHYTA- LAND PLANTS does not necessarily mean that the two phases occur at differ ent points in time; at any given time, both phases may occur The Embryophyta, or embryophytes (commonly known as in a population. land plants), are a monophyletic assemblage within the green The sporophyte can be viewed as forming from the zygote plants (Figures 3.1, 3.6). The first colonization of plants on by the delay of meiosis and spore production. Instead of mei land during the Silurian period, ca. 400 million years ago, osis, the zygote undergoes numerous mitotic divisions, which FIGURE 3.5 A. Diagram of plasmodesmatain cellulosiccell wall, an apomorphy some of green plants, including the land plants. was concomitant with the evolution of several important fea result in the development of a separate entity. The embryo is B. Coleochaete sp., a close relative to the embryophytes. (Photo courtesy of Linda Graham.) C—E.Charales. C. Nitella sp., oogonia and tures. These shared, evolutionary novelties (Figure 3.6) con defined as an immature sporophyte that is attached to or sur antheridia.D. Chara sp., oogonium and antheridium. Note spiral tube cells of oogonia. E. Tectochara helicteres, a fossil oogonium from the adaptations that enabled formerly aquatic rounded by the gametophyte. In many land plants, such as the Eocene, showing remnants of spiral tube cells. stituted major 60 CHAPTER 3 EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS UNIT II EVOLUTION AND DIVERSITY OF PLANTS 61

Sporophyte Body protection of inner tissue and to inhibit water metabolic activities such as respiration, photosynthesis, lat HAPLODIPLONTIC mechanical LIFE CYCLE loss. The cuticle consists of a thin, homogeneous, transparent eral transport, storage, and regeneration/wound healing. mitosis, growth, & differentiation mitosis, growth, & differentiation (“Alternation of Generations”) layer of cutin, a polymer of fatty acids, and functions as a Parenchymacells may further differentiate into other special z preventing excess water loss. Cutin also impregnates ized cell types. It is not clear if the evolution of both apical Embryo sealant, Sporangium the outer cellulosic cell walls of epidermal cells; these are growth and true parenchyma is an apomorphy for the land /% known as a “cutinized” cell wall. The adaptive advantage of plants alone, as shown here (Figure 3.6). Both may be inter mitosis, growth, & dfferenuation initosis, growth, & differentiation cutin and the cuticle is obvious: prevention of desiccation preted to occur in certain closely related green plants, includ outside the ancestral water medium. In fact, plants that are ing the Charales. SPOROPHYTE GENERATION adapted to very dry environments will often have a particu Correlated with the evolution of parenchyma may have Zygote Sporocyte (2N) larly thick cuticle (as in Figure 3.8) to inhibit water loss. been the evolution of a middle lamella in land plants. The A third apomorphy for the land plants was the evolution of middle lamella is a pectic-rich layer that develops between ——fertilization rneiosis—— parenchyma tissue (Figure 3.9). All land plants grow by the primary cell walls of adjacent cells (Figure 3.5A). means of rapid cell divisions at the apex of the stem, shoot, Its function is to bind adjacent cells together, perhaps a thallus or (in most vascular plants) of the root. This region prerequisite to the evolution of solid masses of parenchyma (Sperm nonflagellatein Conifers, GAMETOPHYTE GENERATION and Gnetales, and Angiosperms) (N) of actively dividing cells is the apical meristem. The apical tissue. Egg Sperm meristem of liverworts, hornworts, and mosses (discussed Another evolutionary innovation for the land plants was ) 4) a (Figure 3. antheridium is a type Spores© later), and of the monilophytes (see Chapter have single the antheridium bA). The apical cell (Figure 3.9), probably the ancestral condition for of specialized gametangium of the haploid (n) gametophyte, the land plants. In all land plants the cells derived from the one that contains the sperm-producing cells. It is distin lost by reductionand modificationf Archegonium Antheridium apical meristem region form a solid mass of tissue known as guished from similar structures in the Viridiplantae in being in the Angiosperms mitosis, growth, &/differentiation and some Gnetales parenchyma (Gr. para, “beside” + enchyma, “an infusion”; surrounded by a layer of sterile cells, the antheridial wall. mitosis, growth, & differentiation in reference to a concept that parenchyma infuses or fills up The evolution of the surrounding layer of sterile wall cells, Gametophyte Body space beside and between the other cells). Parenchyma tissue which is often called a sterile “jacket” layer, was probably consists of cells that most resemble the unspecialized, undif adaptive in protecting the developing sperm cells from desic FIGURE 3.7 Haplodiplontic “alternation of generations” in the land plants (embryophytes). ferentiated cells of actively dividing meristematic tissue. cation. In all of the nonseed land plants, the sperm cells are Structurally,parenchyma cells (1) are elongate to isodiamethc; released from the antheridium into the external environment seed plants, the embryo will remain dormant for a period of in the sporophyte, may be “shielded” by dominant alleles, (2)have a primary (1°) cell wall only (rarely a secondary wall); and must swim to the egg in a thin film of water. Thus, a wet time and will begin growth only after the proper environmen but which, in the gametophyte, would always be expressed); and (3) are living at maturity and potentially capable of environment is needed for fertilization to be effected in the tal conditions are met. As the embryo grows into a mature and (2) by permitting increased genetic variability in the continued cell divisions. Parenchyma cells function in nonseed plants, a vestige of their aquatic ancestry. Members sporophyte, a portion of the sporophyte differentiates as the sporophyte generation (via genetic recombination from two of the Charales also have a structure termed an antheridium, spore-producing region. This spore-producing region of the “parents”) upon which natural selection acts, thus increasing which has an outer layer of sterile cells (Figure 3.5C,D). sporophyte is called the sporangium. The sporangium is the potential for evolutionary change. — single apical cell However, because of its differing anatomy, the Charales enveloped by a sporangial wall, which consists of one or A second innovation in land plants was the evolution of antheridium may not be homologous with that of the land more layers of sterile, non-spore-producing cells. A sporan cutin and the cuticle (Figure 3.8). A cuticle is a protective plants, and thus may have evolved independently. gium contains sporogenous tissue, which matures into sporo layer that is secreted to the outside of the cells of the epider Another land plant innovation was the evolution of cytes, the cells that undergo meiosis. Each sporocyte produces, mis (Gr. epi, “upon” + derma, “skin”), the outermost layer the archegonium, a specialized female gametangium by meiosis, four haploid spores (Figure 3.7). of land plant organs. The epidermis functions to provide (Figure 3.lOB). The archegonium consists of an outer layer One adaptive advantage of a sporophyte generation as a of sterile cells, termed the venter, that immediately surround separate phase of the life cycle is the large increase in spore the egg, plus others that extend outward as a tube-like neck. cuticle cell wall production. In the absence of a sporophyte, a single zygote epidernial cell The archegonium is stalked in some taxa; in others the egg (the result of fertilization of egg and sperm) will produce four is rather deeply embedded in the parent gametophyte. The spores. The elaboration of the zygote into a sporophyte and egg cell is located inside and at the base of the archegonium. I sporangium can result in the production of literally millions Immediately above the egg is a second cell, called the of spores, a potentially tremendous advantage in reproductive ventral canal cell, and above this and within the neck region output and increased genetic variation. there may be several neck canal cells. The archegonium may Another possible adaptive value of the sporophyte is have several adaptive functions. It may serve to protect the associated with its diploid ploidy level. The fact that a sporo developing egg. It may also function in fertilization. Before phyte has two copies of each gene may give this diploid phase fertilization occurs, the neck canal cells and ventral canal cell an increasedfitnessin either of two ways: (1)by potentiallypre FIGURE 3.9 Equisetum shoot apex, showingparenchymatous break down and are secreted from the terminal pore of the ventiiig the expression of recessive, deleterious alleles (which, FIGURE 3.8 The cuticle, an apombrphy for the land plants. growth form,from an apical meristem. neck itself; the chemical compounds released function as an 60 CHAPTER 3 EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS UNIT II EVOLUTION AND DIVERSITY OF PLANTS 61

Sporophyte Body protection of inner tissue and to inhibit water metabolic activities such as respiration, photosynthesis, lat HAPLODIPLONTIC mechanical LIFE CYCLE loss. The cuticle consists of a thin, homogeneous, transparent eral transport, storage, and regeneration/wound healing. mitosis, growth, & differentiation mitosis, growth, & differentiation (“Alternation of Generations”) layer of cutin, a polymer of fatty acids, and functions as a Parenchymacells may further differentiate into other special z preventing excess water loss. Cutin also impregnates ized cell types. It is not clear if the evolution of both apical Embryo sealant, Sporangium the outer cellulosic cell walls of epidermal cells; these are growth and true parenchyma is an apomorphy for the land /% known as a “cutinized” cell wall. The adaptive advantage of plants alone, as shown here (Figure 3.6). Both may be inter mitosis, growth, & dfferenuation initosis, growth, & differentiation cutin and the cuticle is obvious: prevention of desiccation preted to occur in certain closely related green plants, includ outside the ancestral water medium. In fact, plants that are ing the Charales. SPOROPHYTE GENERATION adapted to very dry environments will often have a particu Correlated with the evolution of parenchyma may have Zygote Sporocyte (2N) larly thick cuticle (as in Figure 3.8) to inhibit water loss. been the evolution of a middle lamella in land plants. The A third apomorphy for the land plants was the evolution of middle lamella is a pectic-rich layer that develops between ——fertilization rneiosis—— parenchyma tissue (Figure 3.9). All land plants grow by the primary cell walls of adjacent cells (Figure 3.5A). means of rapid cell divisions at the apex of the stem, shoot, Its function is to bind adjacent cells together, perhaps a thallus or (in most vascular plants) of the root. This region prerequisite to the evolution of solid masses of parenchyma (Sperm nonflagellatein Conifers, GAMETOPHYTE GENERATION and Gnetales, and Angiosperms) (N) of actively dividing cells is the apical meristem. The apical tissue. Egg Sperm meristem of liverworts, hornworts, and mosses (discussed Another evolutionary innovation for the land plants was ) 4) a (Figure 3. antheridium is a type Spores© later), and of the monilophytes (see Chapter have single the antheridium bA). The apical cell (Figure 3.9), probably the ancestral condition for of specialized gametangium of the haploid (n) gametophyte, the land plants. In all land plants the cells derived from the one that contains the sperm-producing cells. It is distin lost by reductionand modificationf Archegonium Antheridium apical meristem region form a solid mass of tissue known as guished from similar structures in the Viridiplantae in being in the Angiosperms mitosis, growth, &/differentiation and some Gnetales parenchyma (Gr. para, “beside” + enchyma, “an infusion”; surrounded by a layer of sterile cells, the antheridial wall. mitosis, growth, & differentiation in reference to a concept that parenchyma infuses or fills up The evolution of the surrounding layer of sterile wall cells, Gametophyte Body space beside and between the other cells). Parenchyma tissue which is often called a sterile “jacket” layer, was probably consists of cells that most resemble the unspecialized, undif adaptive in protecting the developing sperm cells from desic FIGURE 3.7 Haplodiplontic “alternation of generations” in the land plants (embryophytes). ferentiated cells of actively dividing meristematic tissue. cation. In all of the nonseed land plants, the sperm cells are Structurally,parenchyma cells (1) are elongate to isodiamethc; released from the antheridium into the external environment seed plants, the embryo will remain dormant for a period of in the sporophyte, may be “shielded” by dominant alleles, (2)have a primary (1°) cell wall only (rarely a secondary wall); and must swim to the egg in a thin film of water. Thus, a wet time and will begin growth only after the proper environmen but which, in the gametophyte, would always be expressed); and (3) are living at maturity and potentially capable of environment is needed for fertilization to be effected in the tal conditions are met. As the embryo grows into a mature and (2) by permitting increased genetic variability in the continued cell divisions. Parenchyma cells function in nonseed plants, a vestige of their aquatic ancestry. Members sporophyte, a portion of the sporophyte differentiates as the sporophyte generation (via genetic recombination from two of the Charales also have a structure termed an antheridium, spore-producing region. This spore-producing region of the “parents”) upon which natural selection acts, thus increasing which has an outer layer of sterile cells (Figure 3.5C,D). sporophyte is called the sporangium. The sporangium is the potential for evolutionary change. — single apical cell However, because of its differing anatomy, the Charales enveloped by a sporangial wall, which consists of one or A second innovation in land plants was the evolution of antheridium may not be homologous with that of the land more layers of sterile, non-spore-producing cells. A sporan cutin and the cuticle (Figure 3.8). A cuticle is a protective plants, and thus may have evolved independently. gium contains sporogenous tissue, which matures into sporo layer that is secreted to the outside of the cells of the epider Another land plant innovation was the evolution of cytes, the cells that undergo meiosis. Each sporocyte produces, mis (Gr. epi, “upon” + derma, “skin”), the outermost layer the archegonium, a specialized female gametangium by meiosis, four haploid spores (Figure 3.7). of land plant organs. The epidermis functions to provide (Figure 3.lOB). The archegonium consists of an outer layer One adaptive advantage of a sporophyte generation as a of sterile cells, termed the venter, that immediately surround separate phase of the life cycle is the large increase in spore the egg, plus others that extend outward as a tube-like neck. cuticle cell wall production. In the absence of a sporophyte, a single zygote epidernial cell The archegonium is stalked in some taxa; in others the egg (the result of fertilization of egg and sperm) will produce four is rather deeply embedded in the parent gametophyte. The spores. The elaboration of the zygote into a sporophyte and egg cell is located inside and at the base of the archegonium. I sporangium can result in the production of literally millions Immediately above the egg is a second cell, called the of spores, a potentially tremendous advantage in reproductive ventral canal cell, and above this and within the neck region output and increased genetic variation. there may be several neck canal cells. The archegonium may Another possible adaptive value of the sporophyte is have several adaptive functions. It may serve to protect the associated with its diploid ploidy level. The fact that a sporo developing egg. It may also function in fertilization. Before phyte has two copies of each gene may give this diploid phase fertilization occurs, the neck canal cells and ventral canal cell an increasedfitnessin either of two ways: (1)by potentiallypre FIGURE 3.9 Equisetum shoot apex, showingparenchymatous break down and are secreted from the terminal pore of the ventiiig the expression of recessive, deleterious alleles (which, FIGURE 3.8 The cuticle, an apombrphy for the land plants. growth form,from an apical meristem. neck itself; the chemical compounds released function as an to plants gametophyte living formally tophyte features; include called are monophyletic presence During (discussed heat cells, nutritional tissue. DIVERSITY embryo/sporophyte effecting the Sperm

attractant,

62 A that Liverworts, The a egg shock paraphyletic flavonoid in phase of the cells land the as the cell lacking recognized. CHAPTER the of the fertilization, acting the in dependence proteins. liverworts, various nonvascular enter hornworts early plants of name, of to Chapter mosses, dominant, lineages OF the the chemical form true as the group, NONVASCULAR land evolution a ultrastructural placed life development share These homing vascular 4). 3 neck a and mosses, the of and diverged plants cycle. FIGURE diploid photosynthetic, land These antheridial compounds, EVOLUTION defined the hornworts other are archegonium in many of device sporophyte tissue quotation of not the was plants It and lineages (2n) possible before is liverworts. land and modifications 3.10 by discussed archegonium thalloid wall hornworts. for likely and differ the zygote. the or and the plants, (sterile A. marks, in persistent, LAND may the serves upon absence AND establishment “bryophytes” apomorphies: that swimming having Antheridia. from a in here. The vascular proliferation collectively nature, In “jacket” “Bryophytes” gametophytic the three is as of and the

DIVERSITY PLANTS sporophyte addition of the no the sperm a ancestral and vascular fertilize derived site similar layer) sperm. B. longer major, game plants sperm free- Archegonia. of cells and the for be of to a OF neck rhizoids, tissue; the anchorage Thalloid dispersal; studies. gametophytes: changes meaning hot enings, Liverworts, elaters, ponents first shaded the Many to (1) been the ephemeral,

LIVERWORTS of B GREEN There The the Both one capsule, distinctive monophyletic gametophyte habitats). land proposed, different liverworts, relationships this found areas another are elongate, As of in that liverworts are plants. uniseriate, as and apomorphies in and AND also is the carrying the moisture (although inside two hornworts oil they likely Among absorption. one relationships attached land sporangium traditionally thalloid and Today, (see nonsporogenous bodies groups mosses, LAND basic change recent consist the of spores to filamentous the plant

later / the of content. some sporangium. the the liverworts to and and that morphological ancestral land and of liverworts, and PLANTS discussion). Pores and flora, with shape of vascular apomorphies dries called mosses, (2) which are among are plants. a hornworts leafy nutritionally thallus, them specialized Elaters adapted descendents in processes out, growing cells and are form, the is the Elaters the plants the mosses, (Figures the seen (Figures relatively Hepaticae, with move upper three a types function gametophyte to based is elaters of flattened in mostly spiral periodically are that structures relatively remain dependent in Figure of liverworts and surface lineages 3.11, hygroscopic, of on response minor some 3.11—3.13). twist function wall hornworts are in liverwort in cladistic mass 3.12K). unclear. 3.6. neck moist, one of out small, called thick of bears spore com upon have dry, the are the of in of of

to r (longitudinal-section) archegoniophore (longitudinal antheridiophore section) (n) (n) thalloid liverwort FIGURE 3.11 antheridium archegonium UNIT (n) Liverwort (n) II morphology EVOLUTION elater (longitudinal-section) archegoniophore and life AND cycle. capsule spore (n) DIVERSITY dorsal dorsal 2 rows view leaves (upper) of leafy OF liverwort sporophyte germinating PLANTS

(2n) C ventral ventral 1 view row (lower) leaves of

spore 63