CK-12 Life Science
Page 23
The gametophyte produces the gametes, sperm and egg, by mitosis. Remember, gametes are haploid.
Then the sperm fertilizes the egg, producing a diploid zygote that develops into the sporophyte.
The sporophyte produces haploid spores by meiosis.
The haploid spores undergo mitosis, developing into the gametophyte.
As we will see in the following lessons, the generation in which the plant spends most of its lifecycle differs between various plants. In the plants that first evolved, the gametophyte takes up the majority of the lifecycle of the plant. During the course of evolution, the sporophyte became the major stage of the lifecycle of the plant. In flowering plants, the female gametophyte is retained within the sporophyte and the male gametophyte is the pollen.
Figure 10.5
In ferns, the sporophyte is dominant and produces spores that germinate into a gametophyte; after fertilization the sporophyte is produced. Ferns will be discussed in further detail in the next lesson.
Classification of Plants
The Plant Kingdom is formally divided into 12 phyla, and these phyla are gathered into four groups:
nonvascular plants
seedless vascular plants
nonflowering plants
flowering plants
Portrayed in Figure below is some of the rich diversity of this kingdom. These four groups are based on the evolutionary history of significant features in plants. The first significant feature to evolve in the Plant Kingdom, after the retention of the embryo, was vascular tissue. Vascular tissue allowed the transport of water and food throughout the plant. The phyla that were around before the evolution of the vascular tissue are known as the nonvascular plants (without vascular tissue to move water, nutrients and food). The next significant step in the evolutionary history of plants was the development of the seed. Plants that evolved vascular tissue but do not have seeds are the seedless vascular plants. The final major evolutionary event in the Plant Kingdom was the evolution of flowers and fruits. Plants with vascular tissue and seeds but without flowers are the gymnosperms. The plants that have all these features and also fruits and flowers are the angiosperms. These four groups are the focus of the next two lessons.
Figure 10.6
The plant kingdom contains a diversity of organisms. Note that in the upper left is a protist, not a plant.
Lesson Summary
Plants are multicellular photosynthetic eukaryotes that evolved from green algae.
Plants have several adaptive features for living on land, including a cuticle, stomata, and vascular tissue.
Plants are informally divided into four groups: the nonvascular plants, the seedless vascular plants, the nonflowering plants (gymnosperms) and the flowering plants (angiosperms).
Review Questions
How are plants necessary for animal life?
Compare and contrast a typical plant to a photosynthetic protist like a diatom.
Plants evolved from green algae. How are they different from green algae?
What strategies have plants evolved for life on land?
What is the purpose of the stomata?
What term describes the plant life cycle?
What is the diploid stage of the alternation of generations?
Which generation of the alternation of generations is dominant in early plants?
What is the term for plants that lack vascular tissue?
What is the term for plants that have flowers and bear fruit?
Further Reading / Supplemental Links
http://www.ucmp.berkeley.edu/plants/plantae.html
http://www.bioedonline.org/slides/slide01.cfm?q=%22Plantae%22
http://www.wisc-online.com/objects/index_tj.asp?objID=BIO804
http://www.perspective.com/nature/plantae
http://plants.usda.gov
http://en.wikipedia.org/wiki
Vocabulary
alternation of generations
The plant lifecycle, which alternates between a haploid gametophyte and a diploid sporophyte.
angiosperms
Plants that flower and bear fruit.
cuticle
Waxy layer that aids water retention in plants.
gamete
Haploid sex cell; egg or sperm
gametophyte
Haploid generation of the alternation of generations life cycle; produces gametes.
gymnosperms
Seed plant where seeds are not enclosed by a fruit.
nonvascular plants
Plants that do not have vascular tissue to conduct food and water.
sporophyte
Diploid generation of the alternation of generations; produces haploid spores.
stomata
Small pores on the underside of leaves that can regulate the passage of gasses and moisture.
vascular tissue
Tissues that conduct food, water, and nutrients in plants.
Points to Consider
Can you think of examples of plants that do not have seeds?
If a plant does not have seeds, how can it reproduce?
Lesson 10.2: Seedless Plants
Lesson Objectives
Name examples of nonvascular seedless plants.
Name examples of vascular seedless plants.
Explain the reproduction strategies of seedless plants.
Describe the ways seedless plants impact humankind.
Check Your Understanding
What is a plant?
How are plants classified?
Introduction
What might you think a forest would have looked like millions of years ago? Or tens of millions of years ago? Or hundreds of millions of years ago? Probably very different than today. In this lesson the focus will be on the very first and most ancient plants: the nonvascular seedless plants and the vascular seedless plants. These plants have had a great impact on all our lives. Over 300 million years ago, during the Carboniferous period, forests looked very different than they do today. Seedless plants grew as tall as today's trees in vast swampy forests (Figure below). The remains of these forests formed the fossil fuel coal that we depend on today. Although most of these giant seedless plants are now extinct, smaller relatives still remain.
Figure 10.7
Seedless plants were dominant during the Carboniferous period, as illustrated by this drawing.
Nonvascular Seedless Plants
Since the nonvascular seedless plants lack vascular tissue, they also do not have true roots, stems, or leaves. Remember that vascular tissue moves water, food and nutrients throughout the plant. By definition, roots, stems and leaves must contain vascular tissue. However, nonvascular plants do often have a “leafy” appearance and can have stem-like and root-like structures. These plants must also remain very short in stature due to their lack of ability to conduct nutrients and water up a stem. The appearances of the nonvascular plants vary, however, and they are classified into three phyla: the mosses, the hornworts, and the liverworts.
The mosses, phylum Bryophyta, are most often recognized as the green “fuzz” on damp rocks and trees in a forest. If you look closely, you will see that most mosses have tiny stem-like and leaf-like structures. This is the gametophyte stage. Remember from lesson 1 that the gametophyte is haploid. The gametophyte produces the gametes that, after fertilization, develop into the diploid sporophyte. The sporophyte forms a distinctive capsule, called the sporangium, which releases spores (Figure below).
Figure 10.8
Sporophytes sprout up on stalks from this bed of moss gametophytes. Notice that both the sporophytes and gametophytes exist at the same time.
The hornworts, phylum Anthocerophyta, get their name from their distinctive hornlike sporophytes, and “wort” which comes from the Anglo-Saxon word for herb. The hornlike sporophytes grow from a base of flattened lobes, which are the gametophytes (Figure below). They tend to grow in moist and humid areas.
Figure 10.9
In hornworts, th
e horns are the sporophytes that rise up from the leaflike gametophyte.
Liverworts, phylum Hepatophyta, have two distinct appearances- they can either be leafy like mosses or flattened and ribbon-like. Liverworts get their name from the type with the flattened bodies which can resemble a liver (Figure below). Liverworts can often be found along stream beds.
Figure 10.10
Liverworts with a flattened, ribbon-like body are called thallose liverworts.
Vascular Seedless Plants
As their name implies, vascular seedless plants have vascular tissue but do not have seeds. Vascular tissue is specialized tissue that conducts water and nutrients throughout the plant. The development of vascular tissue allowed these plants to grow much taller than nonvascular plants, forming the ancient swamp forests mentioned previously. Most of these large vascular seedless plants are now extinct, but their smaller relatives still remain. Seedless vascular plants include the club mosses, the ferns, the horsetails, and the whisk ferns.
Club mosses, in the phylum Lycophyta, are so named because they can look similar to mosses (Figure below). Club mosses are not true mosses, though, because they have vascular tissue. The “club” part of the name comes from club-like clusters of sporangia in some types of club mosses. The resurrection plant is also a club moss. It shrivels and turns brown when it dries out, but then quickly recovers and turns green when watered again.
Figure 10.11
Club mosses can superficially resemble mosses, but they have vascular tissue.
Ferns, in the phylum Pterophyta, are the most common seedless vascular plants (Figure below). They typically have large divided leaves called fronds. In most ferns, fronds develop from a curled-up formation called a fiddlehead (Figure below). The fiddlehead resembles the curled decoration on the end of a stringed instrument, such as a fiddle. Leaves unroll as the fiddleheads grow and expand. Ferns grow in a variety of habitats, ranging in size from tiny aquatic species to giant tropical plants.
Figure 10.12
Ferns are common in the understory of the tropical rainforest.
Figure 10.13
The first leaves of most ferns appear curled up into fiddleheads.
The horsetails, the phylum Sphenophyta, have hollow, ribbed stems and are often found in marshes (Figure below). Whorls of tiny leaves around the stem make the plant look like a horse's tail, but these soon fall off and leave the photosynthetic hollow stem. The stems are rigid and rough to the touch as they are coated with abrasive silicates. Because of their scratchy texture, these plants were once used as scouring pads for cleaning dishes.
Figure 10.14
Horsetails are common in marshes.
The whisk ferns, the phylum Psilophyta, have green branching stems with no leaves, so they resemble a whisk broom (Figure below). Another striking feature of the whisk ferns is their spherical yellow sporangia.
Figure 10.15
Whisk ferns have no leaves and bear yellow sporangia.
Reproduction of Seedless Plants
Seedless plants can reproduce asexually or sexually. Some seedless plants, like hornworts and liverworts, can reproduce asexually through fragmentation. When a small fragment of the plant is broken off, it can form a new plant.
Like all plants, nonvascular plants have an alternation of generations lifecycle. In the lifecycle of the nonvascular seedless plants, the gametophyte is dominant. The gametophyte is photosynthetic and normally described as the plant. The male gametophyte produces flagellated sperm that must swim to the egg formed by the female gametophyte. For this reason, sexual reproduction must happen in the presence of water; hence the nonvascular plants tend to live in moist environments. Following fertilization, the sporophyte forms. The sporophyte is connected to and dependent on the gametophyte. The purpose of the sporophyte is to produce spores that will develop into gametophytes and start the cycle over again.
For the seedless vascular plants, the sporophyte tends to be dominant. For example, in ferns the gametophyte is a tiny heart-shaped structure, and the leafy plant we recognize as a fern is the sporophyte (as shown in Figure above). The sporangia of ferns are often on the underside of the fronds (Figure below). Like the nonvascular plants, ferns also have flagellated sperm that must swim to the egg. But unlike the nonvascular plants, once fertilization takes place, the gametophyte will die and the sporophyte will thrive independently.
Figure 10.16
This fern is producing spores underneath its fronds.
Why Seedless Plants Are Important
The greatest influence of seedless plants on human society was in the formation of the fossil fuel coal millions of years ago. Coal is burned to provide energy. But some seedless plants still have uses in society today. Sphagnum, also called peat moss, is commonly used by gardeners to improve soils since it has a great ability to absorb and hold water (Figure below). Ferns are also a familiar fixture in many gardens. Besides being prized for their ornamental value, the fiddleheads of certain species of ferns are used in gourmet food. Some species of ferns, like the maidenhair fern, are believed by some people to have medicinal qualities.
Figure 10.17
or peat moss, is commonly added to soil to aid water retention.
Lesson Summary
Nonvascular seedless plants include mosses, liverworts, and hornworts.
Vascular seedless plants include club mosses, ferns, whisk ferns, and horsetails.
Nonvascular seedless plants tend to have a dominant gametophyte while vascular seedless plants tend to have a dominant sporophyte.
Mosses and ferns are used commonly in gardening.
Review Questions
What is vascular tissue?
What is an example of a nonvascular seedless plant?
What is an example of a vascular seedless plant?
Compare and contrast the fern gametophyte and sporophyte.
Compare and contrast the whisk fern (Psilophyta) and the ferns (Pterophyta).
Compare and contrast mosses and club mosses.
What are some uses of the seedless plants to gardeners?
What are some of the distinguishing features of horsetails?
What does the sporophyte of the hornwort look like?
Explain reproduction by fragmentation.
Further Reading / Supplemental Links
http://www.cavehill.uwi.edu/FPAS/bcs/bl14apl/bryo1.htm
http://www.microscopy-uk.org.uk/mag/indexmag.html
http://www.microscopy-uk.org.uk/mag/artjul98/jpmoss.html
http://www.biologycorner.com/bio2/notes_plants.html
http://forestencyclopedia.com/p/p1893
http://www.hiddenforest.co.nz/plants/clubmosses/clubmosses.htm
[http://amerfernsoc.org/ http://amerfernsoc.org/
http://www.washjeff.edu/greenhouse/Pnudum
http://en.wikipedia.org/wiki
Vocabulary
club mosses
Seedless vascular plants that resemble mosses.
ferns
Seedless vascular plants that have large, divided fronds.
hornworts
Seedless nonvascular plants with hornlike sporophytes.
horsetails
Seedless vascular plants with hollow, rigid stems.
liverworts
Seedless nonvascular plants that can have flattened bodies resembling a liver.
mosses
Seedless nonvascular plants with tiny stem-like and stem-like structures.
whisk ferns
Seedless vascular plants that have branching stems and yellow globular sporangium.
Points to Consider
Can you think of examples of plants that have seeds?
Can you think of a plant that has seeds but no flowers or fruits?
Why do you think having flowers is beneficial to a plant?
Lesson 10.3: Seed Plants
Lesson Objectives
Describe the importance of the seed.
Explain the ways in which seeds are dispersed.
Define and give examples of Gymnosperms.
Define and give examples of Angiosperms.
Explain some uses of seed plants.
Check Your Understanding
What are the two types of seedless plants?
How do seedless plants reproduce?
Introduction
If you’ve ever seen a plant grow from a tiny seed, then you might realize that seeds are rather amazing structures. The seed allows a plant embryo to survive droughts, harsh winters, and other conditions that would kill an adult plant. The tiny plant embryo can simply stay dormant, in a resting state, and wait for the perfect conditions for growth before it sprouts. In fact, some seeds can stay dormant for hundreds of years! Another impressive feature of the seed is that it provides stored food for the seedling after it sprouts. This greatly increases the chances that the tiny plant will survive. So being able to produce a seed is a very beneficial adaptation, and as a result, seed plants have been very successful. Although the seedless plants were here on Earth first, today there are many more seed plants than seedless plants. Recall that there are two different groups of seed plants: the Gymnosperms, which do not have flowers or fruits, and Angiosperms, which do have flowers and fruits.