Lecture 22: Evolution of Plants

Covers Chapter 21

Taxonomy• Branch of biology concerned with naming and

classifying organisms• 8 major categories and ranks:– Domain– Kingdom– Phylum– Class– Order – Family – Genus– Species

Taxonomy

Example of taxonomy

How do we know where to place an organism?

• Anatomy plays a key role in taxonomy (organisms that are anatomically similar are placed in the same or closely related categories)

• DNA similarities are also useful

Domains• At first, it was thought there were only TWO

domains: plantae and animalia (bacteria were considered plants!)

• Then came the realization of the differences between procaryotes and eucaryotes (no nucleus vs nucleus)

• Bacteria and Archaea were moved to their own domains, and the domain of eukarya was created.

• (Eukarya was later split into Animalia, Protists, Fungi and Plants!)

Plants

• The first thing you notice: plants are green!• *Due to chlorophyll: molecule (a pigment)

that carries out photosynthesis. • Pigment: a material that changes the color of

reflected or transmitted light as the result of wavelength-selective absorption (more in lecture 23)

Chlorophyll

Chemical Structure of Chlorophyll

But….some protists, bacteria and archaea all have chlorophyll!

• The fact that plants do photosynthesis is not unique to them….other organisms also perform photosynthesis.

• So how are plants unique?• *Answer: THE MULTICELLULAR EMBRYO • *Multicellular Embryo: Just like humans, plant

reproduction results in an embryo that is attached to and dependent upon its parent as it grows and develops. Protists, bacteria and archaea do not!

So, what are the ancestors of modern plants?*

• It is now believed that plants evolved from photosynthetic protists like the green algae (stoneworts) that still live in the ocean today.

• Stoneworts are the closest living relative to modern plants.

• Why? – DNA in modern plants and green algae are similar– Both use chlorophyll for photosynthesis– Both store food as starch– Both have cell walls made of cellulose

Stonewort

How exactly did plants evolve?*• Green algae:– Lives in water– Lacks true roots, stems, leaves– Lacks complex reproductive system (flowers, fruit)– Requires water to facilitate reproduction

• As evolution continued and plants moved on land, they needed to solve these problems:– Survive without living IN the water– 1.) Gravity (need structural support to live on land)– 2.) Prevent body from drying out– 3.) Find a way to get nutrients– 4.) Find a way to reproduce outside of the water

1.) Gravity (structural support)*

• Roots: anchor plant in the ground• Tissues evolved that contain rigid cell walls to

allow plant to support itself (more later in anatomy lecture)

2.) Prevent body from drying*

• Cuticle: waxy substance that covers leaves to prevent evaporation

• Stomata: pores that can open for gas exchange and close to prevent drying

3.) Getting nutrients and water*

• Roots: have the ability to absorb nutrients and water from soil

• Xylem & Phloem: specialized plant cells that can transport water and nutrients to all parts of plant (not all plants have X & P, however)

4.) Reproduction on land*

• Seed: protection and nourishment of embryos in dry land environments

• Pollen: grains that contain male gametes (sperm), can be dispersed by wind

• Flowers: attract pollinators…get other organisms to carry pollen to other plants

• Fruit: attracts organisms to eat fruit and poop out seeds

Modern Plants (see handout)*

Timeline of evolution of plants