Download From Water to Land

S E C T I O N
13.1
From Water to Land
E X P E C TAT I O N S
Describe anatomical and
physiological characteristics
of plants.
Learn the life cycle of select
plants.
Classify representative
organisms.
Use techniques of
classification to illustrate
the principles of taxonomy.
Figure 13.1 Mosses can live
on land, but need very wet
conditions to survive and
reproduce.
What are plants, and how do they differ from other
living things? All species of plants are multicellular
eukaryotes that obtain their food by photosynthesis.
You learned that algae and some bacteria also carry
out photosynthesis. Unlike these two groups,
which are sometimes called “the grass of the
waters,” plants such as the mosses in Figure 13.1
live on land. For photosynthetic organisms, one
advantage of living on land instead of in water is
the greater availability of light. Another advantage
is the more rapid diffusion of carbon dioxide and
oxygen into and out of the organism. One
disadvantage of living on land is the risk of
dehydration. Many characteristics of plants are
adaptations to living in a dry environment.
Adaptations to Life on Land
Fossil evidence shows that the first land plants
appeared about 400 million years ago. Their
ancestors were most likely filamentous green algae
(see Figure 13.2). Like green algae, plants have
chlorophylls a and b in their cells. Plants and
green algae also both have cellulose cell walls. A
third similarity is that plants and green algae store
food energy in the form of starch, whereas bacteria,
fungi, and animals store food as glycogen.
Imagine taking a thin filament of algae floating in
a pond and moving it to land. What changes would
have to happen in order for it to survive on land?
One major need is protection from drying out.
Other major needs include a system to transport
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MHR • Diversity of Living Things
water and dissolved substances from the outside
environment to cells within the body of the plant,
and a system to support the body of the plant,
lifting it up into the light and air.
The adaptations of plants to life on land did not
all occur at once. The earliest land plants were
small and delicate. They grew in moist places and
transferred water and dissolved substances from
cell to cell by osmosis and diffusion. These
processes are slow and inefficient, and they limited
the maximum size of these plants as well as the
types of environments in which they could live.
Despite this, land plants flourished, and there are
still many plants that continue to survive using
these adaptations. Of the plants living today,
mosses and their relatives most closely resemble
the first land plants.
Vascular and Non-vascular Plants
You may have learned that most plants consist of
three main parts: roots, stems, and leaves
(as shown in Figure 13.3 on the next page). These
specialized organs are all adaptations to life on
land. Roots penetrate the soil to anchor the plant
and reach sources of water. Leaves provide a
greater surface area to carry out photosynthesis.
Stems supply rigid tissues that raise and support
the leaves.
In order for roots, stems, and leaves to grow,
they need a regular supply of water, energy, and
Vascular plants
Non-vascular plants
Non-vascular plants
mosses,
liverworts,
hornworts
gymnosperms
angiosperms
first seed plants
ancestral
green alga
700 mya
ferns
Seed plants
first vascular plants
500-400 mya
350 mya
140 mya
Figure 13.2 Phylogeny of relationships among land plants showing green algae as
the ancestor (mya = millions of years ago)
nutrients. How does a cell in a tree branch get
water? How do roots receive sugars made in the
leaves? In most plants, these tasks are carried out
by vascular tissue, which is made of cells able to
conduct solutions throughout the plant, and which
links the tips of the roots to the highest leaves. A
plant’s vascular system has a similar function to
the circulatory system that carries blood around in
your body. It transports water, dissolved minerals,
and sugars to all parts of the plant, providing cells
with materials they need to carry out their life
functions. The evolution of vascular tissue has
allowed plants to increase in size, with some
growing to be giants compared to their ancestors.
This characteristic is so significant that it divides
the plant kingdom into two major groups: vascular
and non-vascular plants. Mosses, liverworts, and
hornworts are non-vascular plants; they have no or
poorly developed roots, leaves, and stems. All
other plants contain vascular tissue.
leaves
stem
vascular tissue
(xylem and phloem)
root
Figure 13.3 Most plants have roots, stems, leaves, and
vascular tissue.
FAST FORWARD
For more information on characteristics of plants and their
specialized tissues, turn to Chapter 14, Section 14.1–14.3.
Plants and Animals • MHR
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Reproductive Strategies
A further challenge faced by land plants is how to
carry out sexual reproduction. For fertilization to
occur, gametes must move from one organism to
another. Algae do this by releasing unprotected
gametes into the surrounding water. But gametes
are single-celled structures and cannot survive
exposure to air for very long. To shield their
gametes from drying out, the first land plants
had a layer of thick-walled cells around their
reproductive parts. However, they could only
release their gametes when a film of water covered
the plant. For millions of years, this mechanism
limited the conditions under which plants could
reproduce. Today, mosses, ferns, and their relatives
still depend on external water in the form of dew
or rain in order to reproduce.
How do other plants carry out sexual
reproduction? You can see the answer in the air
each spring. Male gametes in these plants consist of
pollen grains such as those in Figure 13.4 on the
next page. These tiny, waterproof structures are
Investigation
carried to female plants not by water but on the
wind, by insects, or by other animals. After
fertilization, the zygote develops inside another
protective, waterproof coat forming the seed. A
seed is a structure made up of an embryo, stored
food, and a tough waterproof coat. Seeds can remain
dormant for long periods of time, which allows
plants to survive exposure to drought, freezing,
and, in some cases, even fire. As a result of these
characteristics, seed-bearing plants are more
widespread and diverse than seedless plants.
REWIND
To review information on haploid and diploid cells, turn to
Chapter 5, Section 5.2.
SKILL FOCUS
1 3 • A
Performing and recording
How Do Mosses Differ from Algae?
Mosses and green algae are both multicellular eukaryotes that carry out
photosynthesis. Yet mosses are classified in the plant kingdom, while
green algae are not. In this investigation, you will compare the
characteristics of these two types of organisms.
Pre-lab Questions
Describe the environments of a typical moss
and a typical alga.
List two characteristics of each of the
kingdoms to which mosses and algae belong.
Problem
What differences can you observe between
mosses and algae that would explain their
classification in different kingdoms?
Prediction
Predict which characteristics of mosses
and algae will be useful to classify them
to kingdoms.
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MHR • Diversity of Living Things
Analyzing and interpreting
Communicating results
Materials
magnifying lens
slides
dropper
microscope
cover slips
samples of mosses and
green algae
Alternation of Generations
The life cycle of plants consists of two generations
which alternate between a haploid and a diploid
stage. Figure 13.5 on the next page shows this
cycle. The diploid generation of a plant is called
the sporophyte. Through the process of meiosis,
sporophytes produce haploid spores, which can
develop without fertilization. The haploid spore
grows into a plant body called the gametophyte.
Gametophytes produce male and female gametes,
which fuse at fertilization and develop into another
sporophyte. The cycle then repeats itself.
Although all plant life cycles include a
sporophyte and gametophyte generation, one stage
or the other is characteristically dominant in
different plant groups. In non-vascular plants the
dominant stage (the familiar green plant) is the
gametophyte. In vascular groups the sporophyte is
the more dominant generation. In flowering plants,
the gametophyte is reduced to a small group of
cells entirely dependent on the sporophyte.
Figure 13.4 The small flowers of the alder plant shed their
pollen to be dispersed by wind.
Procedure
1. Make a table to record features of both types
of organisms. Include such variables as
overall size, and structures for obtaining
water and nutrients, for support, for
reproduction, and for photosynthesis.
2. Study your samples with a magnifying lens.
Use your textbook or other references
supplied by your teacher to identify
structures. Make labelled sketches of each
structure and record your notes and
observations in your table.
3. Prepare wet mounts of the samples, making
sure that each contains the organism’s
photosynthetic structures. Observe them
under low and high power. Record
observable similarities and differences.
4. Prepare wet mounts of the structures the
organisms use for absorbing water and
nutrients. Observe them under low and high
power. Record similarities and differences.
5. Prepare wet mounts of the structures the
organisms use for reproduction. Observe
them under low and high power. Record
similarities and differences.
Ensure you wash your hands thoroughly at
the end of the procedure.
Post-lab Questions
1. Using your notes and sketches, describe two
pieces of evidence that indicate mosses are
better adapted to life on land than algae are.
Conclude and Apply
2. Based on your observations of their
structure, give a reason why mosses do not
grow to the size of trees.
Exploring Further
3. Obtain samples of other terrestrial and
aquatic plants. Using the same procedure as
above, record your observations. What are
the similarities to your other samples? What
are the differences? Offer explanations for
your findings.
Plants and Animals • MHR
467
BIO
female
gamete (n)
spores (n)
male
gamete (n)
Gametophyte (n)
haploid cells
fertilization
meiosis
Sporophyte (2n)
diploid cells
FACT
The first eukaryote organisms were probably haploid, and
some eukaryotes today are haploid all their lives. Plants and
animals, however, are diploid at some stage in their lives.
Diploid cells seem to have developed early in the history of
life by the fusion of two haploid cells that later divided by
meiosis. This fusion process was the origin of sexual
reproduction. Its value is in producing genetically variable
offspring — the raw material for the evolution of new
characteristics. The alternation of different diploid and
haploid forms found in plants is called the alternation of
generations, but the word “generation” here means simply
“phase in a life cycle.” It does not refer to the more
common meaning relating to offspring and time.
mitosis
PAUSE
Figure 13.5 The life cycle of a plant consists of the
alternation of generations of diploid sporophytes with
haploid gametophytes.
SECTION
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RECORD
Think of the different challenges faced by organisms living
in water versus organisms living on land. Make a list
outlining as many differences as possible. Think of some
examples of organisms that live on land, organisms that live
in water, and organisms that use both land and water
habitats. How have they solved the different challenges
you listed?
REVIEW
1.
K/U Name two pieces of evidence to support the
idea that filamentous green algae were the ancestors
of land plants.
2.
K/U List three changes that had to occur in order
for plants to move from a life in water to a life
on land.
3.
K/U Why is a temperate rainforest the ideal habitat
for mosses? Is this also true for a tropical rainforest?
Explain your answer.
4.
Explain why the alternation of generations is an
adaptation for plants living on land.
5.
K/U How does the size of a plant indicate how
it transports water and nutrients? Explain your
reasoning.
6.
Write a short essay explaining why the evolution
of seeds was so important.
9.
10.
gametophyte
C
C Draw a generalized life cycle of a plant, showing
the alternation of generations. Label your diagram to
fully explain the life cycle.
8.
C Create an illustrated time line to show the
evolution of plants from water to land. Be sure to
indicate what the first land plants looked like. How
does the appearance of plants today differ from
those early land plants?
MHR • Diversity of Living Things
C Based on your knowledge of the alternation of
generations, which of the following life cycles, A, B,
or C describes the condition found in (a) nonvascular plants; (b) non-flowering vascular plants;
and (c) flowering plants?
A sporophyte
K/U
7.
I Suppose, on a field trip with your class, you
collected several samples of plants growing near the
edge of a pond. Can you identify the samples as
mosses? Explain how you would do this.
B sporophyte
gametophyte
C sporophyte
gametophyte
Support your answer with a short written explanation.
11.
MC Consider the different relationships that terrestrial
organisms have with land plants. Detail three
interactions between land organisms and land plants.
Now, write a short story describing the effect it would
have on humans if land plants were to die off. What
effects would this have on other land organisms?
What effects would it have on aquatic plants and
animals?