Chapter 29: Plant Diversity I: The Colonization of Land

 

Numbers Game:

               Oldest land plant fossils are 550 million years old

               More than 280,000 extant species of plants

 

Terrestrial plants evolved from green algae called charophyceans.  The most complex charophyceans, Chara and Coleochaete, are the algae most closely related to plants.  Realize that these modern algae are not the ancestors of plants.

 

Plants are divided into four main groups:

 

Common characteristics of all plants:

·        Five derived characteristics – unique to land plants:

·        Apical meristems – regions of cell division in root and shoot tips

·        Multicellular, dependent embryos – nutrients supplied by parental tissues

·        Alternation of generations – gametophyte and sporophyte both multicellular Fig. 29.6

·        Sporopollenin, (most durable organic material known), makes up walls of spores

·        Multicellular gametangia (which produce eggs or sperm)

·        Other characteristics all plants have in common:

·        multicellular

·        eukaryotic

·        photosynthetic autotrophs

·        also true tissues, sexual and asexual reproduction

·        nearly all terrestrial (some may be secondarily adapted to aquatic environment)

·        gas exchange through microscopic pores (stomata)

·        cells have cell walls of cellulose

·        food reserves stored as starch

 

Features in common with green algae (including charophyceans)

·        chlorophyll a & b, beta-carotene in chloroplasts

·        homologous chloroplasts (thylakoid menbranes stacked into grana)

·        plant starch as their storage molecule

·        homologous cell walls, usually of cellulose

·        various mitotic and molecular similarities

 

Features in common with charophyceans (Fig. 29.2), but not with other algae:

·        rosette cellulose-synthesizing complexes (other algae have linear arrays)

·        same peroxisomes – enzymes in peroxisomes

·        structure of sperm similar

·        phragmoplast formed during cell division

 

Adaptations for life on land = water demands, protection from UV rays, and protection:

1.       waxy cuticle covering outer surfaces

2.      stomata (pores in leaves and stems)--Fig. 35.19

3.      vascular tissue (xylem and phloem)

 

Alternation of generations a common theme in life cycles of plants and higher algae

 

Common features of alternation of generations in all plants) Fig. 29.6:

1. sporophyte generation is diploid (2n), produces haploid (n) spores through meiosis*

2. spores germinate and undergo mitosis to produce (haploid) gametophyte

3. gametophyte eventually forms haploid gametes which fuse during sexual reproduction

    to form a diploid zygote

4. zygote grows via mitosis and reestablishes the sporophyte generation

    * Note that the only stage that meiosis occurs is in the production of spores

 

The sporophyte (diploid) is dominant in all but the mosses and their relatives (the non-vascular plants)

 

The reduction of the gametophyte (haploid) proceeds as evolution of higher plant forms advances

 

4 major evolutionary periods for terrestrial plants – adaptations found in each new group of plants Pay special attention to Fig. 29.1. (also see Table 29.1): 

 

Here are the “The Big Four” plant groups:

(Note that botanists have historically used the term division in place of phylum)

 

1. Non-vascular plants ( Phyla/Divisions Bryophyta, Hepatophyta, Anthocerophyta)

               eg. mosses, liverworts Fig. 29.15

               gametophyte is the dominant generation

               mid-Silurian Period (at least 425 m.y.a.) New fossils found in China

               origin of first plants from green algae

               formation of cuticle and jacketed gametangia, stomata

               probably the only land plants for the first 100 million years after colonizing land

 

2. Seedless vascular plants (Phlya/Divisions Lycophyta, Pterophyta)

               eg. ferns, horsetails (Fig. 29.21)

               from early Devonian (app. 400 m.y.a.)

               begin use of vascular tissue for translocation

 

3. First seed plants - Gymnosperm or “naked seed” (Phyla/Divisions Coniferophyta, Cycadophyta, Ginkgophyta, Gnetophyta)

               began during late Devonian through Carboniferous (360-290 m.y.a.)

               seeds for the protection of the new sporophyte embryo

               pollen for fertilization without water

 

4. Flowering plants (Phylum/Division Anthophyta)

               early Cretaceous period (130 m.y.a.)

               emergence of flowering plants (Angiosperms or “container seeds”)

               developed ovaries to contain and protect seeds during development and dispersal

 

first terrestrial plants probably a filamentous chlorophyte, moved from shallow water to water’s edge, to land during periods of intermittent flooding and draining

 

many advantages on land

·        light bright, unfiltered

·        soil rich in minerals

·        no herbivores, at least at first

              

1st adaptations needed: jacketed gametangia, waxy cuticle on outer body surfaces; stomata

 

1st of the “Big Four”: Non-vascular plants (mosses, liverworts, hornworts)

              

Common Traits of non-vascular plants:

·        gametophyte is the dominant generation

·        waxy cuticle

·        gametes develop within jacketed gametangia

·        male gametophyte forms antheridia which produce flagellated sperm (therefore, water needed for fertilization)

·        female gametophytes form archegonia which produce one ovum each

      ovum develops into embryo and new sporophyte within female parent’s tissues

·        no vascular tissue, so limited in potential size (low-growing)

 

water transported through tissues via:

1.       diffusion

2.      capillary action

3.      cytoplasmic streaming

     

Division Bryophyta (mosses)

·        mosses grow in low dense mats, which absorb and retain water

·        each plant has rhizoids (filaments of cells) for absorbing water and nutrients

·        haploid gametophyte is the dominant generation

·        separate male and female gametophytes (therefore, are heterosporous)

·        sporophyte grows out of the female gametophyte’s archegonium, remains dependent on the gametophyte for its entire life span (long enough to produce 1n spores following meiosis)

·        spores scatter on rupture of the sporangium, (Fig. 29.18) form a protonema and finally a gametophyte via mitosis

·        Importance--mosses such as Sphagnum (peat moss) still cover over 3% of terrestrial Earth (esp. at northern latitudes)

 

Division Hepatophyta (liver worts or “liver herbs”)

flattened lobed body of gametophyte is dominant generation

both asexual and sexual reproduction

asexual via bundles of cells (gemmae) bounced out of splash cups by rain drops

More info:  http://www.hiddenforest.co.nz/bryophytes/liverworts/intro.htm

 

Division Anthocerophyta (hornworts)

like liverworts

elongated horn-shaped spore capsule

each photosynthetic cell has one large chloroplast (like Spirogyra in the algae)

More info: www3.uakron.edu/biology/hornworts/hornworts.html 

 

2nd of the “Big Four”: The Seedless Vascular Plants

 

Further adaptations of plants to land lead to next wave of adaptive radiation

               Increasing regional specialization between absorptive and photosynthetic tissues,               not needed in algae or mosses

 

a. need to support aerial photosynthetic portions of the plant (increasing competition)

               lignin in a cellulose matrix in cell walls provides a “skeleton” of lignified tissues

 

b. need for absorption: soil holds moisture and nutrients, so root cells not covered with                cuticle (must be able to absorb across cell membrane)

 

c. need for transport system for water/dissolved minerals and sugars

               two conducting tissues:

                              1. Xylem  - “pipes” of tube-shaped cell walls (cell walls of dead cells)

                                             carry water and minerals upwards from roots (unidirectional flow)

                                             lignified, so also function in support

 

                              2. Phloem - tubes made of living cells connected end to end for active                                                             transport of sugars, amino acids, other organics (bidirectional)

 

d. need to protect genome from mutations due to increase in ionizing radiation on land    

lead to the increasing dominance of the sporophyte (2n) over the gametophyte (n)

 

earliest terrestrial vascular plants only exist as fossils from late Silurian-early Devonian

               Cooksonia (earliest)--Europe, N. America (from when once joined together)

               simple dichotomous branching, single terminal sporangium (Fig. 29.20)

 

Two extant divisions:

1. Division Lycophyta

lycopods, club mosses (misleading name), ground pines (another misleading name)

dominant during Carboniferous app. 360-290 m.y.a.

 

two lines led to:

               a. woody trees 2 M in diameter, 40 M tall (!), now extinct

               b. smaller herbaceous species, eg. Selaginella, Lycopodium

 

·        have true leaves with vascular tissue

·        sporophyte the most conspicuous generation

·        spores borne in strobili (modified leaves which form a clublike, cone-shaped structure (modified leaves which form the strobilus are called sporophylls)

·        gametophytes live underground (up to 10 years;, non-photosynthetic, nurtured by symbiotic fungi

 

2. Division Pterophyta

Ferns, 12,000 species extant

·        evolved during the Devonian, made up large part of Carboniferous forests (tree ferns)

·        compound leaves are largest of the vascular seedless group--are true megaphylls (branched vascular system within the fronds)

·        stems usually prostrate creeping rhizomes with leaves sprouting vertically, or vertical stems (tree ferns)

 

Horsetails – 1 genus with 15 species extant (Equisetum)

·        epidermis embedded with silica, hence “scouring rush”

·        during Carboniferous Period, were up to 15 M tall

·        underground rhizome with vertical stems growing upward

·        sporangia borne on tips of stems

·        stems segmented simple microphyll leaves (single vascular trace) in whorls at junctions

·        sporophyte dominant, heterosporous

 

The sporophyte is dominant generation.

·        specialized sporophyll leaves bear sporangia on underside; sporangia arranged in clusters called sori (Fig. 29.24)

·        gametophytes flattened, heart-shaped, photosynthetic (free-living)

·        are homosporous, so gametophytes are bisexual, making antheridia and archegonia on same plant

·        young sporophyte grows out of fertilized archegonium

 

These two seedless vascular phyla/divisions formed vast forests during Carboniferous (360-290 m.y.a.) – source of huge coal reserves now in N. America and Europe (were more tropical, near Equator then)--Fig. 29.25

Lost dominance w/ end of Carboniferous, as continents merged to form Pangaea during Permian Period, and the climate became cooler and drier.  These plants were followed by primitive seed plants (evolving at the same time, but less favored by climatic conditions during Carboniferous)