Chapter 17 – Introduction to Plants

Plant Evolution and Diversity:

The Colonization of Land

Plant EvolutionPlants: multicellular, eukaryotic, photosynthetic, autotrophs

1. Nonvascular Plants (Bryophytes)2. Vascular Plants (Tracheophytes)

• Seedless : club mosses and ferns• Seed : Gymnosperms (cones) and

Angiosperms (flowers)Terrestrial colonization needs:• Support – vascular tissue and lignin• Absorption (above & below ground)• Vascular tissue for transport• Waxy cuticle for water retention• Stomata for gas exchange• Gametangia for reproduction

non-vascular

seedless vascular

seeded vascular

hornwort

liverwort

moss

club moss

fern

Plant Origins:

• Charophyceans: green algae (closest living plant ancestor)

• Similarities:

1. Molecular similarities in nuclear and chloroplast genes: chlorophyll a & b

2. Biochemical similarity in cellulose composition; peroxisome enzymes

3. Cell division similarity in mitosis; cytokinesis

4. Sperm similarity

Green Algae versus PlantsGreen Algae

– Supported by water, anchored by holdfast

– Whole body is exposed to water, light & minerals

– Lack organs with specialized functions

– Lack apical meristems, mycorrhizae, cuticle, stomata and vascular tissue

– Embryo develops in water

– Lack seeds and spores

Plants– Leaves and stems

usually contain rigid supporting elements

– Above ground parts exposed to light, roots in contact with soil and nutrients

– Discrete organs (roots, stems and leaves)

– Have apical meristems, mycorrhizae, cuticle, stomata and vascular tissue

– Walled spores develop in sporangium

– Embryo develops in female gametangium

– Many have resistant spores/seeds that aid in dispersal

– Exhibit ALTERNATION OF GENERATIONS

Alternation of Generations

Alternation of Generations: lifecycle in which there is both a multi-cellular diploid (2n) form (the sporophyte) and a multi-cellular haploid (n) form (the gametophyte)

(2n) Sporophyte – makes spores by meiosis

(n) Gametophyte – makes gametes by mitosis

** Unlike animals, the diploid and haploid stages of plants are distinct, multi-cellular generations.

** These 2 generations alternate in producing each other spore = (n) cell that grows into a new organism without fusing with another

Moss LifecycleGametophyte : Dominant Generation1. Gametes develop in gametangia

(archegonia = female, antheridia = male) usually on separate plants. Sperm swim through film of water to egg in archegonium. Fertilization occurs – zygote remains in archegonium.

2, 3. Zygote divides by mitosis sporophyte embryo mature sporophyte (which remains attached to gametophyte)

4. Meiosis occurs at tips of sporophyte stalks

* haploid spores are released from sporangium

5. Spores undergo mitosis and develop into gametophyte plants

Fern LifecycleSporophyte – Dominant

Generation (most plants)

1. Fern gametophyte (heart shaped – prothallus) sperm requires moisture to reach egg. Fertilization occurs.

2. Zygote remains on gametophyte

3. Zygote develops into independent sporophyte by mitosis

4. Mature sporophyte contains clusters of sporangia – which undergo meiosis

5. Haploid spores are released

1. Develop into gametophytes

Gymnosperm LifecycleMale gametophyte – microspore, Female gametophyte - megaspore

• Sporophyte – Dominant Generation

1. Ovulate cone (female) hard and woody, contains scales; each with pair of ovules

2. Pollen cone (male) smaller, contain sporangia – make haploid spores by meiosis

Male gametophytes (pollen grains) develop from spores

3. Pollination occurs when pollen grains enter ovule

4. One surviving haploid spore develops into female gametophyte

Months later – eggs appear in female gametophyte

5. Tube grows out of pollen grain and releases sperm near egg

Fertilization occurs nearly a year after pollination

6. Eggs are fertilized, one zygote develops fully into a sporophyte embryo

Ovule transforms into the seed (contains food supply) and has a tough seed coat

Seeds are shed about 2 years after pollination

7. Seed germinates and embryo grows into pine seedling

Angiosperm Lifecycle

• Sporophyte – Dominant Generation

1. Meiosis in anthers spores undergo mitosis male gametophyte (pollen grains)

2. Meiosis in ovule spores undergo mitosis forms few cells of female gametophyte (one becomes egg)

3. Pollination – pollen grain lands on stigma– Tube grows from pollen grain to

ovule and fertilization occurs

4. Zygote forms

5. Seed develops from each ovule

6. As seed develops, ovary wall thickens forming the fruit that encloses the seeds

7. Seed germinates mature sporophyte

Flower Anatomy

• Floral organs: sepals, petals, stamens, carpels

• Complete flower – all 4 parts• Incomplete flower – lacking 1 or

more organ• Perfect flower – both stamens and

carpels on 1 flower – (Monoecious)• Imperfect – lacking either a stamen or

carpel – (Dioecious)• Sepals – enclose flower before it

opens (calyx – complete circle of sepals)

• Petals – attract pollinators (corolla - complete circle of petals)

• Stamens – male: anther (produces pollen) filament (stalk)

• Carpels (pistil) – female: stigma (sticky, traps pollen), style (stalk between stigma and ovary), ovary (protective chamber containing one or more ovules)

• Receptacle – where stem and flower leaves meet

*sepals and petals = accessory organs *carpels and stamens = essential organs

Important Medications Derived From Plants

Flowers: the sex organs of angiosperms* site of pollination and fertilization* generate fruits with seeds inside* contain gametophytes

(ovules and pollen grains)* fruit is a great seed dispersal in fruit

(via animals)* flower is a great pollen dispersal mechanism

flower’s color/nectar/scent (via insects, birds)Agriculture: many a plant product* gymnosperms: lumber, paper* angiosperms: almost all food, spices, meds,

coffee, chocolate

Co-evolution of plants and animals- a selective process:

• bees drawn to flowers with markings that reflect UV light

• hummingbirds drawn to flowers with nectar in cone-shaped tube

• bats drawn to nocturnal flowers with light colors, strong scent (easy to find)

Plant Diversity: a non-renewable resource:

• massive clearing of forests• 25% of Rx drugs made from

plants• ~50 mill acres cleared yearly

(size of WA)• 40% decrease in America’s

forests (habitation, paper,lumber)

• 20% of rainforests cleared in 20th century