An overview of Plant An overview of Plant EvolutionEvolution

Key Moments in the life of Kingdom Key Moments in the life of Kingdom PlantaePlantae

By Dr. Neil BuckleyBy Dr. Neil BuckleyBuckley, N. (n.d.). Plattsburgh State Faculty and Research Buckley, N. (n.d.). Plattsburgh State Faculty and Research

Web Sites. Retrieved May 21, 2015, from Web Sites. Retrieved May 21, 2015, from http://faculty.plattsburgh.edu/ http://faculty.plattsburgh.edu/

How did we get from here to there?How did we get from here to there?

Key “Moments” in Plant Key “Moments” in Plant EvolutionEvolution

• The Transition to LandThe Transition to Land• Development of Vascular SystemsDevelopment of Vascular Systems• Evolution of HeterosporyEvolution of Heterospory• Evolution of the SeedEvolution of the Seed• Diversification of the AngiospermsDiversification of the Angiosperms

The risks: Harsh environment…The risks: Harsh environment… Heat, dessication, damage by UV raysHeat, dessication, damage by UV rays

The rewards: Great opportunity…The rewards: Great opportunity… Plentiful COPlentiful CO22, sunlight, few competitors or , sunlight, few competitors or

herbivores.herbivores. The importance: paved the way for other The importance: paved the way for other

organismsorganisms

Food for herbivores; First soils!Food for herbivores; First soils!

1. The transition to Land - 1. The transition to Land - ca. 475 myaca. 475 mya

Ancestors of the PlantaeAncestors of the Plantae

The Plantae evolved from green algae, most The Plantae evolved from green algae, most likely a group called the charophytes. likely a group called the charophytes.

Evidence:Evidence: Plants and green algae contain chlorophyll b.Plants and green algae contain chlorophyll b. Chloroplasts of both have a similar structure in which Chloroplasts of both have a similar structure in which

thylakoid membranes are stacked as grana. thylakoid membranes are stacked as grana. Cell wall structure of both is very similar (about 22-Cell wall structure of both is very similar (about 22-

26% cellulose)26% cellulose) DNA sequence data supports close relationship DNA sequence data supports close relationship

between these groups.between these groups.

Challenges of living on landChallenges of living on land

Water is a supportive medium, air is not. Water is a supportive medium, air is not.

Algae are surrounded by a medium that Algae are surrounded by a medium that contains water and minerals and can take contains water and minerals and can take in their requirements across the whole in their requirements across the whole body.body.

Challenges of living on landChallenges of living on land

To survive on land a plant must:To survive on land a plant must: Avoid drying out.Avoid drying out. Be able to hold itself up.Be able to hold itself up. Possess differentiated tissues because air Possess differentiated tissues because air

and soil differ in composition and resources. and soil differ in composition and resources. Exploiting these different media requires Exploiting these different media requires specialized tissues.specialized tissues.

Solve the problem of reproducing outside Solve the problem of reproducing outside water.water.

Transition to landTransition to land

It is believed that ancestral charophytes lived in It is believed that ancestral charophytes lived in shallow water that sometimes dried out (as do shallow water that sometimes dried out (as do modern charophytes).modern charophytes).

Selection would have favored adaptations in Selection would have favored adaptations in these charophytes to resist drying out such as these charophytes to resist drying out such as waxy cuticles and protecting developing waxy cuticles and protecting developing embryos within layers of tissue. These embryos within layers of tissue. These preadaptations preadaptations facilitated the transition onto facilitated the transition onto land.land.

Reproduction on land Reproduction on land

Moving onto land required the Moving onto land required the development of new forms of reproduction.development of new forms of reproduction.

Algae shed their gametes into the water, Algae shed their gametes into the water, but on land gametes must be protected but on land gametes must be protected against desiccation.against desiccation.

Reproduction on landReproduction on land

Plants produce gametes within Plants produce gametes within gametangia (protective layers of tissue gametangia (protective layers of tissue that prevent gametes from drying out).that prevent gametes from drying out).

Egg is fertilized within female Egg is fertilized within female gametangium (called the archegonium) gametangium (called the archegonium) and embryo develops for some time inside and embryo develops for some time inside archegonium.archegonium.

EmbryophytesEmbryophytes

Retention of the developing embryo by Retention of the developing embryo by plants is a fundamental difference from plants is a fundamental difference from algae. Because this difference is so basic, algae. Because this difference is so basic, plants are sometimes described as plants are sometimes described as embryophytes.embryophytes.

Transition to landTransition to land

The ancestor of modern plants once The ancestor of modern plants once established on land had enormous established on land had enormous opportunities. opportunities.

No competition for sunlight or minerals No competition for sunlight or minerals and no herbivores.and no herbivores.

Selection rapidly led to a massive Selection rapidly led to a massive diversification of plants.diversification of plants.

2. Rise of Vascular plants2. Rise of Vascular plants

The first land plants lacked vascular tissue The first land plants lacked vascular tissue (as is true of most mosses today) so they (as is true of most mosses today) so they could not transport water, sugars or could not transport water, sugars or minerals around the plant.minerals around the plant.

Lack of vascular tissue also, of course, Lack of vascular tissue also, of course, limited the size of plants.limited the size of plants.

2. Rise of Vascular plants2. Rise of Vascular plants

Once the first plants moved onto land, selection Once the first plants moved onto land, selection quickly led to the development of specialized roots quickly led to the development of specialized roots and shoots.and shoots.

Roots and shoots required the development of a Roots and shoots required the development of a vascular system to move water and other vascular system to move water and other essentials around the plant and by about 400mya essentials around the plant and by about 400mya early vascular plants had begun to diversify. early vascular plants had begun to diversify.

Large ferns and other Large ferns and other seedless plantsseedless plants came to came to dominate the land in the Carboniferous Period.dominate the land in the Carboniferous Period.

3. Transition from homospory to heterospory

Homospory means spores are the same Homospory means spores are the same size and heterospory that microspores size and heterospory that microspores (male) and megaspores (female) differ in (male) and megaspores (female) differ in size.size.

Microspores develop into male Microspores develop into male gametophytes and megaspores into gametophytes and megaspores into female gametophytes.female gametophytes.

3. Transition from homospory to heterospory

Mosses and most ferns are homosporous. Mosses and most ferns are homosporous. Conifers and flowering plants are heterosporous.Conifers and flowering plants are heterosporous.

Homosporous plants produce spores that Homosporous plants produce spores that develop into bisexual gametophytes that develop into bisexual gametophytes that produce both sperm and eggs.produce both sperm and eggs.

For successful fertilization, homosporous plants For successful fertilization, homosporous plants need water in the form of rainfall when gametes need water in the form of rainfall when gametes are mature. are mature.

3. Transition from homospory to heterospory

Some homosporous plants evolved Some homosporous plants evolved heterospory.heterospory.

With heterospory in which the female With heterospory in which the female gametophyte is enclosed and protected gametophyte is enclosed and protected and there is no need for water to ensure and there is no need for water to ensure fertilization.fertilization.

Heterospory led to the evolution of seeds.Heterospory led to the evolution of seeds.

4. Evolution of the seed4. Evolution of the seed

In mosses the life cycle is dominated by the In mosses the life cycle is dominated by the gametophyte generation.gametophyte generation.

In ferns the sporophyte generation is dominant In ferns the sporophyte generation is dominant and the gametophyte is reduced, but still visible and the gametophyte is reduced, but still visible to the naked eye.to the naked eye.

In seed plants the gametophyte generation is so In seed plants the gametophyte generation is so reduced that in most cases it is microscopicreduced that in most cases it is microscopic

Alternation Alternation of of

GenerationsGenerations

4. Evolution of the seed4. Evolution of the seed

The reduction of size of the female The reduction of size of the female gametophyte has meant that it can be gametophyte has meant that it can be enclosed and protected within sporophyte enclosed and protected within sporophyte tissue (the ovule).tissue (the ovule).

The female gametophyte is not dispersed The female gametophyte is not dispersed and is protected from drying out and other and is protected from drying out and other hazards.hazards.

4. Evolution of the seed4. Evolution of the seed

The male gametophyte is what is The male gametophyte is what is dispersed in seed plants. It is also dispersed in seed plants. It is also protected by sporophyte tissue, the pollen protected by sporophyte tissue, the pollen grain.grain.

Pollen lands on the ovule and eventually Pollen lands on the ovule and eventually fertilizes egg produced by the female fertilizes egg produced by the female gametophyte. Embryo (sporophyte 2n) gametophyte. Embryo (sporophyte 2n) then develops.then develops.

Advantages of seedsAdvantages of seeds

Provides protection and nourishment for Provides protection and nourishment for developing embryo.developing embryo.

Dispersal: seeds can be dispersed more Dispersal: seeds can be dispersed more widely than spores by enclosing them in a widely than spores by enclosing them in a bribe (fruit) and having animals move bribe (fruit) and having animals move them.them.

Dormancy: the developing embryo is Dormancy: the developing embryo is protected and can wait a long time to protected and can wait a long time to germinate when conditions are good.germinate when conditions are good.

Seeds vs sporesSeeds vs spores

Seeds are better than spores because Seeds are better than spores because spores have a short lifetime. spores have a short lifetime.

Spores are thinner walled and more Spores are thinner walled and more vulnerable to pathogens and damage.vulnerable to pathogens and damage.

Angiosperm diversificationAngiosperm diversification

The angiosperms have been enormously The angiosperms have been enormously successful. successful.

There are now about 235,000 species in There are now about 235,000 species in comparison to just over 700 comparison to just over 700 gymnosperms.gymnosperms.

Flowers and fruitFlowers and fruit

The key to the success of the The key to the success of the Angiosperms has been that they have Angiosperms has been that they have evolved flowers and fruit.evolved flowers and fruit.

Fruit protects the seeds and aids in their Fruit protects the seeds and aids in their dispersal.dispersal.

The fruit is a bribe. Animals eat the fruit The fruit is a bribe. Animals eat the fruit and spread the seeds.and spread the seeds.

Flowers and pollinationFlowers and pollination

A major advantage of flowers is that they have A major advantage of flowers is that they have allowed angiosperms to use other organisms to allowed angiosperms to use other organisms to move their pollen about.move their pollen about.

Bees, bats, birds and others all transport pollen. Bees, bats, birds and others all transport pollen. They are attracted to flowers by the nectar and They are attracted to flowers by the nectar and pollen [bribes] provided by the plant and when pollen [bribes] provided by the plant and when they visit multiple flowers they move pollen from they visit multiple flowers they move pollen from one to the nextone to the next