Lecture #5

Plant Diversity I: Non-vascular plants

&Seedless Vascular plants

• 1.2 billion years ago (BYA) – appearance of cyanobacteria on land• 500 million years ago (MYA) – appearance of plants, fungi and animals• more than 290,000 known plant species today• plants inhabit all but the harshest environments

– such as some mountaintops, deserts areas and polar regions

• many plants have returned to their aquatic “roots”– e.g. some species of sea grasses

• most present-day plants are terrestrial• presence of plants has enabled other life forms to survive on land

– through their production of O2

Plants and Algae• evolution of plants proposed from algae

– closest relatives are located with the clade Charaophycea

– these share a common ancestor with the clade Chlorophyta – include the green algae

– similarities with algae: • multicellular • photosynthetic autotrophs• cell walls with cellulose• chlorophylls a and b

(Opisthokonta) (Viridiplantae)

Rho

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Archaeplastida

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Unikonta

Ancestral eukaryote

4 key traits of plants– four key traits of plants (and charophyceans)

• provided by not only morphologic evidence but genetic evidence• 1. rose-shaped complexes for cellulose synthesis – both charophyceans and land

plants have rosette cellulose-synthesizing complexes – protein arrays found in the plasma membrane that synthesize cellulose microfibrils of the

cell wall– plants and charophyceans have a higher percentage of cellulose in their cell walls than the

chlorophytans• 2. peroxisome enzymes – peroxisomes have enzymes that help minimize the loss of

organic production as a result of photorespiration• 3. flagellated sperm structure – similar to the charophyceans• 4. formation of a phragmoplast – group of microtubules that forms between the

daughter nuclei of the dividing plant cell during mitosis– synthesis of a new cell plate occures - divides the two daughter cells

Rosettes

Adaptations by Land plants• advantages of terrestrial life:

– stronger exposure to sunlight for photosynthesis– atmosphere offered more CO2 for photosynthesis– soil rich in nutrients– initially relatively few herbivores

• movement onto land would require protection of the zygote from drying out– development of layer of durable polymer called sporopellenin – prevents exposed zygote from dessication

• movement onto land resulted in the development of adaptations– facilitated survival and reproduction on land– e.g. development of a structural system to withstand the forces of gravity– e.g. changes adapting to the relative scarcity of water

• these adaptations have defined the plant kingdom• but what adaptations are unique to plants?• depends on how you draw the boundary separating plants from algae• some traits related to terrestrial life

– for the earliest land plants – mycorrhizal associations with fungi for nutrient absorption– epidermis with a waxy covering called a cuticle– production of secondary compounds that are products of secondary metabolic pathways

• primary metabolic paths produce lipids, carbohydrates, amino acids – not unique to plants– secondary paths produce compounds such as tannins, terpenes and alkaloids (defense against

herbivores and parasites), plus phenolics (flavonoids – absorb UV radiation, deter attacks by pathogenic microbes)

Viridiplantae

Streptophyta

Plantae

Red algae Chlorophytes Charophyceans Embryophytes

Ancestral alga

• Kingdom Plantae contains the plants called embryophytes – plants with embryos• however, current debate advises some changes – 2 options:

– Kingdom Streptophytae – Embryophytes (land plates) + Charophyceans OR– Kingdom Viridiplantae – Embryophytes + Charophyceans + Chlorophytes

• botanists do not use the term phyla when classifying the plant kingdom – use divisions• currently accepted organization: development of two lineages or divisions: non-vascular

and vascular (390 MYA) – called the Bryophyta and Tracheophyta– vascular lineage developed into the seedless vascular and seed vascular plants (360 MYA)– seed vascular plants developed into the gymnosperms and angiosperms (130 MYA)

** plants can be divided into 2major categories1. non-vascular2. vascular – subdivided into 2 more categories:

a. seedlessb. seed

Land plants: characteristics

• 4 key derived traits found in plants:–1. alternation of generations & multicellular,

dependent embryos–2. walled spores produced in sporangia–3. multicellular gametangia–4. apical meristems

Mitosis

Spores

Mitosis

Mitosis

Zygote

Gametes

Haploid multicellularorganism (gametophyte)

Diploid multicellularorganism (sporophyte)

MEIOSIS FERTILIZATION

-sporophytes – multicellular, diploid, produce spores via meiosis-gametophytes – multicellular, haploid, produce gametes via mitosis

Land plants: 4 characteristics– 1. alternation of generations: alternation between multicellular haploid and diploid

stages in a life cycle• seen also in some chlorophytans – but not in the charophyceans• must be multicellular!!• alternates between the gametophyte (haploid) and sporophyte (diploid)• diploid sporophyte (mature plant) produces haploid spores via meiosis• mitotic division of the haploid spore produces a multicellular gametophyte

(reproductive organ) which is still haploid!!• the gametophyte produces haploid gametes by mitosis• gametes fuse via syngamy to produce the zygote• zygote grows via mitosis to develop a new sporophyte• in ferns (non-vascular plants) – the sporophyte and gametophyte have distinct phenotypic

appearances – but they are forms of the same species• in vascular plants – the gametophyte is microscopic

• 1. Alternation of generations and multicellular dependent embryos cont….– in a life cycle with alternation of

generations – the multicellular embryos develop from zygotes retained within the female gametophyte

– maternal tissue provides nutrients– embryos are called embryophytes– embryo receives nutrition during

development from placental transfer cells

• development of elaborate ingrowths of the plasma membrane and cell wall of the embryo

• lined with transfer cells• enhance the transfer of nutrients from

parent to embryo• analogous to the placental relationship

to the mather in eutherian animals

Multicellular,Dependent Embryos

Maternaltissue

Embryo

2 µm 10 µm

Wallingrowths

Placentaltransfercell (blue line)

Land plants: 4 characteristics

Walled SporesProduced in Sporangia

MulticellularGametangia

Longitudinal section ofSphagnum sporangium (LM)

Spores

Sporangium

Sporophyte

Gametophyte

Sporophyte and sporangiumof Sphagnum (a moss)

Archegonia and antheridiaof Marchantia (a liverwort)

Male gametophyte Antheridiumwith sperm

Female gametophyte

Archegoniumwith egg

– 3. walled spores in sporangia• developing within the diploid sporophyte are multicellular organs called sporangia

(singular = sporangium) – production of haploid spores via meiosis• within a sporangium are diploid cells called sporocytes or spore mother cells –

undergo meiosis to generate the haploid spores of the sporangium• the spores are protected by sporoporellin – key adaptation to terrestrial life

– 4. multicellular gametogangia• the haploid gametophyte undergoes production of gametes within multicellular

gametogania (singular = gametoganium) • the production of gametes is through mitotic division • female gametogania = archegonium - produces a single egg• male gametogania = antheridium – produces many flagellated sperm

ApicalMeristemof shoot

Developingleaves

Shoot Root

– 4. apical meristems• light and CO2 are available above ground, water and minerals are found

mainly in the soil• must be a way of collecting these components• plants do this by growing in length – through the production of stems and roots• apical meristem – localized regions of cell division located at the tips of

shoots and roots• e.g. shoot apical meristem – cells divide by mitosis and cytokinesis to

produce progenitor cells for the rest of the stem• progenitor cells are the source for the tissues of the stem and root:

– protoderm (epidermis, cork),– provascular tissue (xylem and phloem, vascular cambium) – ground meristem (pith and cortex)

Land plants: characteristics

Plant Diversification• plant fossils dating back to 475 MYA• one major way to distinguish groups of plants is to classify them as: vascular &

non-vascular– vascular tissue – extensive system formed by cells joined into tubes– conduct water and nutrients– those without these tubes – non-vascular plants

• bryophytes: term used to refer to all non-vascular plants– do not form a monophyletic group or clade– known popularly as the mosses, liverworts and hornworts– debate as to how they are related to each other– don’t possess the advanced adaptations of vascular plants (e.g. roots & leaves)– they do share many characteristic with vascular plants – see the slide on plant

characteristics• vascular plants: clade that includes 93% of all surviving plant species

– categorized into smaller clades:• 1. lycophytes – club mosses• 2. pteryophytes – ferns• 3. gymnosperms• 4. angiosperms

Ancestralgreen alga

Origin of land plants(about 475 mya)

Origin of vascular plants(about 420 mya)

Origin of seed plants(about 360 mya)

Land plants

Vascular plants

Seed plantsSeedless vascular plantsBryophytes

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Non-vascular plants

• commonly known as the bryophytes– even though Bryophyta is one of the 3 phyla in this group

• three phyla –– 1. Phylum Hepatophyta: liverworts

• gametophytes are flattened into a thalloid or a leafy shape– 2. Phylum Anthocerophyta – hornworts

• sporophyte can grow quite tall – sporangium along the length– 3. Phylum Bryophyta – mosses

• mosses are not to be confused with the vascular mosses – lycophytes• life cycle is dominated by the gametophyte stage

– gamete forming stage– gametophyte is only a few cells thick– anchored to the ground by rhizoids – long tubular single cells– NOT roots – not composed of tissues (cells only), lack specialized conducting

cells and are not responsible for water and mineral absorption• some mosses are NOT mosses at all – Irish moss (red seaweed), reindeer

moss (lichen), club mosses (seedless vascular plant)

Marchantia polymorpha = moss

Plagiochila deltoidea = liverwort

General Life Cycle: The Gametophyte

antheridium

archegonium

Gametophore offemale gametophyte

Marchantia polymorpha,a “thalloid” liverwort

FootSeta

Sporangium

500 µm

Marchantia sporophyte (LM)

• dominant stage in these three phlya• when bryophyte spores land on favorable habitats – germinate and grow into gametophytes• the spore develops into a threadlike protonema – covers a large surface area for absorption or water

and minerals• each protonema produces a bud with an apical meristem (stem-cell like tissue for growth)

– the AM generates gamete-producing structures known as gametophores– gametophore bears the male or female gametangia– the protonema + gametophore = gametophyte

• at the tip of the gametophore develops the reproductive structures = gametangia (singular = gametangium)– multiple gametangia develop on each plant– some bryophyte gametangia are bisexual – both antheridium and archegonium on the same gametophyte =

monoecious– most mosses have separate antheridium and archegonium located on separate gametophytes - dioecious

• “male and female” gametophytes

• production of the gametes by mitosis since the gametophyte is haploid already

General Life Cycle: The Sporophyte• in the plant kingdom – the sporophyte is the mature plant• fertilization is followed by development of the embryo within the archegonium• the embryo grows into a small sporophyte (diploid) - remains attached to the archegonium via a foot

– for absorption of nutrients• the sporophyte grows in length upward to produces a seta (stalk) • at the tip of this stalk forms a sporangium surrounded by a capsule• haploid spores develop in this sporangium via meiosis• in most mosses the upper part of the capsule forms a peristome – for gradual spore discharge• when the capsule matures – the peristome “pops” off and the spores are dispersed• from these spores comes new protonemata (singular = protonema)• hornwort and moss sporophytes tend to be large and more complicated• their sporophytes also have specialized pores = stomata

– support photosynthesis by allowing the exchange of CO2 and O2– also allow for the evaporation of water– also found in vascular plants

sporophyte

Malegametophyte

“Bud” Spores develop intothreadlike protonemata.

Protonemata“Bud”

The haploid protonemata produce “buds” that grow into gametophytes.

Raindrop

Sperm

Antheridia

Most mosses have separate male and female gametophytes, with antheridia and archegonia, respectively.

Egg

Haploid (n)

Diploid (2n)

Key

A sperm swims through a film of moisture to an archegonium and fertilizes the egg.

Archegonia

Rhizoid

Femalegametophyte

GametophoreSpores

Sporangium

Peristome

MEIOSIS

Meiosis occurs and haploid spores develop in the sporangium of the sporophyte. When the sporangium lid pops off, the peristome “teeth” regulate gradual release of the spores.

The sporophyte grows a long stalk, or seta, that emerges from the archegonium.

FERTILIZATION

(within archegonium)

Archegonium

Zygote

Embryo

Calyptra

Youngsporophyte

Attached by its foot, the sporophyte remains nutritionally dependent on the gametophyte.

The diploid zygote develops into a sporophyte embryo within the archegonium.

Capsule(sporangium)

Seta

Foot

Maturesporophytes

Capsule withperistome (SEM)

Femalegametophytes

Life Cycle of a Moss

http://www.sumanasinc.com/webcontent/animations/content/moss.html

Moss Life Cycle

The Economics of Moss• mosses have very lightweight spores• easy distribution has allowed for the establishment of

mosses around the globe• very common and diverse in moist forests and wetlands• can help retain nitrogen in the soil

– many species harbor cyanobacteria that increase the availability of nitrogen to the moss

• many species can survive drought and rehydrate when moisture reappears

• one wetland moss = Sphagnum or “peat moss”– peat moss = partially decayed remnants of Sphagnum– major component of partially decayed organic material

called peat– regions with thick layers of peat = peatlands (3% of Earth’s

surface)• peat contains 30% of world’s soil carbon• 450 billion tons of carbon is stored as peat

– Sphagnum does not decay easily – phenolic compounds in its cell walls

– peat – fuel source in northern Europe rather than wood– overharvesting of Sphagnum – could alter CO2 levels

globally

Sphagnum moss

Seedless Vascular Plants• bryophytes prominent during the first 100 million years of

plant evolution– but they are not very tall– rarely over 20 cm in height

• those plants that could achieve heights would have better access to sunlight, better spore dispersal

• height would mean the need for a transport system for water and nutrients

• would also need a structural support system• ferns are example of the evolution of plants that began to

develop height and a vascular system• fossils of present day vascular plants date back 425 MYA

Seedless Vascular Plants• 4 major characteristics of vascular plants:

– 1. dominant phase in the alternation of generations life cycle is the sporophyte

• e.g. ferns – the leafy plant is the sporophyte• the sporophyte becomes the larger and more complex stage of the life cycle • dramatic reduction in gametophyte stage – may be under the soil• sporophyte no longer dependent on the gametophyte for nutrition

– 2. development of vascular tissues – xylem and phloem• xylem – conduction of water and minerals

– included tracheids – dead, tube-shaped cells for the conduction of water and minerals up from the roots

– so vascular plants are often referred to as tracheophytes– water conducting cells contain a phenolic polymer – lignin– cells are said to be lignified– this permits vascular plants to grow tall – lignin strengthens the walls

• phloem – conduction of sugars and other nutrients– living cells– arranged into tubes for the distribution of sugars, amino acids and other

organic products

Seedless Vascular Plants• 4 major characteristics:

– 3. development of sporophylls: modified leaves that bear sporangia• vary in structure• two types: microphyll and megaphyll• e.g. in ferns – megaphylls with clusters of sporangia called sori• e.g. in lycophytes and gymnosperms – microphylls that form cone-like strobili

– most seedless vascular plants are homosporous – one type of sporangium that produces one type of spore

• this spore produces the two types of gametes = bisexual

– heterosporous species has two types of sporangia that develop into two types of spores

• megasporangium - megaspore = egg• microsporangium - microspore - sperm

Seedless Vascular Plants• 4 major characteristics of vascular plants:

– 4. development of roots and leaves• rather than rhizoids – the sporophytes of vascular plants have evolved roots• roots – organs for the anchorage of the plant & absorption of water and nutrients

– resembles the stem tissues of fossilized plants –evolved from them?

• leaves – organs for the increase of vascular surface area to capture more solar energy– the sporophyte has two types: either megaphylls or microphylls– megaphylls are larger and have a highly branched vascular system (of veins) running

through them» greater photosynthetic capacity

– microphylls are spine-like» supplied by a single, unbranched vein» appeared to have evolved first

Evolution of Leaves• evolution of microphylls from clusters of sporangia• evolution of megaphylls from an accumulation of

branches on a stem– one branch with overtopping growth – smaller branches flattened and fused to one another and to the

overtop branch

Seedless Vascular plants

• two clades: Phylum Lycophyta and Phylum Pterophyta• have modified leaves called sporophylls that bear sporangia • two types of sporophylls: microphylls and megaphylls• most seedless vascular plants are homosporous – one type of

sporophyll producing one type of spore that develops into a bisexual gametophyte

• Phylum Pterophyta – ferns, horsetails and whisk ferns– the pterophytes are divided by some botanists into separate phyla:

• phylum Sphenophyta – horsetails• phylum Psilophyta – whisk ferns and relatives• phylum Pterophyta – ferns• most recent information consider these groups now to be one

clade• Phylum Lycophyta – club mosses, spike mosses and quillworts

Phylum Lycophyta• club mosses, spike mosses and quillworts• 1200 species today• NOT true mosses since they have vascular tissue• most ancient line of vascular plants• microphyll line of evolution

– distinct line of evolution that came out of the first land plants– development of leaves from clusters of sporangia– earliest lycophytes formed primitive leaves = enations– enations were small (4 cm) and contained a single trace of

vascular tissue – also very effective at photosynthesis– enations are now called microphylls– “micro” refers to the evolution from small enations not their

size– evolution of true roots – increased the size of the sporophyte– sporangia became clustered into compact cones or strobili– many species evolved heterospory

• modern lycophytes grow on tropical trees as epiphytes – BUT they are NOT parasites

Diphasiastrum tristachyum, a club moss

Strobili(clusters ofsporangia)

epiphytic ferns

Phylum Pterophyta

opposite

whorled

alternate

• megaphyll line of evolution – development of leaves from a branching system of stems– seen in all seed plants, ferns and arthrophytes (horsetails)– telome theory: main stem with dichotomously branching

lateral stems– the lateral branches developed subdivisions – all on one

plant– the last lateral branches = telomes– during evolution - tissue grew in between (webbing) – the telomes also acquired spore-forming ability – positioning of the branches became very regular and

controlled– development of phyllotaxy – arrangement of leaves on a

stem• four basic patterns developed

– some lateral branches became arranged in a spiral pattern = spiral phyllotaxy

– some phyllotaxy developed an alternating or an opposite pattern

– others became more complicated (e.g. whorled pattern)

Phylum Pterophyta• ferns

– leaves are known as megaphylls– fern sporophyte is comprised of

underground, horizontal stems called rhizomes

– from these come vertical shoots that give rise to large leaves called fronds divided into leaflets

– frond grows as the fiddlehead• leaf primordial cells as they

grow curl inward– mature frond is called the

megaphyll– megaphyll is a compound leaf

with a center rachis and multiple leaflets

– although some fern species – e.g. staghorn fern – have a simple leaf structure

– gametophyte is very small and shrivels and dies after the young sporophyte develops

– the diploid sporophyte bears sporangia (singular = sporangium) clustered under the leaflets in structures called sori (singular = sorus)• a sporangium contains spore mother cells (2n)

– the spore mother cells undergo meiosis to produce spores (n)– these spores are for development of a haploid gametophyte

Phylum Pterophyta

Annulus and spore dispersal• http://www.youtube.com/watch?v=-

xF83pHEx6Q• the sporangium is stalked with

spring-like devices that disperse the spores = annulus

• annulus, a row of cells that bisects the sporangium like a sturdy spine.

• annulus walls are permeable to water

• as the sporangium dries, evaporating water is drawn out from the annulus, causing the cells to shrink – this pries the sporangium open

• the presence of water within the sporangium propels the spores out like a catapult

Spore

Sperm

Antheridium

Egg

Haploid (n)Diploid (2n)

Key

Younggametophyte

Sorus

Sporangium

MEIOSIS

FERTILIZATION

Archegonium

ZygoteNewsporophyte

Maturesporophyte

Sporangium

Gametophyte

Fiddlehead

Fern Life Cycle

• meiosis of spores within a sporangium• spore release from the sorus and germination• the spore develops into a young gametophyte – bisexual• bisexual gametophyte develops male and female gametogania

– male antheridium – female archegonium

• antheridium produces and releases sperm – swims to the egg within the archegonium – fertilization and development into a zygote

• the zygote develops into the diploid sporophyte – emerges from the gametophyte• growth of the sporophyte produces fronds or megaphylls

– young, developing frond is called the fiddlehead• gametophyte disappears• fronds contain sporangia for the production of spores (meiosis)• heterosporous species have megasporangium and microsporangium –production of distinct

spores for male and female gametophyte• almost all fern species are homosporous

http://www.youtube.com/watch?v=9c9Zi3WFVRc

Fern Life Cycle