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BIOL1262

Lecture 7

Vascular seedless plants II

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Evolution of today’s plants: note ferns

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 Ancient (extinct) relative of vascular plants – a rhyniophyte

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Phylum Pteridophyta

Classification

Ferns & fern allies: most ferns, horsetails & whisk ferns

The major groups (orders) within this phylum are largelydetermined by:

1. whether homosporous (one type of spore) or 

heterosporous (two types of spores) in life cycle.

most members are homosporous (few exceptions)

2. the structure and method of development of sporangia, i.e.whether they are

eusporangiate

leptosporangiate

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Seven Orders in Phylum Pteridophyta (ferns)

Eusporangiate and heterosporous Ophioglossales (e.g. Botrichum)

Marratiales

Leptosporangiate and homosporous

Filicales

Psilotales (whisk ferns, protostelic e.g. Psilotum)

Equisetales (horsetails, single genus Equisetum)

Leptosporangiate and heterosporous

Marsileales (water ferns)

Salvinales (water ferns)

Phylum Pteridophyta - Classification

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strobili

Equisetum - Horsetails

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Phylum Pteridophyta: ORDER Filicales

Contains the familiar ferns

more than 10,500 species 320 genera in 35 families

homosporous and leptosporangiate

Sporophyte has well-developed

body with:

true stems true leaves

true roots

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Development of Roots

probably from branches that penetratedunderground

developed further branching

exposed to different environment & selection

processes than stems

evolved over millennia to the forms present

today (different from shoots)

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Phylum Pteridophyta: Filicales

Roots are adventitious primary root develops from the

embryo but is short-lived

adventitious roots grow from the stem

Stems are generally siphonostelicrhizomes

Leaves/fronds are: megaphylls

most conspicuous structures of thesporophyte

commonly compound

with a rachis (extension of thepetiole)

pinnae (leaflets) [sing. pinna]

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Leaves/fronds (cont’d):

Show circinate vernation:-

type of leaf development

where the young leaves arecoiled;

the maturing leaf uncoils dueto more rapid growth on inner than outer surface.

Young leaves and rhizomesusually covered with hair or 

waxy scales:- epidermal outgrowths; used in fern

classification

Phylum Pteridophyta: Filicales

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LIFE CYCLE OFHOMOSPOROUS FERN

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Phylum Pteridophyta: Order Filicales – Life Cycle

Sporophyte produces one type of spore (i.e. homosporous)

Spores are produced in sporangia on the lower surface of leaves OR specially modified leaves (sporophylls) ORseparate stalks (sporangiophores).

Sporangia occurs in clusters called sori.

may be yellow, orange-brown or blackish.

may appear as broad patches, lines or dots

Indusia cover the young sori (sing. indusium)

these are specialized epidermal outgrowths

they shrivel when sporangia are ready to shed spores

Stalked

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Types of sori

indusia covering

the sporangia

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The sporangium possesses:

tapetum food-rich layer of cells

nourishes the developing spores

thick wall annulus (differentially-thickened

cell walls stomium (thin-walled cells)

Dehiscence liberates (ejects)spores which are dispersed bywind

Phylum Pteridophyta: Order Filicales – Life Cycle

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Dehisced sporangia of homosporous fern

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Gametophyte:-

develops from the spores

found in moist places

flat, thin (one cell thick), leaf-like structure (prothallus)

free living

green and nutritionallyindependent

numerous rhizoids on lower surface.

Phylum Pteridophyta: Order Filicales – Life Cycle

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Gametophyte (cont’d)

Monoecious (both male and female gametangia on sameindividual)

 Antheridia develop on ventral surface

occur among the rhizoids

consist of a sterile jacket containing sperm

 Archegonia develop on ventral surface

occur near the apical notch. flask-shape

swollen lower portion (venter) sunken in gametophyteneck

Phylum Pteridophyta: Order Filicales – Life Cycle

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2. Transfer of gametes

& Fertilization

•  When sperm are mature and there is

an adequate supply of water,

antheridia burst and sperm arereleased.

•  The multi-flagellated sperm swim to

the archegonia

•  Water is still essential for gametic

transfer

•  A sperm enters through the neck 

canal and fertilizes egg.

•  The zygote begins to divide

immediately.

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3. Development of the young Sporophyte

•  Through mitotic cell divisions embryo develops into ayoung sporophyte.

•  At the early stage, it is dependent on gametophyte (receives

nutrition from the gametophyte through foot).

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Fern gametophyte with young developing sporophyte in notch

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Fern gametophyte with developing

sporophyte in notch

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4. Maturation of the

sporophyte

•  The young sporophyte rapidly develops.

•  The gametophyte disintegrates

once the sporophyte achieves alevel of photosynthesis that is

sufficient to maintain itself and it

has rooted in the soil.

• Sporophyte soon becomes mature.

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Spore production by sporophyte

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club mosses, spike mosses and

quill-worts

Phylum Lycophyta

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Phylum Lycophyta

Living members (10 genera) are herbaceous, generally less than 20 cm,

BUT large tree lycophytes were among dominant plants in coal-

forming forest of the Carboniferous period.

Possess only microphylls (simple leaves)

in contrast to the megaphylls of Pteridophyta and seed

plants often arranged spirally on stem.

Stems:

branched (dichotomous) or unbranched, erect, creeping or hanging.

Roots

dichotomously branched

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Lycopodiaceae: club mosses most familiar living members of 

this phylum

mostly tropical species

sporangia aggregated in strobili

(sing. strobilus) or on

sporophylls

homosporous.

Isoetaceae: quill-worts

only one genus, Isoetes

heterosporous.

Lycopodium sp.

Phylum Lycophyta

Isoetessp.

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Selaginellaceae: spike mosses only one genus in this family

mostly tropical

grow mostly in moist places

(few grow in dry places)

heterosporous.

Creeping & erect stems

Roots arise where stem

branches

Dichotomously branched

Protosteles

Selaginella sp.

Phylum Lycophyta - Selaginella 

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Stem bears four rows of leaves (microphylls)

2 smaller dorsally 2 larger ventrally

Sporophylls arranged instrobili at tips of branches.

each sporophyll bears a

single sporangium on itssurface.

¾ Sporophyll: modified leaf or leaf-like organ that bearssporangia.

Selaginella sp.

Phylum Lycophyta - Selaginella 

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Megasporangia are borne

on megasporophyll

Microsporangia are borneon microsporophyll

Both kinds of sporangia

occur in the same strobilus

L.S. strobilus of 

Selaginella sp.

megasporangium

megasporophyll

microsporophyll

microsporangium

Phylum Lycophyta - Selaginella 

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microsporophyll

Microsporangiumcontainingnumerous

microspores

Megasporangiumcontaining four

megaspores

megasporophyll

 Axis of strobilus

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Microsporangiaand

larger megasporangia

of 

Selaginella sp.

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Life Cycle of 

Selaginella

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Selaginella : Life cycle

Spore Production through meiosis of sporogenous tissue

- microspores produced in the microsporangium

- megaspores produced in the megasporangium

• Gametophytes develop (from mitotic divisions) within the

microspore/megaspore walls: much reduced

• Microgametophyte produces biflagellate sperm in the antheridia

• Megagametophyte produces eggs in the archegonia (nourished

by food previously stored in megaspore)

• Sperm must swim in water to reach egg for fertilization

• Zygote → sporophyte embryo (dependent on megagametophyte

initially) → independent mature sporophyte

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Important developments in Selaginella 

Heterospory The megagametophyte stores food for the developing embryo

Improves survival in harsh terrestrial environment

Endospory The development of the gametophytes from spores takes place

on the parent sporophyte

Shift in the balance with gametophyte becoming dependenton the sporophyte as is seen in the seed plants

The new sporophyte generation begins development within themegagametophyte

Similar to the seed habit of seed plants

Range – greater; some species in arid habitats

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economic importance

Bryophyte mosses

Sphagnum peat – fuel,

(preservation of artefacts, bodies, seeds... )

Seedless vascular plants (Palaeozoic era)

coal