Lesson Overview

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Lesson Overview

• 22.1 What is a Plant?

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• The Plant Kingdom

Plants are classified as members of the kingdom
Plantae.   Plants are eukaryotes that have cell
walls containing cellulose and carry out
photosynthesis using chlorophyll a and b.
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• What Plants Need

All plants have the same basic needs sunlight, a
way to exchange gases with the surrounding air,
water, and minerals.
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• Sunlight

Plants use the energy from sunlight to carry out
photosynthesis.   Leaves are typically broad
and flat and are arranged on the stem so as to
maximize light absorption.
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Plants require oxygen to support cellular
respiration, as well as carbon dioxide to carry
out photosynthesis.   Plants must exchange these
gases with the atmosphere and the soil without
losing excessive amounts of water through
evaporation.

• Gas Exchange

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• Water and Minerals

Land plants have evolved structures that limit
water loss and speed the uptake of water from the
ground.   Minerals are nutrients in the soil that
are needed for plant growth.
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Many plants have specialized tissues that carry
water and nutrients upward from the soil and
distribute the products of photosynthesis
throughout the plant body.
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• Origins in the Water

The ancestors of todays land plants were
water-dwelling organisms similar to todays green
algae.   Although not as large and complex as
many plants, green algae have cell walls and
photosynthetic pigments that are identical to
those of plants. Green algae also have
reproductive cycles that are similar to
plants.   Studies of the genomes of green algae
suggest that they are so closely related to other
plants that they should be considered part of the
plant kingdom.
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• The First Land Plants

The greatest challenge that early land plants
faced was obtaining water. They met this
challenge by growing close to the ground in damp
locations.   Fossils suggest the first true
plants were still dependent on water to complete
their life cycles. One of the earliest fossil
vascular plants was Cooksonia, shown here.
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• The First Land Plants

Several groups of plants evolved from the first
land plants.   One group developed into
mosses.   Another lineage gave rise to ferns,
cone-bearing plants, and flowering plants.
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• An Overview of the Plant Kingdom

Botanists divide the plant kingdom into five
major groups based on four important features
embryo formation, specialized water-conducting
tissues, seeds, and flowers.
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• An Overview of the Plant Kingdom

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• The Plant Life Cycle

The life cycle of land plants has two alternating
phases, a diploid (2N) phase and a haploid (N)
phase.   The shift between the haploid phase and
the diploid phase is known as the alternation of
generations, as shown in the figure.
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• The Plant Life Cycle

  The multicellular diploid phase is known as the
sporophyte, or spore-producing plant.
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• The Plant Life Cycle

The multicellular haploid phase is known as the
gametophyte, or gamete-producing plant.
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• The Plant Life Cycle

A sporophyte produces haploid spores through
meiosis. These spores grow into multicellular
structures called gametophytes.
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• The Plant Life Cycle

Each gametophyte produces reproductive cells
called gametessperm and egg cells.   During
fertilization, a sperm and egg fuse with each
other, producing a diploid zygote that develops
into a new sporophyte.
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• Trends in Plant Evolution

An important trend in plant evolution is the
reduction in size of the gametophyte and the
increasing size of the sporophyte.