Chapter 18

Lecture Outline

The Seedless Vascular Plants: Ferns and Their

Relatives

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Outline Introduction

Phylum Psilotophyta – The Whisk Ferns

Phylum Lycophyta – The Ground Pines, Spike Mosses and Quillworts

Phylum Equisetophyta – The Horsetails and Scouring Rushes

Phylum Polypodiophyta – The Ferns

Fossils

Introduction During early stages of vascular plant

evolution:• Internal conducting tissue developed.

• True leaves appeared.

• Roots that function in absorption and anchorage developed.

• Gametophytes became progressively smaller.

Four phyla of seedless vascular plants: Psilotophyta, Lycophyta, Equisetophyta, Polypodiophyta

Introduction Psilotophyta

• Sporophytes have neither true leaves, nor roots.

• Stems and rhizomes fork evenly.

Lycophyta• Plants covered with

microphylls.– Microphylls - Leaves with

single vein whose trace is not associated with a leaf gap

Psilotum

Lycopodium

Introduction Equisetophyta

• Sporophytes have ribbed stems containing silica.

• Have whorled, scalelike microphylls that lack chlorophyll

Polypodiophyta• Sporophytes have

megaphylls that are often large and much divided.– Megaphylls - Leaves with

more than one vein and a leaf trace associated with leaf gap

Equisetum

A fern

Phylum Psilotophyta – The Whisk Ferns

Structure and form:• Sporophytes:

– Dichotomously forking stemso Above ground stems arise

from rhizomes beneath surface of ground.

– Have neither leaves nor roots

– Enations along stems.o Enations - Tiny, green,

superficially leaflike, veinless, photosynthetic flaps of tissue

– Rhizoids, aided by mycorrhizal fungi, scattered along rhizomes.

Resemble small, green whisk brooms

Phylum Psilotophyta – The Whisk Ferns Reproduction:

• Sporangia fused in threes and produced at tips of short branches.

• Gametophytes develop from spores beneath ground.– Branch dichotomously

– No chlorophyll

– Rhizoids aided by mycorrhizal fungi.

– Archegonia and antheridia scattered on surface.

• Zygote develops foot and rhizome.

• Rhizome separates from foot.

Phylum Psilotophyta – The Whisk Ferns Reproduction:

Phylum Psilotophyta – The Whisk Ferns Fossil whisk fern look-alikes:

• Silurian, 400 million years ago– Cooksonia and Rhynia

o Naked stems and terminal sporangia

• Devonian, 400-350 million years ago– Zosterophyllum

o Naked stems and rounded sporangia along stemo Thought to be ancestral to club mosses

Phylum Lycophyta – The Ground Pines, Spike Mosses, and Quillworts

Collectively called club mosses

• Two living major genera– Lycopodium

– Selaginella

• Two living minor genera

• Several genera that became extinct about 270 million years ago

Sporophytes have microphylls.

Have true roots and stems

Phylum Lycophyta

Lycopodium - Ground pines

• Often grow on forest floors

• Stems are simple or branched.– Develop from

branching rhizomes

• Leaves usually less than 1 cm long.

• Roots develop along rhizomes.

Phylum Lycophyta

• Sporangia in axils of sporophylls.

– Sporophyll - Sporangium-bearing leaves

– In some species, sporophylls have no chlorophyll, are smaller than other leaves and clustered into strobili (singular: strobus).

• In sporangia, sporocytes undergo meiosis, producing spores.

Lycopodium reproduction:

Phylum Lycophyta Lycopodium reproduction:

• Spores grow into independent gametophytes.

– In some species, gametophytes resemble tiny carrots, develop in the ground and are associated with mycorrhizal fungi.

– In others, gametophytes develop on surface and are green.

– Archegonia and antheridia produced on gametophytes.

– Sperm are flagellated and water is essential for fertilization.

Phylum Lycophyta Lycopodium

reproduction:

Phylum Lycophyta

Selaginella - Spike mosses

• Especially abundant in tropics

• Branch more freely than ground pines

• Leaves have a ligule on upper surface.

Phylum Lycophyta

Selaginella reproduction:

• Produce two different kinds of gametophytes = heterospory.– Microsporophylls bear microsporangia containing

microsporocytes, producing tiny microspores.o Microspore becomes male gametophyte, consisting

of an antheridium within microspore wall.

– Megasporophylls bear megasporangia containing megasporocytes, producing 4 large megaspores.o Megaspore develops into female gametophyte

consisting of many cells inside megaspore.o Several archegonia produced where spore wall

ruptures.

Phylum Lycophyta

Selaginella reproduction:

Phylum Lycophyta

Isoetes - Quillworts

• Most found in areas partially submerged in water for part of year.

• Microphylls are arranged in a tight spiral on a stubby stem.

• Ligules occur towards leaf bases.

• Corms have vascular cambium.

• Plants generally less than 10 cm tall.

Phylum Lycophyta

Isoetes reproduction:• Similar to spike

mosses, except no strobili

• Sporangia at bases of leaves.

Phylum Lycophyta

Ancient relatives of club mosses and quillworts:

• Dominant members of forests and swamps of Carboniferous, 325 million years ago– Large, tree-like, up

to 30 meters tall - Lepidodendron

Surface of Lepidodendron, showing microphyll bases

Phylum Equisetophyta – The Horsetails and Scouring Rushes

Equisetum Branched and

unbranched forms, usually less than 1.3 meters tall

Stems jointed and ribbed.• If branched, then branches

in whorls.

• Scalelike leaves in whorls at nodes.

• Stomata in grooves between ribs.

Phylum Equisetophyta Stem anatomy:

• Hollow central cavity from break down of pith

• Two cylinders of smaller canals outside pith.– Carinal canals

conduct water with xylem and phloem to outside.

– Vallecular canals outside carinal canals contain air.

Silica deposits on walls of stem epidermal cells.

Phylum Equisetophyta

Equisetum reproduction:

• Asexual by fragmentation of rhizomes

• Sexual reproduction:– Strobili at tips of stems with

sporangia connected to sporangiophores.

– Spores green with 4 ribbon-like elaters attached.o Aid in spore dispersal

– Gametophytes lobed, green, cushionlike, up to 8 mm in diameter.

Spores with elaters

Phylum Equisetophyta Equisetum reproduction:

Phylum Equisetophyta

Ancient relatives of horsetails:

• Flourished in Carboniferous, 300 million years ago.

Human and ecological relevance:

• Many giant horsetails used for food by humans and other animals.

• Scouring rush stems used for scouring and sharpening.

Reconstruction of fossil giant horsetail, Calamites

Phylum Polypodiophyta – The Ferns

Structure and form:

• Vary in size from tiny floating forms less than 1 cm to giant tropical tree ferns up to 25 m tall– Fern leaves are megaphylls - Referred to as fronds.

o Typically divided into smaller segments

– Require external water for reproduction

Phylum Polypodiophyta – The Ferns Reproduction:

• Sporophyte is conspicuous phase.– Fronds, rhizomes, roots

– Fronds first appear coiled in crozier (fiddlehead), and then unroll and expand.o Fronds often divided into

segments called pinnae (singular: pinna).

Crozier

Phylum Polypodiophyta – The Ferns Reproduction:

• Sporangia stalked.– May be scattered on lower

leaf surface, confined to margins, or found in discrete clusters called sori (singular: sorus).o Sori may be protected by

indusia (singular: indusium).

– With row of heavy-walled, brownish cells = annuluso Annulus catapults spores

out of sporangium.

Sorus covered by indusium

Phylum Polypodiophyta – The Ferns

Reproduction:• Meiosis forms spores in sporangia.

• Spores released and grow into gametophytes called prothalli (singular: prothallus).

• Prothalli are one cell thick, and have archegonia and antheridia.

• Zygote develops into young sporophyte.

• Gametophyte, or portion of it, dies and leaves sporophyte growing independently.

Phylum Polypodiophyta – The Ferns

Reproduction:

Phylum Polypodiophyta – The Ferns

Fossil relatives of ferns:

• Devonian, 375 million years ago - Possible ancestors of ferns– Resemble ferns in

growth habit, but look more like whisk ferns

Possible ancestors: Aglaophyton and Psilophyton

Phylum Polypodiophyta – The Ferns

Fossil relatives of ferns

• Carboniferous, 320-250 million years ago - Tree ferns abundant– Seeds found on some of

fossil tree ferns.

Phylum Polypodiophyta

Human and ecological relevance:

• House plants– Function well as air filters

• Outdoor ornamentals

• Cooked rhizomes as food

• Folk medicine

• Fronds used in thatching for houses.

• Basketry and weaving

Fossils

A fossil - Any recognizable prehistoric organic object preserved from past geological ages.

• Conditions of formation almost always include quick burial and an accumulation of sediments.– Hard parts more likely preserved than soft parts.

Fossils

Molds, casts, compressions, and imprints:• After being buried in sediment and hardened into

rock, organic material slowly washed away.– If air space remains - Mold

– If silica fills space - Cast

• Compression - Objects buried by layers of sediment and overlying sheer weight compresses them to thin film of organic material and an outline.– Image of an impression = imprint

– Coal is a specific type of compression.

Compression fossil

Fossils Petrifactions - Uncompressed rock-like material

in which original cell structure has been preserved• Chemicals in solution infiltrate cells and cell walls, where

they crystallize and harden, preserving original material.

Coprolites - Dung of prehistoric animals and humans

Unaltered fossils - Organisms fell into oil or water that lacked oxygen and did not permit decay.

Petrified wood

Review Introduction

Phylum Psilotophyta – The Whisk Ferns

Phylum Lycophyta – The Ground Pines, Spike Mosses and Quillworts

Phylum Equisetophyta – The Horsetails and Scouring Rushes

Phylum Polypodiophyta – The Ferns

Fossils