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Overview of Plant Diversity

Chapter 29

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Outline

• The Evolutionary Origins of Plants• Plant Life Cycles• Mosses, Liverworts, and Hornworts• Features of Vascular Plants• Seedless Vascular Plants• Seed Plants• Gymnosperms• Angiosperms

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The Evolutionary Origins of Plants

• Defining characteristic of plants is protection of their embryos.

• Some biologists list three kingdoms of plants.– green, red, and brown

Land plants can be divided into two groups based on the presence or absence of vascular tissue.

Vascular plants have water-conducting xylem and food-conducting phloem strands of tissues in their stems.

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Major Plant Groups

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The Evolutionary Origins of Plants

• Adaptations to land– Most plants are protected from desiccation

by a waxy cuticle secreted on exposed surfaces.

limits gas exchange essential for respiration and photosynthesis

occurs via stomata evolution of leaves resulted in

increased photosynthetic area

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Plant Life Cycles

• The plant life cycle is haplodiplontic.– diploid generation, sporophyte, alternates

with haploid generation, gametophyte– diploid sporophyte produces haploid spores

by meiosis Spores divide by mitosis, producing a

multicellular, haploid gametophyte.haploid gametophyte is produced by

mitosis, and is the source of gametes gametes fuse to form diploid zygote

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Generalized Plant Life Cycle

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Mosses, Liverworts, and Hornworts

• Bryophytes– highly adapted to terrestrial environments– gametophytes are photosynthetic– sporophytes are attached to, and

nutritionally dependent on, gametophytes– require water to reproduce sexually– most are small

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Mosses, Liverworts, and Hornworts

• Mosses (Bryophyta)– Gametophytes typically consist of small

leaf-like structures arranged spirally or alternately around a stem-like axis.

anchored to substrate by rhizoids– most of water used by plant travels up

outside of plant.

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Mosses

– Multicellular gametangia are formed at the tips of leafy gametophytes.

archegonia - female gametangia antheridia – male gametangia

– Many species can withstand prolonged periods of drought, and most mosses are very sensitive to air pollution.

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Typical Moss Lifecycle

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Liverworts, and Hornworts

• Liverworts (Hepaticophyta)– sexual reproduction similar to mosses

• Hornworts (Anthocerotophyta)– among earliest land plants

Sporophyte has stomata, is photosynthetic, and provides much of plant’s energy.

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Features of Vascular Plants

• Vascular tissues consist of specialized cylindrical or elongated cells that form a network throughout plant.

– xylem - conducts water and dissolved minerals upward from roots

– phloem - conducts sucrose and hormone signals throughout the plant

• Seeds are resistant structures suited to protect an embryo from drought.

– occur in heterosporous plants

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Seedless Vascular Plants

• Earliest vascular plants lacked seeds– club mosses (Lycophyta)

relicts of earliest vascular plants most abundant in tropics and moist

temperate regions

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Whisk Ferns, Horsetails, and Ferns (Pterophyta)

• Whisk ferns – simplest of all extant vascular plants

evenly forking green stems without roots• Horsetails

– single genus, Equisetum Sporophytes consist of photosynthetic

stems arising from underground rhizomes.

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Whisk Ferns and Horsetails

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Ferns

• Ferns are the most abundant group of seedless vascular plants.

– about 11,000 living species, 75% occurring in tropics

– life cycle differs from that of a moss greater development, independence, and

dominance of fern’s Sporophyte

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Typical Fern Lifecycle

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Seedless Vascular Plants

• Fern sporophytes typically have a horizontal underground stem, the rhizome.

– Fronds usually develop at the tip of the rhizomes as tightly rolled-up coils.

• most homosporous, producing sporangia in clusters, sori, on back of fronds

• Diploid spore mother cells in each sporangium undergo meiosis, producing haploid spores.

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Seed Plants

• Seed Plants first appeared about 425 mya.– drought protection– enhanced dispersal– dormant phase

increase embryo survival by waiting for favorable environmental conditions

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Gymnosperms

• Ovule rests exposed on a scale and is not completely enclosed by sporophyte tissues at time of pollination

• Lack flowers and fruits of angiosperms• Four living groups

– conifers– cycads– gnetophytes– Ginkgo

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Gymnosperms

• Conifers (Coniferophyta)– include pines, spruces, cedars, hemlocks,

yews, larches, cypresses and others– pines

currently more than 100 speciesnative to N. America

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Pines

• Tough, needlelike leaves produced in clusters– thick cuticle and recessed stomata

retard water loss– leaves have canals into which surrounding

cells secrete resin deters insect and fungal attack

produce turpentine and rosin• Wood consists primarily of xylem tissue.

– “soft” wood

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Pines

• Heterosporous– Female cones typically produced on upper

branches of the same tree that produces male cones.

Two ovules develop on each scale. Each ovule contains a megasporangium (nucellus).

surrounded by integument opening - micropyle one layer becomes seed coat

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Pines

• While scales of female cones are open, pollen grains drift down, and some are caught in fluid from the micropyle.

– drawn to top of nucellus• While female gametophyte is developing, a

pollen tube emerges from pollen grain and digests its way to the archegonia.

– Fifteen months after pollination, pollen tube reaches archegonium and discharges its contents.

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Typical Pine Lifecycle

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Gymnosperms

• Cycads (Cycadophyta)– slow-growing - tropical and sub-tropical

Sporophytes resemble palm trees.• Gnetophytes (Gnetophyta)

– only gymnosperms with vessels in their xylem

– half of all species are in genus Ephedra, common in arid regions of western US

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Gymnosperms

• Ginkgo (Ginkgophyta)– maidenhair tree (Gingko biloba) only living

species

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Angiosperms

• Ovules are enclosed within diploid tissues at time of pollination

– carpel, modified leaf encapsulating seed, develops into fruit

• Monocots and eudicots– eudicots and monocots differ in:

number of cotyledons leaf venation presence lateral meristems number of flower parts

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Angiosperms

• Structure of flowers– Flower originates as a primordium that

develops into a bud at the end of a pedicel. expands at tip into a receptacle to which

remaining flower parts are attachedattached in whorls

outermost whorl - sepals second whorl - petals

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Angiosperms

• Third whorl - stamens (androecium)– Each stamen consists of pollen-bearing

anther and a stalk (filament). gynoecium - consists of one or more

carpels located at flower’s center regions of carpel

ovary - contains ovules and develops into fruit

stigma - sticky top style - connects stigma and ovary

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Angiosperm Flower

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Angiosperms

• Angiosperm life cycle– eight nuclei arranged in two groups of four– integuments differentiate into seed coat– one nucleus from each group migrates

toward center and functions as polar nuclei one cell functions as the egg, and the

other two are synergids– pollination - mechanical transfer of pollen

from anther to stigma may be followed by fertilization

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Angiosperms

• As the pollen tube enters the embryo sac, it discharges its contents.

– double fertilization One sperm unites with the egg and forms

a zygote. The other sperm and and two polar

nuclei unite, forming a triploid primary endosperm nucleus.

nutrient supply for embryo

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Typical Angiosperm Lifecycle

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Summary

• The Evolutionary Origins of Plants• Plant Life Cycles• Mosses, Liverworts, and Hornworts• Features of Vascular Plants• Seedless Vascular Plants• Seed Plants• Gymnosperms• Angiosperms

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