Plant Diversity II: Evolution by Seed Plants

• cyanobacteria on land – 1.2 billion years ago• 500 MYA – colonization by plants• closest relatives of land plants = charophyceans

• molecular comparison of both nuclear and chloroplast genes confirms morphological and biochemical conclusions that the charophyceans are ancestors of plants

• plant share characteristics with other more primitive organisms– multicellular, eukaryotic– photosynthetic autotrophs – brown, red, green algae– cell walls made of cellulose – green algae, dinoflagellates, brown algae– chloroplasts with chlorophyll a and b – green algae, euglenids and a few

dinoflagellates

• A reminder: four unique traits seen in plants and only charophyceans– 1. rose-shaped complexes for cellulose synthesis –called

rosettes• synthesize cellulose microfibrils for the cell walls

– 2. peroxisome enzymes – peroxisomes contain enzymes that help minimize the loss of organic products as a result of photorespiration

– 3. flagellated sperm – some species of land plants have flagellated sperm

– 4. formation of a phragmoplast – involved in the synthesis of new cell walls during mitosis - via the formation of new cross walls called cell plates

Defining the Plant Kingdom• traditional classification schemes equates the plant

kingdom with the presence of embryophytes• vascular plants form a clade – 93% of all plant species

– categorized into three smaller clades• 1. lycophytes – club mosses and relatives• 2. pterophytes – ferns and relatives• 3. seed vascular plants

– A. gymnosperms - “naked seed” plants– B. angiosperms – flowering plants

• four key traits define plants – absent in charophyceans– 1. apical meristems– 2. alternation of generations & multicellular, dependent embryos– 3. walled spores in sporangia– 4. multicellular gametogangia

Seed plants

– three key reproductive adaptations evolved in seed plants:

– 1. increasing dominance of the sporophyte generation – reduced gametophyte

– 2. advent of the seed – ovules and eggs– 3. evolution of pollen as an airborne agent

• gametophytes of mosses and ferns are the dominant stage

• gametophytes of seed plants are mostly microscopic

• miniaturization allows for the development of their gametophytes within the sporangium of the parental sporophyte

• protects the delicate egg-forming gametophyte from environmental stress

• moist environment of the sporophyte shields the gametophytes from drought and UV radiation

• also allows the growing gametophyte to derive nourishment directly from the sporophyte

1. Reduced Gametophytes

Sporophyte(2n)

Gametophyte(n)

Sporophyte dependent on gametophyte (mosses and other bryophytes)

Gametophyte(n)

Sporophyte(2n)

Large sporophyte and small, independent game-tophyte (ferns and other seedless vascular plants)

Microscopic femalegametophytes (n) inovulate cones(dependent)

Microscopic malegametophytes (n) ininside these partsof flowers(dependent)

Sporophyte (2n),the flowering plant(independent)

Microscopic malegametophytes (n)in pollen cones(dependent)

Microscopic femalegametophytes (n) ininside these partsof flowers(dependent)

Sporophyte (2n),(independent)

Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms)

Bryophytes Seedless Vascular

SeedVascular

Mosses and Ferns vs. Seed plants• mosses:

– development of a sporangium – releases spores for development into gametophyte• homosporous – one type of spore that ends up developing into female and male gametophytes

– development of a protonema with buds – develop into large gametophytes (moss)– most mosses have separate female and male gametophytes (bisexual gametophyte)

• some gametophytes bear both male and female gametangia• multiple gametangia per gametophyte plant

– sperm are released (flagellated) and they fertilize the egg developing within the archegonium – development into a new sporangium and release of spores

• vascular seedless plants:– development of a sporangium – releases spores for development into gametophyte

• homosporous – one type of spore that ends up developing into female and male gametophytes– development of the bisexual gametophyte bearing multiple archegonium and antheridium – fertilization of the egg within the archegonium - development of the sporophyte– development into large sporophytes (plant) – sporophyte bears multiple microphylls (leaves) bearing spore-

forming structures on the underside – sporangium in clusters called sori• in gymnosperms and angiosperms – vascular seed plants:

– development of the sporophyte (plant) bearing separate female and male sporangia located in specialized reproductive structures – e.g. cones or flowers

– development of a sporangium – releases spores for development into gametophyte• heterosporous – development of distinct spores that develop into either male or female• microsporangium – microspore – male gametophyte • megasporangium – megaspore – female gametophyte

– microscopic gametophytes bearing either archegonium or antheridium (unisex gametophyte)– fertilization of the egg within the archegonium - development of the sporophyte

Heterospory• the rule among seed plants• nearly all nonvascular plants are homosporous – produce

one kind of spore which gives rise to a bisexual gametophyte

• with the evolution of seed plants – development of heterospory– megasporangium located on leaves called megasporophylls

produce megaspores – female gametophytes– microsporangium located on leaves called microsporophylls –

produce microspores – male gametophytes– both are found on specialized reproductive structures

• e.g cones, flowers

• these sporangium can either be located on the same plant = monoeicous

• or they can be located on “male” and “female” plants = dioecious

2. Ovaries & Seeds

Integument

Megasporangium(2n)Megaspore (n)

Unfertilized ovule Fertilized ovule

Spore wall

Male gametophyte(within germinatingpollen grain) (n)

Micropyle

Femalegametophyte (n)

Egg nucleus (n)

Dischargedsperm nucleus (n)Pollen grain (n)

Seed coat(derived fromintegument)

Embryo (2n)(new sporophyte)

Gymnosperm seed

Food supply(femalegametophytetissue) (n)

• seed plants are unique in that the megasporangium is retained within the parent sporophyte

• the megasporangium containing the developing megaspore is surrounded by layers of sporophyte tissue called integuments– in gymnosperms – the megaspore is surrounded by only one integument– angiosperms usually have two integuments

• the megaspore + megasporangium + integuments = ovule• inside each ovule is a future female gametophyte that develops from the

megaspore– gametophyte can produce one or more egg cells within the ovule

2. Ovaries & Seeds

Integument

Megasporangium(2n)Megaspore (n)

Unfertilized ovule Fertilized ovule

Spore wall

Male gametophyte(within germinatingpollen grain) (n)

Micropyle

Femalegametophyte (n)

Egg nucleus (n)

Dischargedsperm nucleus (n)Pollen grain (n)

Seed coat(derived fromintegument)

Embryo (2n)(new sporophyte)

Gymnosperm seed

Food supply(femalegametophytetissue) (n)

• seed = ovule after fertilization - development into the embryo– seed = embryo + food supply + seed coat (from the integuments) – allow for the developing embryo to resist harsh conditions– multicellular structure - in contrast to the spore

• evolutionary advantage of seeds:– until seeds – the spore was the only protective stage in the life cycle– spores are more “hardy” than the parental plant

• e.g. moss spores survive better in harsh conditions better than the moss

– but most spores can only survive under certain environmental conditions– unlike spores – seeds carry their own food supply– unlike spores - a seed can remain dormant for years following its release

3. Evolution of Pollen

• the microsporangium produces microspores• microspores develop into pollen grains • a pollen grain contains the male gametophyte enclosed within a

pollen wall• outer wall is made by the sporophyte, inner wall is made by the

gametophyte within– outer wall = protected by a coating of sporopollenin– composition of sporopollenin is still not known!– designed to protect the pollen grain on its “travels”

• transfer of pollen to the ovule = pollination• pollen grains are carried away from the parent plant by wind,

insects• or they can travel to the female reproductive structures within

the same sporophyte• in order to fertilize, the pollen grain begins to germinate (grow)

and produces a pollen tube – allows for the discharge of two sperm (gametes) into the ovule – unites with the egg developing within female gametophyte (within the ovule)

• in non-vascular plants (bryophytes) and seedless vascular plants (ferns) – the sperm is flagellated and swims to the female gametophyte in order to fertilize the egg which is also free living

• in vascular seed plants – the female gametophyte produces an egg which never leaves the sporophyte ovule

Gymnosperms• “naked seed” – seeds are not enclosed in ovaries• seeds are exposed on modified leaves (sporophylls) that form

cones– in the ferns – development of sporophyll leaves that bear the

sporangium (microphylls and sori)– in gymnosperms – development of modified sporophylls that cluster

together to form cones or strobili• 380 MYA – development of heterosporous trees with woody stems

– but did not bear seeds = progymnosperms• first seed plant in the fossil record – 360 MYA

– now extinct• earliest fossils of gymnosperms – 305 MYA• drier environment favored gymnosperms over the bryophytes and

ferns– start to see a die off of many of the bryophyte and fern species

• gymnosperms with their thick cuticles and reduced leaves as needles – adapted well to the dry climates

• 251 MYA – boundary between the Paleozoic (“old life”) and Mesozoic (“middle life”) eras

• early Mesozoic era - domination by gymnosperms• toward the end of the Mesozoic (65 MYA) – increased dominance

of angiosperms in some ecosystems• most common existing gymnosperms are the conifers – spruce, pin,

fir and redwood

• four gymnosperm phyla: Cycadophyta, Ginkgophyta, Gnetophyta and Coniferophyta– Phylum Cycadophyta – cycads

• second largest group of gymnosperms• large cones and palmlike leaves• 130 species survive

– Phylum Ginkgophyta - ginkos• only one species left – Ginkgo biloba• deciduous leaves - fanlike formation• tolerates air pollution well• trees bear fleshy seeds that smell rancid

– Phylum Gnetophyta – three genera alive today• tropical and desert species• Gnetum – 35 species of tropical trees, shrubs and vines (Africa and Asia)• Welwitschia – one species, Welswitchia (Africa)• Ephedra – 40 species, desert shrubs

– produce the compound called ephedrine– Phylum Coniferophyta – largest group

• “cone-bearing”• 600 species of conifers• many are large trees• most are evergreens – retain their leaves throughout the year

Gymnosperms

Cycas revoluta Ginko biloba

Ephedra.

Welwitschia mirabilis.

Phylum Coniferophyta• also called Division Coniferophyta• 575 species• largest genus – Pinus• leaves of conifers are always simple needles or scales• pine leaves – needle-like

– arranged in clusters or bundles of two to five leaves each bundle

– cluster = fascicle– fascicle forms a cylindrical rod if the needles are held together– needle is comprised of a outer epidermis coated with a thick

cuticle– below that is one to two layers of cells = hypodermis– stomata are recessed in sunken cavities– veins and associated tissues run down the center of the needle

and are surrounded by an endodermis– also contain resin canals – occur in other parts of the pine– these canals are lined with special cells that secrete a resin –

aromatic and antiseptic• combination of turpentine and a waxy rosin• rosin prevents water loss and fungal attacks• deters insects• although most pine roots have mycorrhizal fungi • fossilized resin = amber

Life Cycle: The Pine

• pine tree is the sporophyte • sporangia are located on scale-like leaves

(sporophylls) packed into cones – strobili (single = strobilus)

• two types of cones produce two types of spores– small pollen cones produce microspores – pollen– larger ovulate cones produce megaspores – egg– ovulate cones also known as seed cones – most are woody– but those of the juniper can resemble a fruit (berry)

Life Cycle: The Pine

Maturesporophyte(2n)

Pollencone

Microsporocytes(2n)

Longitudinalsection of pollen cone

Ovulatecone

Longitudinalsection ofovulate cone

Micropyle

Ovule

Key

Haploid (n)Diploid (2n)

Megasporocyte (2n)Integument

MegasporangiumGerminatingpollen grainPollen

grains (n)(containing malegametophytes)

MEIOSIS

MicrosporangiumSporophyll

MEIOSIS

Germinatingpollen grain

Femalegametophyte

Archegonium

Egg (n)

Germinatingpollen grain (n)

Integument

Dischargedsperm nucleus (n)

Pollentube

Egg nucleus (n)FERTILIZATIONEmbryo(new sporophyte)(2n)

Food reserves(gametophytetissue) (n)

Seed coat(derived fromparentsporophyte) (2n)

Seedling

Seeds on surfaceof ovulate scale

Survivingmegaspore (n)

• pollen cones: bear modified leaves or sporophylls each containing 2 microsporangia– the microsporangium is comprised of cells

called microsporocytes (2n) – microsporocytes are also known as

microspore mother cells– microsporocytes divide by meiosis to form

pollen grains which are haploid– pollen grains contain the male

gametophyte – for the production of sperm– grains travel to the ovulate cone where it

begins to germinate and forms a pollen tube through which the sperm will travel

– pollen tube “digests” its way into and through the female reproductive structure through an opening called a micropyle

– development results in the production of 2 sperm cells within the pollen tube

Pollen Cones

Male Pine Cone

microspore(pollen)

microsporan-gium

microsporophyll

• considered to be simple cones– one single stem axis bearing

microsporophylls• cones typically occur in clusters near

the ends of branches• pollen is liberated to the wind and

blown away• pollen has one cell and two large air

bladders that increase its buoyancy in air

• wind dispersal is inefficient – so few pollen grains actually land on the ovulate cone

• but conifer forests are very dense

Life Cycle: The Pine• ovulate cones: bear modified leaves - each contains two ovules

• each ovule contains one megasporangium– the megasporangiumcontains a cell called a

megasporocyte (2n) – megasporocytes are also known as megaspore

mother cells (2n)– the megasporocyte undergo meiosis to form 4

haploid cells– only one survives as the megaspore (n)– the remaining degenerate– the surviving megaspore develops into the female

gametophyte – the female gametophyte develops two or three

separate archegonia - each will form an egg– as the eggs mature – the pollen tube is developing

its two sperm cells– the eggs and sperm mature at the same time– all eggs may be fertilized by the sperm – only one

egg nuclei with fuse with a sperm nuclei to form the zygote

– the ovule is now the seed & its developing embryo is retained within the female gametophyte

Maturesporophyte(2n)

Pollencone

Microsporocytes(2n)

Longitudinalsection of pollen cone

Ovulatecone

Longitudinalsection ofovulate cone

Micropyle

Ovule

Key

Haploid (n)Diploid (2n)

Megasporocyte (2n)Integument

MegasporangiumGerminatingpollen grainPollen

grains (n)(containing malegametophytes)

MEIOSIS

MicrosporangiumSporophyll

MEIOSIS

Germinatingpollen grain

Femalegametophyte

Archegonium

Egg (n)

Germinatingpollen grain (n)

Integument

Dischargedsperm nucleus (n)

Pollentube

Egg nucleus (n)FERTILIZATIONEmbryo(new sporophyte)(2n)

Food reserves(gametophytetissue) (n)

Seed coat(derived fromparentsporophyte) (2n)

Seedling

Seeds on surfaceof ovulate scale

Survivingmegaspore (n)

Seed cones• more complex than pollen cones• compound cones• each consists of a cone axis with axillary buds• the buds bear leaves that are called bracts

rather than sporophylls• associated with each bract are fused

megasporophylls that form a scale (ovuliferous)

bract

megasporophyll

Pollination & The Embryo

• conifer pollen arrives before the egg is mature• more than a year may pass between pollination & fertilization• mitosis within the pollen produces three cells – 2 small

(degenerate) and one large cell• this large cell divides to form: a generative cell and a tube

cell• the tube cell elongates to form the pollen tube• the generative cell forms 2 sperm• following fertilization – the zygote does not immediately form

– the first cells to form elongate as a suspensor – it pushes the other cells deep into the megasporocyte – these will form a

proembryo – will become the embryo• embryonic development is similar to angiosperms• the embryo combined with the integuments (derived from the

ovule) and its food source – known as the seed• so the seeds are also borne on the cone• germination of a new seed – new sporophyte• from the time pollen and ovulate cones appear - takes three to

four years for the male and female gametophytes to be produced!!!

• seeds of nearly all species of pines are edible• high protein content in the seed

germinating pollen grainswith pollen tube

air bladder

generative cell

tube cell

Life Cycle: The Pine