20.1 Origins of Plant Life KEY CONCEPT Plant life began in the water and became adapted to land.

20.1 Origins of Plant Life

KEY CONCEPT Plant life began in the water and became adapted to land.


Land plants evolved from green algae. • Plants and green algae have many common traits.

– both are photosynthetic eukaryotes – both have the same types of chlorophyll – both use starch as a storage product – both have cell walls with cellulose


• Genetic analysis points to the common ancestor of all plants.

– extinct green algae species in class Charophyceae– modern charophyceans common in lakes and ponds


• Important plant characteristics likely originated in charophyceans.– multicellular body allowing for specialization of

cells and tissues

– cell division that allows for chemical communication between cells

– reproduction involving sperm swimming to egg


– Ancestral charophyceans lived in areas of shallow water.

• True plants evolved through natural selection.

– Those that could survive longer dry periods were favored.

– First true plants probably grew at edges of water.– True plants have embryos that develop while attached

to female parent.


• True plants evolved through natural selection.


Plants have adaptations that allow them to live on land. • Challenges of living on land have selected for certain

plant adaptations.• A cuticle allows plants to retain moisture.

– waxy, waterproof layer

– holds moisture in


• Stomata are tiny holes in the cuticle.

stoma

– can open and close (regulated by guard cells)– allow air to move in and out


• A vascular system allows resources to move to different parts of the plant.

sugars

water and mineral nutrients

– collection of specialized tissues– brings water and mineral nutrients up from roots

– disperses sugars from the leaves – allows plants to grow higher off the ground


• Lignin allows plants to grow upright.

– hardens cell walls of some vascular tissues– provides stiffness to stems

plant cells

lignin


• Pollen grains allow for reproduction without free-standing water.

– pollen grains contain a cell that divides to form sperm

– pollen can be carried by wind or animals to female structures


• A seed is a storage device for a plant embryo. – seed coats protect

embryos from drying wind and sunlight

– embryo develops when environment is favorable


Plants evolve with other organisms in their environment. • Plants and other organisms can share a mutualistic

relationship. – a mutualism is an interaction in which two species

benefit– plant roots and certain fungi and bacteria – flowering plants and their animal pollinators


• Plants have adaptations that prevent animals from eating them.

– defensive chemicals

– spines and thorns


KEY CONCEPT Plants have specialized cells and tissue systems.

21.1 Plant Cells and Tissues


Plant tissues are made of three basic cell types.

• Parenchyma cells are the most common plant cell type. – store starch, oils and

water– help heal wounds to

the plant – have thin flexible walls



– they are strong and flexible.– celery strings are strands of collenchyma.– they have unevenly thick cell walls.

• Collenchyma cells provide support to a growing plant.



– second cell wall hardened by lignin– die when they reach maturity – used by humans to make linen and rope

• Sclerenchyma cells are the strongest plant cell type.



Plant organs are made of three tissue systems.

• Dermal tissue covers the outside of a plant. – protects the plant – secretes cuticle of leaves – forms outer bark of trees



– provides support – stores materials in roots and stems – most commonly made of parenchyma

• Ground tissue is found inside a plant.



stem

leaf

root

– two networks of hollow tubes

– xylem transports water and minerals

– phloem transports photosynthetic products

• Vascular tissue transports water, minerals and organic compounds.



KEY CONCEPT The vascular system allows for the transport of water, minerals, and sugars.

21.2 The Vascular System


Water and dissolved minerals move through xylem.

• Xylem contains specialized cells. – vessel elements are short and wide– tracheid cells are long and narrow – xylem cells die at maturity

vesselelement

tracheid



– Plants passively transport water through the xylem.– Cohesion is the tendency of water molecules to bond

with each other.

• The cohesion-tension theory explains water movement.

– Adhesion is the tendency of water molecules to bond with other substances.



– absorption occurs at roots

• Water travels from roots to the top of trees.

– cohesion and adhesion in xylem– transpiration at leaves



– water vapor exits leaf stomata– helps pull water to the top

branches

• Transpiration is the loss of water vapor through leaves.



Phloem carries sugars from photosynthesis throughout the plant.

• Phloem contains specialized cells. – sieve tube elements have

holes at ends– companion cells help sieve

tube elements – unlike xylem, phloem tissue is

alive



– plants actively transport sugar from the source– sugar flows to the sink due to pressure differences

sugars

phloem xylem

water

Sugars move from their source, such as photosynthesizing leaves, into the phloem.

1

The sugars move into the sink, such as root or fruit, where the are stored.

3

Water moves from the xylem into the phloem by osmosis, due to the higher concentration of the sugars in the phloem. The water flow helps move sugars through the phloem.

2

• The Pressure-flow model explains sugar movement.



KEY CONCEPT Reproduction of flowering plants takes place within flowers.

22.1 Plant Life Cycles


Plant life cycles alternate between producing spores and gametes.

• A two-phase life cycle is called alternation of generations. – haploid phase – diploid phase – alternates between

the two

fertilizationfertilization

meiosismeiosis

SPOROPHYTE PHASE

GAMETOPHYTE PHASE



– sporophyte phase is diploid – begins with fertilized egg – spores produced through

meiosis

• The gamete-producing plant is the mature gametophyte.

• The spore-producing plant is the mature sporophyte.

– gametophyte phase is haploid– begins with spore – gametes produced through

mitosis



sepal

Flowers contain reproductive organs protected by specialized leaves.

• Sepals and petals are modified leaves.– Sepals are outermost

layer that protectsdeveloping flower

22.2 Reproduction of Flowering Plants


– Petals can help to attract animal pollinators

petal



• A stamen is the male structure of the flower.

– anther produces pollen grains – filament supports the anther

stamen

filament anther



carpel style

stigma

ovary

• The innermost layer of a flower is the female carpel.

– stigma is sticky tip – style is tube leading from stigma to ovary – ovary produces female gametophyte



Flowering plants can be pollinated by wind or animals.

• Flowering plants pollinated when pollen grains land on stigma.

• Wind pollinated flowers have small flowers and large amounts of pollen.



• Animal pollinated flowers have larger flowers and less pollen.

pollen grains

– many flowering plants pollinated by animal pollinators

– pollination occurs as animal feeds from flower to flower– animal pollination more efficient than wind pollination



Fertilization takes place within the flower.

• Male gametophytes, or pollen grains, are produced in the anthers. – male spores produced in

anthers by meiosis– each spore divides by

mitosis to form twohaploid cells

– two cells form asingle pollen grain pollen grain



– four female spores produced in ovule by meiosis – one spore develops into female gametophyte

- female gametophyte contains seven cells · one cell has two nuclei, or polar nuclei · one cell will develop into an egg

• One female gametophyte can form in each ovule of a flower’s ovary.



• Pollination occurs when a pollen grain lands on a stigma.

pollen tubepollen tube

spermsperm

stigmastigma

– one cell from pollen grain forms pollen tube– other cell forms two sperm that travel down tube



• Flowering plants go through the process of double fertilization.

femalegametophyte

ovule

egg

sperm

polar nuclei



endosperm

seed coat

embryo

– one sperm fertilizes the egg

– other sperm unites with polar nuclei, forming endosperm

– endosperm provides food supply for embryo

• Flowering plants go through the process of double fertilization.



• Each ovule becomes a seed.• The surrounding ovary grows into a fruit.


20.1 Origins of Plant Life KEY CONCEPT Plant life began in the water and became adapted to land.

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Transcript of 20.1 Origins of Plant Life KEY CONCEPT Plant life began in the water and became adapted to land.