Plant DiversityAn introduction to plants

What is a plant?

Plants are multicellular eukaryotes that have cell walls made of cellulose.

They develop from multicellular embryos and carry out photosynthesis using the green pigments chlorophyll a and b.

They are “stationary animals that eat sunlight”

Alternation of Generations

Plant life cycles have two alternating phases:› A diploid phase—sporophyte generation

Haploid Spores are produced by meiosis and grow into the haploid gametophyte

› A haploid phase—gametophyte generation Haploid Gametes are produced by mitosis. A

male and female gamete fuse together to produce a diploid sporophyte.

Plant Life Cycle

What plants need to survive

Sunlight› For photosynthesis› Leaves are broad and flat to maximize sunlight absorption

Water and Minerals› Water is required for photosynthesis. Plants have developed

structures that absorb water and prevent water loss. Nutrients are absorbed from the soil. These nutrients are needed for plant growth.

Gas Exchange› Plants need oxygen for cellular respiration and carbon dioxide for

photosynthesis. They must exchange these gases with the atmosphere.

Movement of Water and Nutrients› Plants get water and nutrients through their roots, but make food in

their leaves, so they must have a way of getting the nutrients and water from their roots to their leaves. Mosses use diffusion, other plants have specialized structures for transport water and nutrients.

Early Plants

The first plants evolved from an organism much like the multicellular green algae living today.› Green algae and plants have similar

reproductive cycles, cell walls and photosynthetic pigments.

› From those early plants come the mosses and their relatives.

› Then plants with better water transportation systems evolved.

The Plant Kingdom

Plants are divided into four groups based on three important features:› Water conducting tissues

Mosses and their relatives do not have vascular tissue

Ferns and all other plants do.› Seeds—gymnosperms—cone bearing› Flowers—angiosperms—flowering plants

Bryophytes

All nonvascular plants, including mosses, are called bryophytes.

They have reproductive cycles that depend on water.

Since they have no vascular tissue, they draw water up by osmosis.

For this reason, they can only grow a few centimeters above the ground.

Groups of Bryophytes

Mosses-the most common. Grown in swamps, bogs, near streams and in rain forests. › They do not have true roots. They have

rhizoids which are long, thin cells that anchor them in the ground and absorb water and nutrients from the soil.

Liverworts Hornworts

Life Cycle of Bryophytes

Gametophyte (1N) is dominant and is the stage that carries out most of the photosynthesis.

Gametes (1N) are formed in reproductive structures at the tip of the gametophyte. Sperm are produced in antheridia and eggs are produced in archegonia.

Fertilization results in a diploid zygote wich develops into the sporophyte (2N)

Haploid spores are produced in the sporophyte, are scattered by the wind and will develop into gametophytes.

Moss Life Cycle

Sporophyte generation

Gametophyte generation

Seedless Vascular Plants

Vascular tissue is specialized to conduct water and nutrients through the plant› First specialized cells were tracheids.

These are the key cells in the xylem.› Xylem transports water upward from the

roots to all parts of the plant.› Phloem is another tissue in plants

specialized to transport solutions of nutrients and carbohydrates produced by photosynthesis throughout the plant.

Zylem and Ploem

Xylem and phloem can move fluids through the plant body, even against the force of gravity.

Thick walls of xylem and lignin (a substance that makes the cell walls rigid) enable vascular plants to grow upright and reach great heights.

Ferns and Their Relatives

Seedless vascular plants include club mosses, horsetail and ferns.

Ferns are the most numerous of this group. Ferns and their relatives have true roots, leaves and

stems.› Roots—underground organs that absorb water and

minerals. › Leaves—photosynthetic organs that contain one or

more bundles of vascular tissue (veins made of xylem and phloem) Ferns leaves are called fronds

› Stems—supporting structures that connect roots to leaves, carrying water and nutrients between them. Ferns have underground stems called rhizomes

Life Cycle of Ferns

The diploid sporophyte stage is dominant.› Fern sporophytes develop haploid spores on the

underside of their fronds in tiny containers called sporangia. The sporangia are grouped together in clusters called sori.

› Spores make haploid gametophytes.› The small gametophyte grows rhizoid like roots then

flattens into a thin, heart-shaped green structure.› The antheridia and archegonia are found on the

underside of the gametophyte. The sperm swim to the egg producing a diploid zygote which will grow into a sporophyte.

› As the sporophyte grows, the gametophyte withers away.

Fern Life Cycle

Copyright Pearson Prentice Hall

Seed Plants

Seed plants are divided into two groups› Gymnosperms—bear their seeds directly

on the surfaces of cones Examples: pines and spruces, cycads,

ginkgoes› Angiosperms (flowering plants)—bear their

seeds within a layer of tissue that protects the seed. Examples: grasses, flower trees and shrubs,

wildflowers and cultivated flowers

Reproduction without water

Life cycle that includes alternation of generations, but does not require water for fertilization of gametes.

Hence, seed plants can live just about anywhere.

Adaptations that allow seed plants to reproduce without water include:› Cones and flowers› Pollination› Embryos in seeds

Gymnosperms—Cone Bearers

Gymnosperms include gnetophytes, cycads, ginkgoes and conifers

These plants all reproduce with seeds that are exposed—gymnosperm means “naked seed”

Angiosperms—Flowering Plants

Angiosperms develop unique reproductive organs known as flowers

Flowers contain ovaries, which surround and protect the seeds.

Angiosperm means enclosed seed After pollination, the ovary develops

into a fruit which protects the seed and aids in its dispersal.

Diversity of Angiosperms

Two groups of angiosperms—classified by the number of seed leaves (cotyledons) in the plant embryo.

A cotyledon is the first leaf or the first pair of leaves produced by the embryo of a seed plant.› Monocot—one seed leaf—includes corn,

wheat, lilies, orchids and palms› Dicot—two seed leaves—includes roses,

clover, tomatoes, oaks and daisies

Lifespans of Angiosperms

Annuals—flowering plants that complete a life cycle within one growing season. Plant dies at end of cycle.

Biennials—complete their life cycle in two years. In first year, grow roots, short stems and sometimes leaves. In second year, grow new stems and leaves, then produce flowers and seed. Then the plant dies.

Perennials—live for more than two years, often for many years. › Some have stems that die each year and are replaced in

the spring.› Most have wood stems—like palm trees, maple trees and

honeysuckle