Plants dominate most of the land on Earth. A small patch of forest contains thousands of trees (Figure 1(a)), while a golf course is covered in millions of individual grass plants (Figure 1(b)). Plants are ubiquitous in cities as well as in rural settings, indoors and outside. Even if you stay inside all day, you will almost certainly use plant products and eat a variety of plant foods (Figure 1(c)). Plants are so abundant that we may forget just how important they are.
Figure 1 (a) From forests to (b) golf courses, much of Earth's surface is covered by plants. (c) Plants and plant products are all around us, in the products we use and the foods we eat.
Although we are surrounded and supported by ecosystems dominated by plants, most people know very little about them. But as you are about to learn, with more than 270 000 known species, plants are fascinating and incredibly diverse. The largest plants tower more than 100 m into the air, and the smallest are little more than a millimetre across. Today's oldest living tree has a root system that is over 9550 years old! This section gives an introduction to the characteristics and diversity of plants as well as an overview of how they adapted to life on land. The Plants unit provides a more detailed study of plant anatomy, growth, and function.
Why Plants Are Important
Plants can survive in many different climatic conditions, from hot and humid tropics to parched deserts and the extreme cold of the High Arctic (Figure 2). Because plants are producers of food, other terrestrial life forms follow wherever plants live. In fact, the rich diversity of terrestrial ecosystems depends heavily on the diversity of plants. Without plants to supply food through photosynthesis, there would be little life on land.
In addition to supporting food webs, plants also provide other organisms with places to live, such as nesting locations for birds and supports for a spider's web. Humans are particularly dependent on plants for a wide range of valuable substances including medicines, clothing, wood, and paper products.
Unfortunately, many of the world's plants are threatened with extirpation or extinction. Habitat destruction, invasive species, pollution, and climate change all pose serious threats to plants and the organisms that depend on them. Today, 77 of Ontario's native plant species are listed as "species at risk." Of these, three species are extirpated—no longer living anywhere in the province.
Figure 2 These Arctic poppies are found in Arctic and High Arctic climates, including Baffin Island.
Image: A plant, the Arctic poppies in snow and ice.
Classification and Phylogeny
Plants are thought to have evolved from charophytes, a group of green algae (which belong to the kingdom Protista), between 425 million and 490 million years ago. There is very strong evidence supporting this evolutionary relationship. Plants and green algae both contain chlorophyll a and chlorophyll b, two forms of the green pigment used in photosynthesis. They also contain pigments that are not found in other photosynthetic eukaryotes. Plants and green algae share several other characteristics. At the end of mitosis, only the cells of plants and green algae begin cytokinesis by building a cell plate across the middle of the cell. Their cell walls contain large amounts of cellulose, a complex sugar molecule. Plants and green algae also store excess food as starch.
charophyte: the common name for organisms in the order Charophyceae; green algae in the kingdom Protista
LEARNING TIP
“phyte”?
Many plant terms end with the suffix –phyte. Phyte is derived from the ancient Greek word phut (όn) meaning “plant.”
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Today there are more than 270 000 living species of plants. These are classified into four major groups. The phylogenetic tree in Figure 3 shows the evolutionary relationships between the major groups of plants.
Figure 3 A simplified phylogenetic tree of the Plant Kingdom. Many biologists now include green algae within the Plant kingdom and refer to the other groups as the "higher plants."
Image: A phylogenetic tree. - early green algae -- green algae -- early nonvascular plants -- nonvascular plants, bryophytes -- early vascular plants -- seedless vascular plants, pteridophytes -- first seed plants -- gymnosperms -- diversification of flowering plants -- angiosperms
At top of tree is a table called Millions of Years Ago, runs across top of tree starting from left 500, 400 (Paleozoic), 300, 200 (Mesozoic), 100, (Cronozoic), 0.
Characteristics
All higher plants are multicellular eukaryotic organisms. Unlike animals and fungi, almost all plants perform photosynthesis. All have cell walls composed primarily of cellulose. Like fungi, all terrestrial plants are sessile—they cannot move from place to place. Although all plants share these basic features, plants show an incredible diversity of characteristics (Figure 4). GO TO NELSON SCIENCE
CAREER LINK
Botanist
Botanists specialize in the study of plants. To learn more about botanists, GO TO NELSON SCIENCE
Figure 4 (a) In tropical rainforests, a great variety of plant species cover every square metre of available space, including the branches of trees. (b) Sundews are carnivorous plants native to Ontario bogs. Their leaf hairs secrete a sweet, sticky liquid that attracts and traps insect prey. These hairs and the leaf blade then slowly curl inward on the prey.
Alternation of Generations
Plants have a very different life cycle than that of animals. This life cycle is known as an "alternation of generations." This life cycle has diploid and haploid states, or generations. The diploid generation produces spores, and the haploid generation produces gametes.
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UNIT TASK BOOKMARK
Compare and contrast the life cycles of the organisms in your chosen group. Do the conservation efforts aimed at this group need to take into account any features of this life cycle?
Typical animal body cells are diploid (2n), containing two sets of chromosomes. These cells undergo meiosis, producing haploid sex cells, or gametes. Two haploid (n) cells fuse in the process of fertilization, forming a diploid zygote that then grows into the adult animal (Figure 5(a)).
A plant in the diploid stage is called a sporophyte (Figure 5(b)). As in animals, the sporophyte's diploid cells divide by meiosis to produce haploid cells. In plants, however, these haploid cells form asexual spores. As the haploid generation begins, the spores grow into gametophyte individuals. The gametophytes matufe, then produce haploid sex cells—gametes. These gametes undergo fertilization to form diploid zygotes. The zygotes grow into sporophyte individuals, and the cycle continues.
Figure 5 (a) A typical animal life cycle consists only of diploid individuals that produce haploid sex cells. (b) Plant life cycles alternate between a diploid sporophyte generation and a haploid gametophyte generation.
Image: (a) The Animal Life Cycle: Adult (multicellular)(2n), then; meiosis, then, sex cells (n), then, fertilization, then, zygote (2n), then back to Adult (multicellular)(2n). A quarter of the inner circle is shaded haploid (n). In middle of circle is diploid (2n).
(b) Plant Life Cycle: Sporophyte (multicellular) (2n), then, melosis, spores (n), then, gametophyte (multicellular) (n), then, sex cells (n), then, fertilization, then, zygote (2n), then back to Sporophyte (multicellular) (2n). Half of the inner circle is shaded haploid (n). In the middle of the circle is diploid (2n).
Key
K/U: Knowledge and Understanding T/I: Thinking and Investigation C: Communication A: Application Research This
Climate has a dramatic influence on the distribution and success of plant communities. Each plant species is ideally suited for a particular range of temperatures, water availability, and seasons. The greenhouse gases that humans release into the atmosphere are contributing to climate change. The main human-generated greenhouse gas is carbon dioxide—a key reactant in photosynthesis. In this activity you will research some of the ways in which the changing climate and increasing levels of atmospheric carbon dioxide are likely to affect plant distribution and diversity.
1. Use print and online resources to research the following topics:
(i) How might an earlier spring and later fall influence plants?
(ii) Explain how climate change is causing Ontario's plant communities to move northward.
(iii) What effects could a changing climate have on plant pests and disease? On forest fires?
(iv) How are increasing concentrations of carbon dioxide in the atmosphere likely to influence plants? A. Based on your research, how will climate change and increasing levels of carbon dioxide in the atmosphere likely affect plant distribution? T/I
B. How will climate change and the resulting changes in plant distribution affect biodiversity? Consider the effects on the biodiversity of plants as well as the ecosystems and food webs they support. T/I A
Early Adaptations for Life on Land
Most plant species living today live on the land. There are some aquatic species, but most are freshwater—only a few are marine. The primitive ancestors of today's plants, however, lived in the water. They were probably very much like green
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algae—relatively small, photosynthetic, multicellular organisms. These early plants, like green algae living today, would have reproduced sexually using eggs and sperm that swam through the water.
When plants first began to live on land, they developed characteristics that allowed them to live and reproduce in that new environment. Although life on land gave plants lots of light and carbon dioxide—both essential for photosynthesis—it forced plants to adapt in other ways.
The first requirement for life on land was the ability to prevent water loss. When exposed to dry air and warm temperatures, unprotected living cells quickly lose water through osmosis and evaporation. Plants developed a flexible, waxy cuticle on their outer surfaces that effectively prevents water loss (Figure 6(a)).
cuticle: a waterproof, waxy coating produced by the epidermis of most plants
Although preventing water loss is critical for land plants, they must be able to take in small amounts of carbon dioxide gas from the atmosphere in order to perform photosynthesis. To meet this need, plants developed stomata (singular: stoma), tiny openings between specialized cells on the plant's surface (Figure 6(b)). These stomata are like windows that can be opened and closed. When they are open, gases can diffuse into and out of the plant. When they are closed, the plant is effectively sealed off from the surrounding air.
stoma: a small opening in the epidermis of a plant that allows gas exchange
Figure 6 (a) Almost all plant surfaces produce a waterproof, waxy cuticle. (b) Stomata are small openings in the epidermis that allow gas exchange with the air. Most leaves have thousands of stomata per square centimetre.
Images: (a) A diagram showing a plant’s epidermal cell, and a waxy cuticle on the outside of the plant; (b) A microscopic image showing the epidermal cell and the stoma.
Bryophytes: The Mosses
The bryophytes are the simplest of land plants. This group includes the mosses, liverworts, and hornworts. The most recognizable of these are the mosses—there are more than 1000 species of moss in North America alone! The peat mosses, of the genus Sphagnum, may be the most numerous plants on Earth. In many ecosystems, including Canada's boreal forest, Sphagnum mosses can form a continuous ground cover over hundreds and thousands of square kilometres (Figure 7).
bryophyte: a small seedless plant that lacks vascular tissue
Figure 7 Various species of sphagnum moss form a thick covering in this bog.
Like the earliest of land plants, bryophytes have a protective cuticle and stomata for gas exchange. They do not have specialized vascular tissue or true leaves, roots, or seeds. Most bryophytes are only a few centimetres in height.
The gametophyte generations of bryophytes produce swimming sperm in structures called antheridia (singular: antheridium) and eggs (in structures called archegonia (singular: archegonium). Bryophytes can therefore only live and reproduce in places with at least occasional wet conditions, such as rains or heavy dew.
antheridium: the specialized structure on a gametophyte that produces sperm
archegonium: the specialized structure on a gametophyte that produces eggs
The green plants we recognize as mosses are the photosynthetic gametophyte individuals. After fertilization, the new, non-photosynthetic sporophyte individuals grow out of the archegonia on the female gametophyte and get their nourishment from the gametophyte. These sporophytes seem to be part of the gametophyte but are actually the offspring of the gametophyte plants. The sporophytes grow a tall stalk that bears a structure called a sporangium (plural: sporangia), in which haploid spores are produced. These tiny spores are easily carried by winds and dispersed over a large area.
sporangium: the structure in which spores are produced
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Figure 8 illustrates the alternation of generations life cycle of a typical moss. Some moss gametophytes can also reproduce asexually. Small clumps of photosynthetic haploid cells called gemmae (singular: gemma) are produced in cup-shaped structures on the surface of the gametophytes. The gemmae are dispersed by splashes of rain and grow into new gametophyes.
gemma: a small clump of haploid photosynthetic cells-produced in little cup-shaped structures on the gametophyte plant; dispersed by splashes of rain to grow into another gametophyte plant
Figure 8 The life cycle of a moss. In order to fertilize the eggs, sperm must swim through water on the surface of the gametophytes.
Image: The life cycle of a typical moss in list format.
Key: Haploid-* Diploid-
Haploid Gametophyte Generation* - Archegonia with eggs (n) Image: Sperm and eggs
Diploid Sporophyte Generation - young sporophyte plant (2n)
- sporangium (2n) (sporophyte plant) - stalk (2n)
- top of gametophyte plant♀ - meiosis - spores (n)
- mature gametophyte plant (n) - ♀ plant leads back to Archegonia with eggs (n) - ♂ plant leads to Archegonia with sperm (n)
Lycophytes and Pterophytes: The Ferns
The next major adaptation by land plants was the development of vascular tissue. Vascular tissue consists of xylem and phloem. It is specialized for the transportation of water and nutrients. Plants with vascular tissue can grow to great heights, thereby accessing more sunlight. Lignin is a key chemical component of vascular tissue. It is an extremely strong compound that makes cell walls more rigid. It is responsible for the great strength of woody tissues.
lignin: an important structural compound found in the vascular tissues of plants; it is responsible for the strength of wood
The lycophytes (club mosses) and pterophytes (ferns and their relatives) are groups of seedless vascular plants that still have many characteristics of the earliest vascular plants. During the Carboniferous period, about 360 million to 300 million years ago, these plants dominated the landscape. Some grew as large as 2 m in diameter and more than 40 m in height! Today's 13 000 or so species remain widespread and common, but most are relatively small.
lycophyte: a seedless vascular plant; club mosses are examples of lycophytes
pterophyte: a seedless vascular plant; ferns are examples of pterophytes
Among the simplest of vascular plants are members of the genus Equisetum called horsetails. They are considered "living fossils" because they are the only remaining genus of what was once a very diverse group of plants. As they grow, silica crystals form on their stems.
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Similar to bryophytes, the gametophyte individuals of lycophytes and pterophytes reproduce sexually using sperm and eggs. Unlike the bryophytes, however, the sporophytes are photosynthetic and much larger than the gametophytes (Figure 9). Both groups have simple roots and stems. The stems of ferns are usually in the form of rhizomes that grow horizontally just below the surface of the ground. Ferns also have large green leaves called fronds.
rhizome: a horizontal underground stem
frond: a fern leaf; young curled fronds are often called "fiddleheads" because of their distinctive shape; some fiddleheads are edible, but many are toxic
Figure 9 Life cycle of a fern. Note that the sporophyte is photosynthetic and much larger than the gametophyte. The gametophyte (less than 1 cm across) is greatly enlarged in this diagram.
Image: The life cycle of a fern in list format.
Key: Haploid-* Diploid-
Haploid Gametophyte Generation* - mature gametophyte plant with rhizoids and sex organs (n), showing ♀- one of many archegonia with eggs (n), and ♂- one of many antheridia with sperm (n)
Diploid Sporophyte Generation - fertilization - young sporophyte plant (2n) growing out of gametophyte plant - mature sporophyte plant (2n): (a) frond, (b) rhizome (c) roots
Image: The sporangium on underside of frond, resembling a shower head.
Haploid Gametophyte Generation* - meiosis - spores (n) - germination - young gametophyte plant with rhizoids (n) - cycle starts over
In addition to simple roots, many lycophytes and pterophytes have also developed symbiotic mycorrhizal relationships that help them obtain water and other nutrients from the soil. In fact, the gametophytes of lycophytes are non-photosynthetic and live hidden underground, where they get nutrients directly from symbiotic fungi.
Gymnosperms and Angiosperms: The Seed Plants
Today almost all lycophytes and pterophytes grow in the shadows of seed plants. In most of these plants, the entire male gametophyte is carried from one plant to another by the wind or by animals, instead of travelling through water. Seed plants are therefore not restricted to reproduction over short distances in wet conditions, and they have become the dominant land plants on Earth.
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Pollen grains are waterproof capsules that contain microscopic haploid male gametophytes (Figure 10(a)). Pollination occurs when the male gamete in a pollen grain penetrates an ovule containing a female gametophyte. The male gamete fertilizes the egg, producing a diploid zygote. This diploid zygote, or embryo, becomes a seed.
pollen: small structures called "grains" that contain a microscopic haploid male gametophyte
ovule: a small structure that contains a microscopic haploid female gametophyte
seed: a plant structure containing an embryo and a food supply, surrounded by a protective outer covering called the seed coat
Seeds contain a food supply for the embryo inside a seed coat (Figure 10(b)). They vary dramatically in size from the gigantic seeds of the Coco de Mar, or sea coconut, with a mass of more than 17 kg, to the microscopic seeds of certain orchids, with a mass of less than 0.001 g! Seeds can remain dormant, allowing the embryo to survive for extended periods of time until conditions are suitable for it to germinate.
Figure 10 (a) Pollen grains are microscopic and have variable shapes and surface features. (b) Seeds contain a young sporophyte embryo, a nutrient-rich tissue, and an outer protective coating.
Image: (a) Pollen grains; (b) The inside of a seed showing the seed coat which is the outside of the seed, the nutritive tissue, just inside the seed coat, and the embryo sporophyte, the center of the seed.
The food supply within the seed provides the young plant embryo with the nutrients it needs to grow a small root, a stem, and leaves before it is able to rely on photosynthesis. This food supply is a concentrated mix of starch, plant oils, and some protein. It is therefore a very important food source for many animals—including humans. More than 70 % of all human food supplies are derived from the seeds of just three plants: wheat, rice, and corn.
You will study the life cycles, structure, and functions of seed plants in much greater detail in the Plants unit. What follows now is an introduction to the most important members of the Plant kingdom.
Gymnosperms: The Conifers
Gymnosperms include the coniferous trees such as pines, spruce, cedars, and junipers, and other less well known groups of plants including the cycads and ginkgoes.
gymnosperm: a vascular plant that produces seeds in special structures called cones; gymnosperms are a major plant group
Cones are the reproductive structures of conifers. Male cones produce and release pollen, and female cones produce eggs. When an egg is successfully pollinated and fertilized within a female cone, an embryo develops within a seed in the cone.
cone: in plants, the reproductive structure of a conifer; produce either pollen or ovules
There are about 800 species of modern gymnosperms. Many of these are woody trees, and most have needle- or scale-like leaves. These narrow leaves and their thick cuticles are adaptations that help the trees reduce water loss. Gymnosperms have large, shallow root systems that form a mycorrhizal relationship with symbiotic fungi (Figure 11). Many gymnosperms are well adapted to resist hot dry summers and cold winters.
Gymnosperms are very valuable. They provide about 85 % of all wood used in construction and furniture manufacturing and are also the source of almost all pulp and paper.
Some gymnosperms are gigantic. The largest single conifer on Earth is in California. This giant sequoia named "General Sherman" is 83 m tall and has a circumference at its base of 33 m. Its estimated mass of 1400 tonnes is equivalent to the mass of about 10 000 average-sized adult humans! This single tree contains enough lumber to build more than 100 average-sized homes.
Figure 11 This cross-section shows the mycelium of the fungus extending far beyond the root system of the seedling.
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The boreal forests of northern Ontario are dominated by spruce trees and are a major source of lumber and pulp and paper. They also support many large ecosystems—they are home to many species of wildlife. Some gymnosperms can live for hundreds of years and are the foundation of well-established and complex ecosystems. For example, the Three Sisters (Figure 12) live in Carmanah Walbran Provincial Park in Vancouver, British Columbia. They are three Sitka spruces that share a root system. These 80 m tall trees are part of an ecosystem that is hundreds of years old!
Figure 12 The Three Sisters
Threats to the boreal forest, including from climate change and harvesting practices, have raised some serious concerns about future sustainability. In May 2007 more than 1500 scientists from around the world endorsed a document called the Canadian Boreal Forest Conservation Framework. Continued efforts have since led to a truly remarkable agreement between the Forest Products Association of Canada and nine leading environmental organizations. In May 2010 these organizations signed what is considered the largest forest conservation plan in history. The plan calls for strict environmental safeguards for 700 000 km2 of boreal forest, including 300 000 km2 where cutting will be halted to protect threatened woodland caribou.
Angiosperms: The Flowering Plants
More than 90 % of all modern plant species are angiosperms, or flowering plants. The more than 260 000 species of angiosperms dominate the modern world of plants. With the exception of coniferous trees, mosses, and ferns, virtually all familiar trees, shrubs, and herbaceous plants are angiosperms.
angiosperm: a plant that produces flowers; angiosperms form the largest group of living plants
As their common name suggests, angiosperms have specialized reproductive structures called flowers. Flowers perform the same function as cones in producing both pollen and eggs. However, in female flower parts, the eggs are protected in an enclosed ovary. After fertilization, seeds form within the ovary and the outer tissues of the ovary become a fruit. The main function of the fruit is to help disperse the seeds. The development of flowers and fruits is key to the success of the angiosperms.
flower: the specialized reproductive structure of an angiosperm; produces pollen and/or ovules
fruit: the mature ovary (or ovaries) of flowering plants that contain seeds; fruit help protect and disperse seeds
There are two types of seeds in angiosperms. Each seed contains either one ("mono") or two ("di") cotyledons. Cotyledons are structures that store food used by the growing embryo during germination.
cotyledon: a structure in the seeds of flowering plants that stores nutrients
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The two largest groups of flowering plants are the monocots and the eudicots. The monocots, comprising some 60 000 species, include more than 10 000 species of grasses and 20 000 species of orchids. The eudicots, with more than 200 000 species, account for most of the rest. Almost all flowering trees are eudicots. There are a number of smaller groups that do not belong to either the eudicots or monocots. These plants, which include magnolias, cinnamon, black pepper, and water lilies (Figure 13), also have seeds with two cotyledons.
Figure 13 Water lilies belong to a small group of flowering plants called Nymphaeales.
Flowers are extremely diverse (Figure 14(a) to (d)). These structures are specialized for the way the plant is pollinated. Many flowering plants are wind pollinated. The flowers on such plants are typically small and drab looking. Examples include the flowers of grasses. Other flowers are pollinated by animals such as bees, bats, and hummingbirds. These are often colourful and fragrant and produce nectar. These special features attract and reward pollinating animals. By carrying pollen from one plant to another, animal pollinators enable plants of the same species to engage in sexual reproduction.
Fruits are equally diverse (Figure 14(e) to (h)). Each fruit is adapted to protect and disperse the seeds within it. Dispersal methods include using wind, water, and other organisms. These adaptations help ensure that the plant reproduces successfully.
Figure 14 Flowers and fruits reflect the great diversity of the flowering plants. (a) Irises, (b) forget-me-nots, (c) yellow lady's-slippers, and (d) leatherleafs are all pollinated by animals. (e) Some fruits stick to animals, such as burdock, (f) some are eaten by animals, such as these twisted stalk berries, (g) some float in the air, such as this goat's beard, and (h) some float in the water, like these coconuts.
Conquering the Land
When plants colonized land more than 400 million years ago, it was one of the most dramatic events in the history of life on Earth. It created a diversity of living habitats and food sources for countless terrestrial animals, including you. Unfortunately this diversity is threatened by human actions. The most widespread threat is climate change, which has the potential to affect plants living everywhere on Earth. Overharvesting plants threatens many species, including Ontario's goldenseal, wild ginseng, and prickly pear cactus. Some plants are also particularly sensitive to various forms of pollution. For example, ground-level ozone—a major pollutant of urban environments—interferes with photosynthesis in some plant species.
UNIT TASK BOOKMARK
As you work on the Unit Task, consider threats to plants and other organisms that support your chosen group. Are any conservation efforts being taken to conserve the ecosystems the group is part of?
3.2 Summary
- Plants are large, multicellular eukaryotes that evolved from a group of green algae more than 400 million years ago.
- As producers, plants support virtually all terrestrial food webs.
- Plant life cycles alternate between haploid and diploid generations.
- Bryophytes are simple plants with a waxy cuticle and stomata to reduce water loss and allow gas exchange.
- Ferns are seedless vascular plants with large leaves and simple roots.
- Gymnosperms are seed plants that reproduce with specialized cones that produce pollen grains and ovules.
- Angiosperms are the most diverse group of plants and reproduce using flowers. Angiosperms produce seeds within a specialized structure called a fruit.
- Flowering plants provide most of the food consumed by humans and our domesticated animals. The three most important food crops are wheat, rice, and corn. All are monocot grasses.
- Plants are threatened by a number of human actions, and by climate change.
Key
K/U: Knowledge and Understanding T/I: Thinking and Investigation C: Communication A: Application
3.2 Questions
1. Review the evidence that links plants to charophytes. K/U (a) List features shared by plants and charophytes that are not shared with most other eukaryotes. (b) Based on your understanding of endosymbiosis and the evolution of eukaryotes, would you expect any photosynthetic bacteria to share the same pigments as charophytes and plants?
2. Explain how each of the following adaptations dramatically enhanced the success of land plants: K/U C (a) vascular tissue (b) animal pollination (c) pollen grains (d) seeds (e) fruit (f) mycorrhiza
3. MWhich of the adaptations from Question 2 occurs in the following groups of plants? List the adaptations beside and the plant groups. K/U C (a) mosses (b) ferns (c) conifers (d) flowering plants
4. Consider the sporophytes and gametophytes of mosses and ferns: K/U C (a) In which group(s) does the sporophyte become independent and photosynthetic? (b) Which forms require liquid water in order to reproduce? (c) Which are able to spread farther, their spores or their gametes? Explain.
5. Why are seeds a particularly valuable source of food for humans and other animals? K/U
6. Many colourful flowers reward animal pollinators with a supply of sweet nectar. This encourages animals to visit the flowers and, in so doing, carry pollen from one flower to another. Many fruits, containing the seeds of the plant, are also colourful and sweet. Brainstorm the possible benefit(s) to the plant when animals feed on these fruits. A
7. A group of 34 regions on Earth, referred to as biodiversity hotspots, contain more than half of all plant species but cover only 2.3 % of Earth's land area. Use the Internet and other resources to find out more about these special places. Choose one hotspot and find out where it is located and what types of plants and animals live there. Report your findings to the class. T/I C
Plants dominate most of the land on Earth. A small patch of forest contains thousands of trees (Figure 1(a)), while a golf course is covered in millions of individual grass plants (Figure 1(b)). Plants are ubiquitous in cities as well as in rural settings, indoors and outside. Even if you stay inside all day, you will almost certainly use plant products and eat a variety of plant foods (Figure 1(c)). Plants are so abundant that we may forget just how important they are.
Figure 1 (a) From forests to (b) golf courses, much of Earth's surface is covered by plants. (c) Plants and plant products are all around us, in the products we use and the foods we eat.
Although we are surrounded and supported by ecosystems dominated by plants, most people know very little about them. But as you are about to learn, with more than 270 000 known species, plants are fascinating and incredibly diverse. The largest plants tower more than 100 m into the air, and the smallest are little more than a millimetre across. Today's oldest living tree has a root system that is over 9550 years old! This section gives an introduction to the characteristics and diversity of plants as well as an overview of how they adapted to life on land. The Plants unit provides a more detailed study of plant anatomy, growth, and function.
Why Plants Are Important
Plants can survive in many different climatic conditions, from hot and humid tropics to parched deserts and the extreme cold of the High Arctic (Figure 2). Because plants are producers of food, other terrestrial life forms follow wherever plants live. In fact, the rich diversity of terrestrial ecosystems depends heavily on the diversity of plants. Without plants to supply food through photosynthesis, there would be little life on land.
In addition to supporting food webs, plants also provide other organisms with places to live, such as nesting locations for birds and supports for a spider's web. Humans are particularly dependent on plants for a wide range of valuable substances including medicines, clothing, wood, and paper products.
Unfortunately, many of the world's plants are threatened with extirpation or extinction. Habitat destruction, invasive species, pollution, and climate change all pose serious threats to plants and the organisms that depend on them. Today, 77 of Ontario's native plant species are listed as "species at risk." Of these, three species are extirpated—no longer living anywhere in the province.
Figure 2 These Arctic poppies are found in Arctic and High Arctic climates, including Baffin Island.
Image: A plant, the Arctic poppies in snow and ice.
Classification and Phylogeny
Plants are thought to have evolved from charophytes, a group of green algae (which belong to the kingdom Protista), between 425 million and 490 million years ago. There is very strong evidence supporting this evolutionary relationship. Plants and green algae both contain chlorophyll a and chlorophyll b, two forms of the green pigment used in photosynthesis. They also contain pigments that are not found in other photosynthetic eukaryotes. Plants and green algae share several other characteristics. At the end of mitosis, only the cells of plants and green algae begin cytokinesis by building a cell plate across the middle of the cell. Their cell walls contain large amounts of cellulose, a complex sugar molecule. Plants and green algae also store excess food as starch.
charophyte: the common name for organisms in the order Charophyceae; green algae in the kingdom Protista
LEARNING TIP
“phyte”?
Many plant terms end with the suffix –phyte. Phyte is derived from the ancient Greek word phut (όn) meaning “plant.”
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Today there are more than 270 000 living species of plants. These are classified into four major groups. The phylogenetic tree in Figure 3 shows the evolutionary relationships between the major groups of plants.
Figure 3 A simplified phylogenetic tree of the Plant Kingdom. Many biologists now include green algae within the Plant kingdom and refer to the other groups as the "higher plants."
Image: A phylogenetic tree.
- early green algae
-- green algae
-- early nonvascular plants
-- nonvascular plants, bryophytes
-- early vascular plants
-- seedless vascular plants, pteridophytes
-- first seed plants
-- gymnosperms
-- diversification of flowering plants
-- angiosperms
At top of tree is a table called Millions of Years Ago, runs across top of tree starting from left 500, 400 (Paleozoic), 300, 200 (Mesozoic), 100, (Cronozoic), 0.
Characteristics
All higher plants are multicellular eukaryotic organisms. Unlike animals and fungi, almost all plants perform photosynthesis. All have cell walls composed primarily of cellulose. Like fungi, all terrestrial plants are sessile—they cannot move from place to place. Although all plants share these basic features, plants show an incredible diversity of characteristics (Figure 4). GO TO NELSON SCIENCE
CAREER LINK
Botanist
Botanists specialize in the study of plants. To learn more about botanists, GO TO NELSON SCIENCE
Figure 4 (a) In tropical rainforests, a great variety of plant species cover every square metre of available space, including the branches of trees. (b) Sundews are carnivorous plants native to Ontario bogs. Their leaf hairs secrete a sweet, sticky liquid that attracts and traps insect prey. These hairs and the leaf blade then slowly curl inward on the prey.
Alternation of Generations
Plants have a very different life cycle than that of animals. This life cycle is known as an "alternation of generations." This life cycle has diploid and haploid states, or generations. The diploid generation produces spores, and the haploid generation produces gametes.
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UNIT TASK BOOKMARK
Compare and contrast the life cycles of the organisms in your chosen group. Do the conservation efforts aimed at this group need to take into account any features of this life cycle?
Typical animal body cells are diploid (2n), containing two sets of chromosomes. These cells undergo meiosis, producing haploid sex cells, or gametes. Two haploid (n) cells fuse in the process of fertilization, forming a diploid zygote that then grows into the adult animal (Figure 5(a)).
A plant in the diploid stage is called a sporophyte (Figure 5(b)). As in animals, the sporophyte's diploid cells divide by meiosis to produce haploid cells. In plants, however, these haploid cells form asexual spores. As the haploid generation begins, the spores grow into gametophyte individuals. The gametophytes matufe, then produce haploid sex cells—gametes. These gametes undergo fertilization to form diploid zygotes. The zygotes grow into sporophyte individuals, and the cycle continues.
Figure 5 (a) A typical animal life cycle consists only of diploid individuals that produce haploid sex cells. (b) Plant life cycles alternate between a diploid sporophyte generation and a haploid gametophyte generation.
Image: (a) The Animal Life Cycle: Adult (multicellular)(2n), then; meiosis, then, sex cells (n), then, fertilization, then, zygote (2n), then back to Adult (multicellular)(2n). A quarter of the inner circle is shaded haploid (n). In middle of circle is diploid (2n).
(b) Plant Life Cycle: Sporophyte (multicellular) (2n), then, melosis, spores (n), then, gametophyte (multicellular) (n), then, sex cells (n), then, fertilization, then, zygote (2n), then back to Sporophyte (multicellular) (2n). Half of the inner circle is shaded haploid (n). In the middle of the circle is diploid (2n).
Key
K/U: Knowledge and Understanding
T/I: Thinking and Investigation
C: Communication
A: Application
Research This
Climate Change and Plant Communities
Skills: Researching, Analyzing, Evaluating, Communicating
SKILLS HANDBOOK A2.1, A5.1
Climate has a dramatic influence on the distribution and success of plant communities. Each plant species is ideally suited for a particular range of temperatures, water availability, and seasons. The greenhouse gases that humans release into the atmosphere are contributing to climate change. The main human-generated greenhouse gas is carbon dioxide—a key reactant in photosynthesis. In this activity you will research some of the ways in which the changing climate and increasing levels of atmospheric carbon dioxide are likely to affect plant distribution and diversity.
1. Use print and online resources to research the following topics:
(i) How might an earlier spring and later fall influence plants?
(ii) Explain how climate change is causing Ontario's plant communities to move northward.
(iii) What effects could a changing climate have on plant pests and disease? On forest fires?
(iv) How are increasing concentrations of carbon dioxide in the atmosphere likely to influence plants?
A. Based on your research, how will climate change and increasing levels of carbon dioxide in the atmosphere likely affect plant distribution? T/I
B. How will climate change and the resulting changes in plant distribution affect biodiversity? Consider the effects on the biodiversity of plants as well as the ecosystems and food webs they support. T/I A
Early Adaptations for Life on Land
Most plant species living today live on the land. There are some aquatic species, but most are freshwater—only a few are marine. The primitive ancestors of today's plants, however, lived in the water. They were probably very much like green
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algae—relatively small, photosynthetic, multicellular organisms. These early plants, like green algae living today, would have reproduced sexually using eggs and sperm that swam through the water.
When plants first began to live on land, they developed characteristics that allowed them to live and reproduce in that new environment. Although life on land gave plants lots of light and carbon dioxide—both essential for photosynthesis—it forced plants to adapt in other ways.
The first requirement for life on land was the ability to prevent water loss. When exposed to dry air and warm temperatures, unprotected living cells quickly lose water through osmosis and evaporation. Plants developed a flexible, waxy cuticle on their outer surfaces that effectively prevents water loss (Figure 6(a)).
cuticle: a waterproof, waxy coating produced by the epidermis of most plants
Although preventing water loss is critical for land plants, they must be able to take in small amounts of carbon dioxide gas from the atmosphere in order to perform photosynthesis. To meet this need, plants developed stomata (singular: stoma), tiny openings between specialized cells on the plant's surface (Figure 6(b)). These stomata are like windows that can be opened and closed. When they are open, gases can diffuse into and out of the plant. When they are closed, the plant is effectively sealed off from the surrounding air.
stoma: a small opening in the epidermis of a plant that allows gas exchange
Figure 6 (a) Almost all plant surfaces produce a waterproof, waxy cuticle. (b) Stomata are small openings in the epidermis that allow gas exchange with the air. Most leaves have thousands of stomata per square centimetre.
Images: (a) A diagram showing a plant’s epidermal cell, and a waxy cuticle on the outside of the plant; (b) A microscopic image showing the epidermal cell and the stoma.
Bryophytes: The Mosses
The bryophytes are the simplest of land plants. This group includes the mosses, liverworts, and hornworts. The most recognizable of these are the mosses—there are more than 1000 species of moss in North America alone! The peat mosses, of the genus Sphagnum, may be the most numerous plants on Earth. In many ecosystems, including Canada's boreal forest, Sphagnum mosses can form a continuous ground cover over hundreds and thousands of square kilometres (Figure 7).
bryophyte: a small seedless plant that lacks vascular tissue
Figure 7 Various species of sphagnum moss form a thick covering in this bog.
Like the earliest of land plants, bryophytes have a protective cuticle and stomata for gas exchange. They do not have specialized vascular tissue or true leaves, roots, or seeds. Most bryophytes are only a few centimetres in height.
The gametophyte generations of bryophytes produce swimming sperm in structures called antheridia (singular: antheridium) and eggs (in structures called archegonia (singular: archegonium). Bryophytes can therefore only live and reproduce in places with at least occasional wet conditions, such as rains or heavy dew.
antheridium: the specialized structure on a gametophyte that produces sperm
archegonium: the specialized structure on a gametophyte that produces eggs
The green plants we recognize as mosses are the photosynthetic gametophyte individuals. After fertilization, the new, non-photosynthetic sporophyte individuals grow out of the archegonia on the female gametophyte and get their nourishment from the gametophyte. These sporophytes seem to be part of the gametophyte but are actually the offspring of the gametophyte plants. The sporophytes grow a tall stalk that bears a structure called a sporangium (plural: sporangia), in which haploid spores are produced. These tiny spores are easily carried by winds and dispersed over a large area.
sporangium: the structure in which spores are produced
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Figure 8 illustrates the alternation of generations life cycle of a typical moss. Some moss gametophytes can also reproduce asexually. Small clumps of photosynthetic haploid cells called gemmae (singular: gemma) are produced in cup-shaped structures on the surface of the gametophytes. The gemmae are dispersed by splashes of rain and grow into new gametophyes.
gemma: a small clump of haploid photosynthetic cells-produced in little cup-shaped structures on the gametophyte plant; dispersed by splashes of rain to grow into another gametophyte plant
Figure 8 The life cycle of a moss. In order to fertilize the eggs, sperm must swim through water on the surface of the gametophytes.
Image: The life cycle of a typical moss in list format.
Key:
Haploid-*
Diploid-
Haploid Gametophyte Generation*
- Archegonia with eggs (n)
Image: Sperm and eggs
Diploid Sporophyte Generation
- young sporophyte plant (2n)
- sporangium (2n)
(sporophyte plant)
- stalk (2n)
- top of gametophyte plant♀
- meiosis
- spores (n)
Haploid Gametophyte Generation*
- germination spore (n)
- young gametophyte plant (n)
- mature gametophyte plant (n)
- ♀ plant leads back to Archegonia with eggs (n)
- ♂ plant leads to Archegonia with sperm (n)
Lycophytes and Pterophytes: The Ferns
The next major adaptation by land plants was the development of vascular tissue. Vascular tissue consists of xylem and phloem. It is specialized for the transportation of water and nutrients. Plants with vascular tissue can grow to great heights, thereby accessing more sunlight. Lignin is a key chemical component of vascular tissue. It is an extremely strong compound that makes cell walls more rigid. It is responsible for the great strength of woody tissues.
lignin: an important structural compound found in the vascular tissues of plants; it is responsible for the strength of wood
The lycophytes (club mosses) and pterophytes (ferns and their relatives) are groups of seedless vascular plants that still have many characteristics of the earliest vascular plants. During the Carboniferous period, about 360 million to 300 million years ago, these plants dominated the landscape. Some grew as large as 2 m in diameter and more than 40 m in height! Today's 13 000 or so species remain widespread and common, but most are relatively small.
lycophyte: a seedless vascular plant; club mosses are examples of lycophytes
pterophyte: a seedless vascular plant; ferns are examples of pterophytes
Among the simplest of vascular plants are members of the genus Equisetum called horsetails. They are considered "living fossils" because they are the only remaining genus of what was once a very diverse group of plants. As they grow, silica crystals form on their stems.
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Similar to bryophytes, the gametophyte individuals of lycophytes and pterophytes reproduce sexually using sperm and eggs. Unlike the bryophytes, however, the sporophytes are photosynthetic and much larger than the gametophytes (Figure 9). Both groups have simple roots and stems. The stems of ferns are usually in the form of rhizomes that grow horizontally just below the surface of the ground. Ferns also have large green leaves called fronds.
rhizome: a horizontal underground stem
frond: a fern leaf; young curled fronds are often called "fiddleheads" because of their distinctive shape; some fiddleheads are edible, but many are toxic
Figure 9 Life cycle of a fern. Note that the sporophyte is photosynthetic and much larger than the gametophyte. The gametophyte (less than 1 cm across) is greatly enlarged in this diagram.
Image: The life cycle of a fern in list format.
Key:
Haploid-*
Diploid-
Haploid Gametophyte Generation*
- mature gametophyte plant with rhizoids and sex organs (n), showing ♀- one of many archegonia with eggs (n), and ♂- one of many antheridia with sperm (n)
Diploid Sporophyte Generation
- fertilization
- young sporophyte plant (2n) growing out of gametophyte plant
- mature sporophyte plant (2n): (a) frond, (b) rhizome (c) roots
Image: The sporangium on underside of frond, resembling a shower head.
Haploid Gametophyte Generation*
- meiosis
- spores (n)
- germination
- young gametophyte plant with rhizoids (n)
- cycle starts over
In addition to simple roots, many lycophytes and pterophytes have also developed symbiotic mycorrhizal relationships that help them obtain water and other nutrients from the soil. In fact, the gametophytes of lycophytes are non-photosynthetic and live hidden underground, where they get nutrients directly from symbiotic fungi.
Gymnosperms and Angiosperms: The Seed Plants
Today almost all lycophytes and pterophytes grow in the shadows of seed plants. In most of these plants, the entire male gametophyte is carried from one plant to another by the wind or by animals, instead of travelling through water. Seed plants are therefore not restricted to reproduction over short distances in wet conditions, and they have become the dominant land plants on Earth.
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Pollen grains are waterproof capsules that contain microscopic haploid male gametophytes (Figure 10(a)). Pollination occurs when the male gamete in a pollen grain penetrates an ovule containing a female gametophyte. The male gamete fertilizes the egg, producing a diploid zygote. This diploid zygote, or embryo, becomes a seed.
pollen: small structures called "grains" that contain a microscopic haploid male gametophyte
ovule: a small structure that contains a microscopic haploid female gametophyte
seed: a plant structure containing an embryo and a food supply, surrounded by a protective outer covering called the seed coat
Seeds contain a food supply for the embryo inside a seed coat (Figure 10(b)). They vary dramatically in size from the gigantic seeds of the Coco de Mar, or sea coconut, with a mass of more than 17 kg, to the microscopic seeds of certain orchids, with a mass of less than 0.001 g! Seeds can remain dormant, allowing the embryo to survive for extended periods of time until conditions are suitable for it to germinate.
Figure 10 (a) Pollen grains are microscopic and have variable shapes and surface features. (b) Seeds contain a young sporophyte embryo, a nutrient-rich tissue, and an outer protective coating.
Image: (a) Pollen grains; (b) The inside of a seed showing the seed coat which is the outside of the seed, the nutritive tissue, just inside the seed coat, and the embryo sporophyte, the center of the seed.
The food supply within the seed provides the young plant embryo with the nutrients it needs to grow a small root, a stem, and leaves before it is able to rely on photosynthesis. This food supply is a concentrated mix of starch, plant oils, and some protein. It is therefore a very important food source for many animals—including humans. More than 70 % of all human food supplies are derived from the seeds of just three plants: wheat, rice, and corn.
You will study the life cycles, structure, and functions of seed plants in much greater detail in the Plants unit. What follows now is an introduction to the most important members of the Plant kingdom.
Gymnosperms: The Conifers
Gymnosperms include the coniferous trees such as pines, spruce, cedars, and junipers, and other less well known groups of plants including the cycads and ginkgoes.
gymnosperm: a vascular plant that produces seeds in special structures called cones; gymnosperms are a major plant group
Cones are the reproductive structures of conifers. Male cones produce and release pollen, and female cones produce eggs. When an egg is successfully pollinated and fertilized within a female cone, an embryo develops within a seed in the cone.
cone: in plants, the reproductive structure of a conifer; produce either pollen or ovules
There are about 800 species of modern gymnosperms. Many of these are woody trees, and most have needle- or scale-like leaves. These narrow leaves and their thick cuticles are adaptations that help the trees reduce water loss. Gymnosperms have large, shallow root systems that form a mycorrhizal relationship with symbiotic fungi (Figure 11). Many gymnosperms are well adapted to resist hot dry summers and cold winters.
Gymnosperms are very valuable. They provide about 85 % of all wood used in construction and furniture manufacturing and are also the source of almost all pulp and paper.
Some gymnosperms are gigantic. The largest single conifer on Earth is in California. This giant sequoia named "General Sherman" is 83 m tall and has a circumference at its base of 33 m. Its estimated mass of 1400 tonnes is equivalent to the mass of about 10 000 average-sized adult humans! This single tree contains enough lumber to build more than 100 average-sized homes.
Figure 11 This cross-section shows the mycelium of the fungus extending far beyond the root system of the seedling.
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The boreal forests of northern Ontario are dominated by spruce trees and are a major source of lumber and pulp and paper. They also support many large ecosystems—they are home to many species of wildlife. Some gymnosperms can live for hundreds of years and are the foundation of well-established and complex ecosystems. For example, the Three Sisters (Figure 12) live in Carmanah Walbran Provincial Park in Vancouver, British Columbia. They are three Sitka spruces that share a root system. These 80 m tall trees are part of an ecosystem that is hundreds of years old!
Figure 12 The Three Sisters
Threats to the boreal forest, including from climate change and harvesting practices, have raised some serious concerns about future sustainability. In May 2007 more than 1500 scientists from around the world endorsed a document called the Canadian Boreal Forest Conservation Framework. Continued efforts have since led to a truly remarkable agreement between the Forest Products Association of Canada and nine leading environmental organizations. In May 2010 these organizations signed what is considered the largest forest conservation plan in history. The plan calls for strict environmental safeguards for 700 000 km2 of boreal forest, including 300 000 km2 where cutting will be halted to protect threatened woodland caribou.
Angiosperms: The Flowering Plants
More than 90 % of all modern plant species are angiosperms, or flowering plants. The more than 260 000 species of angiosperms dominate the modern world of plants. With the exception of coniferous trees, mosses, and ferns, virtually all familiar trees, shrubs, and herbaceous plants are angiosperms.
angiosperm: a plant that produces flowers; angiosperms form the largest group of living plants
As their common name suggests, angiosperms have specialized reproductive structures called flowers. Flowers perform the same function as cones in producing both pollen and eggs. However, in female flower parts, the eggs are protected in an enclosed ovary. After fertilization, seeds form within the ovary and the outer tissues of the ovary become a fruit. The main function of the fruit is to help disperse the seeds. The development of flowers and fruits is key to the success of the angiosperms.
flower: the specialized reproductive structure of an angiosperm; produces pollen and/or ovules
fruit: the mature ovary (or ovaries) of flowering plants that contain seeds; fruit help protect and disperse seeds
There are two types of seeds in angiosperms. Each seed contains either one ("mono") or two ("di") cotyledons. Cotyledons are structures that store food used by the growing embryo during germination.
cotyledon: a structure in the seeds of flowering plants that stores nutrients
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The two largest groups of flowering plants are the monocots and the eudicots. The monocots, comprising some 60 000 species, include more than 10 000 species of grasses and 20 000 species of orchids. The eudicots, with more than 200 000 species, account for most of the rest. Almost all flowering trees are eudicots. There are a number of smaller groups that do not belong to either the eudicots or monocots. These plants, which include magnolias, cinnamon, black pepper, and water lilies (Figure 13), also have seeds with two cotyledons.
Figure 13 Water lilies belong to a small group of flowering plants called Nymphaeales.
Flowers are extremely diverse (Figure 14(a) to (d)). These structures are specialized for the way the plant is pollinated. Many flowering plants are wind pollinated. The flowers on such plants are typically small and drab looking. Examples include the flowers of grasses. Other flowers are pollinated by animals such as bees, bats, and hummingbirds. These are often colourful and fragrant and produce nectar. These special features attract and reward pollinating animals. By carrying pollen from one plant to another, animal pollinators enable plants of the same species to engage in sexual reproduction.
Fruits are equally diverse (Figure 14(e) to (h)). Each fruit is adapted to protect and disperse the seeds within it. Dispersal methods include using wind, water, and other organisms. These adaptations help ensure that the plant reproduces successfully.
Figure 14 Flowers and fruits reflect the great diversity of the flowering plants. (a) Irises, (b) forget-me-nots, (c) yellow lady's-slippers, and (d) leatherleafs are all pollinated by animals. (e) Some fruits stick to animals, such as burdock, (f) some are eaten by animals, such as these twisted stalk berries, (g) some float in the air, such as this goat's beard, and (h) some float in the water, like these coconuts.
Conquering the Land
When plants colonized land more than 400 million years ago, it was one of the most dramatic events in the history of life on Earth. It created a diversity of living habitats and food sources for countless terrestrial animals, including you. Unfortunately this diversity is threatened by human actions. The most widespread threat is climate change, which has the potential to affect plants living everywhere on Earth. Overharvesting plants threatens many species, including Ontario's goldenseal, wild ginseng, and prickly pear cactus. Some plants are also particularly sensitive to various forms of pollution. For example, ground-level ozone—a major pollutant of urban environments—interferes with photosynthesis in some plant species.
UNIT TASK BOOKMARK
As you work on the Unit Task, consider threats to plants and other organisms that support your chosen group. Are any conservation efforts being taken to conserve the ecosystems the group is part of?
3.2 Summary
- Plants are large, multicellular eukaryotes that evolved from a group of green algae more than 400 million years ago.
- As producers, plants support virtually all terrestrial food webs.
- Plant life cycles alternate between haploid and diploid generations.
- Bryophytes are simple plants with a waxy cuticle and stomata to reduce water loss and allow gas exchange.
- Ferns are seedless vascular plants with large leaves and simple roots.
- Gymnosperms are seed plants that reproduce with specialized cones that produce pollen grains and ovules.
- Angiosperms are the most diverse group of plants and reproduce using flowers. Angiosperms produce seeds within a specialized structure called a fruit.
- Flowering plants provide most of the food consumed by humans and our domesticated animals. The three most important food crops are wheat, rice, and corn. All are monocot grasses.
- Plants are threatened by a number of human actions, and by climate change.
Key
K/U: Knowledge and Understanding
T/I: Thinking and Investigation
C: Communication
A: Application
3.2 Questions
1. Review the evidence that links plants to charophytes. K/U
(a) List features shared by plants and charophytes that are not shared with most other eukaryotes.
(b) Based on your understanding of endosymbiosis and the evolution of eukaryotes, would you expect any photosynthetic bacteria to share the same pigments as charophytes and plants?
2. Explain how each of the following adaptations dramatically enhanced the success of land plants: K/U C
(a) vascular tissue
(b) animal pollination
(c) pollen grains
(d) seeds
(e) fruit
(f) mycorrhiza
3. MWhich of the adaptations from Question 2 occurs in the following groups of plants? List the adaptations beside and the plant groups. K/U C
(a) mosses
(b) ferns
(c) conifers
(d) flowering plants
4. Consider the sporophytes and gametophytes of mosses and ferns: K/U C
(a) In which group(s) does the sporophyte become independent and photosynthetic?
(b) Which forms require liquid water in order to reproduce?
(c) Which are able to spread farther, their spores or their gametes? Explain.
5. Why are seeds a particularly valuable source of food for humans and other animals? K/U
6. Many colourful flowers reward animal pollinators with a supply of sweet nectar. This encourages animals to visit the flowers and, in so doing, carry pollen from one flower to another. Many fruits, containing the seeds of the plant, are also colourful and sweet. Brainstorm the possible benefit(s) to the plant when animals feed on these fruits. A
7. A group of 34 regions on Earth, referred to as biodiversity hotspots, contain more than half of all plant species but cover only 2.3 % of Earth's land area. Use the Internet and other resources to find out more about these special places. Choose one hotspot and find out where it is located and what types of plants and animals live there. Report your findings to the class. T/I C