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S E C T I O N 13.2 Kingdom Plantae 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 plants. Demonstrate an understanding of the diversity of living organisms. Figure 13.6 Members of the plant kingdom are able to thrive in unexpected locations, such as this epiphyte, which lives on the surface of another plant. Imagine you are walking beside a stream in a forest. Beds of soft green moss cover rocks beside the water and clumps of ferns spring from the decaying trunk of a fallen tree. Seeing a patch of sunlight ahead, you leave the shade of the tall pines and enter a clearing filled with the pinkishpurple spikes of fireweed. This short journey has taken you past examples of all four major groups within the plant kingdom. In fact, a short walk through most places on Earth will take you past members of the same four groups: mosses and their relatives, ferns and their relatives, cone-bearing plants, and flowering plants. Classification of Plants There is a huge variety of plants in the world, but they can all be arranged into a few major groups on the basis of several fundamental characteristics, such as the presence or absence of vascular tissue and seeds. Figure 13.7 shows members of these four groups. Each large group contains several Divisions, which are each roughly equivalent to a Phylum. Figure 13.7 The Kingdom Plantae is divided into four major groups. Plants range in size from tiny mosses to giant pine trees. Plants and Animals • MHR 469 Non-vascular Plants (Mosses and Their Relatives) Instead, they have small root-like structures called rhizoids, which develop from their lower surfaces. Non-vascular plants are the only groups of plants in which the life cycle is dominated by the gametophyte phase. The identifiable green plants that are common in moist shady areas are gametophytes. Male gametophytes produce sperm, and female gametophytes produce eggs. For fertilization to occur, there must be enough moisture on the plant surface for the sperm to swim to an egg. After fertilization, the zygote remains on the female plant and develops into a sporophyte. There are three divisions of non-vascular plants: mosses (Bryophytes), hornworts (Anthocerophytes), and liverworts (Hepatophytes). These plants do not have vascular tissue, and they are dependent on the processes of diffusion and osmosis to transport nutrients. They usually grow in mats of low, tangled vegetation that can hold water like a sponge, allowing them to survive periods of cold and dry. Non-vascular plants have no roots. A The gametophyte generation produces sporophytes that grow up on tall stalks above the gametophyte. meiosis spores sporophyte (2n) B Within the end capsule of the sporophyte, spores are produced through the process of meiosis. Spores are released when the capsule bursts and they disperse with the wind. The spore germinates on the ground and develops into a male or female gametophyte. gametophyte (n) spore Sporophyte generation Gametophyte generation zygote (2n) F A zygote is produced, which undergoes mitosis and forms a new sporophyte. male gametophyte (n) female gametophyte (n) fe rti liz at io n antheridium sperm archegonium (n) Egg (n) E The antheridium releases sperm, which swim to the archegonium and fertilization takes place. sperm archegonium Figure 13.8 The life cycle of a moss 470 MHR • Diversity of Living Things egg D The male gametophyte develops an antheridium, which is the structure that produces sperm. C The female gametophyte develops an archegonium, which is the structure that produces eggs. Mosses (Bryophytes) Bryophytes, commonly known as mosses, lack vascular tissue and do not have well-developed roots, so they absorb most of their water directly through their surface. When the air is dry, mosses become dry; when wet conditions return, mosses quickly absorb water. Although they seem to have disadvantages compared with other plants, mosses are very successful and widespread. They thrive in such diverse habitats as bogs, tundra, on bare exposed rocks, and in deep shade where other plants may be unable to grow. In fact, mosses are the most diverse group of plants after flowering plants, and there are twice as many species of mosses as of mammals. A typical moss sporophyte consists of a sporebearing capsule growing on the end of a stalk called the seta. Sporophytes do not contain chlorophyll and receive all their nourishment from the gametophyte. Spores are produced by meiosis in the capsule of the sporophyte as shown in Figure 13.8 on the previous page. As the sporophyte dries out, the capsule releases spores. If these spores germinate once they land on the ground, they will become either a male gametophyte, called an antheridium (plural antheridia) or a female gametophyte, called an archegonium (plural archegonia). Sperm are released from the antheridium and swim to the archegonium, where they fertilize the egg. A zygote develops, and grows to become a new sporophyte. You can often see a scattering of sporophytes standing up above a bed of green moss gametophytes as shown in Figure 13.9. BIO FACT Scientists are still discovering how some mosses withstand extreme drying. The cells of Tortula ruralis appear similar to those in the leaves of vascular plants. When this moss is dried out, it folds up tightly and its cells lose much of their cytoplasm. Their mitochondria and chloroplasts shrink and the cells lose their stored starch. Photosynthesis ceases and respiration slows or stops. Despite such apparently serious damage to their cellular structure, the plants unfold and return to their normal hydrated state within two minutes after water is added. Photosynthesis resumes about 24 h later. Much of the cell repair appears to be controlled by one or more genes. If scientists can locate and copy these genes, they may be able to add them to crop plants to increase their drought tolerance. Liverworts (Hepatophytes) Hepatophytes, known commonly as liverworts, grow flat and close to the ground and are rarely more than 30 cells thick. There are two types of liverworts, with different appearances shown in Figure 13.10. About 80% of species have “leafy” gametophytes that resemble mosses. Leafy liverworts are most abundant in tropical forests and in humid climates. In appearance, their gametophyte generation resembles mosses. The remaining species are thallose liverworts, called this because the gametophytes are made up of flattened, lobed bodies called thalli (singular thallus). The common name, liverwort, comes from the appearance of these gametophytes, which resemble the lobes of animals’ livers. A Figure 13.9 The spore-bearing capsules of a moss stand B up on brown stalks above the leafy gametophyte. Figure 13.10 Liverworts may be (A) thallose or (B) leafy. The leafy liverwort resembles moss. Plants and Animals • MHR 471 BIO FACT Using DNA analysis, scientists have uncovered evidence that liverworts are more closely related to the first land plants than any other plants living today. Researchers searched the DNA from 352 species of plants for three pieces of genetic material called introns. Nearly all of the mosses, hornworts, ferns, conifers, and flowering plants the scientists examined had at least two of the three introns. Only liverworts, like aquatic green algae, lacked all three. This suggests that liverworts are the closest relatives of algae on land. As land plants evolved and diversified, they acquired the introns missing in liverworts and green algae. Investigation Liverworts produce sporophytes that grow on the body of the gametophyte. Spores are formed inside an egg-shaped capsule held at the top of a translucent or white stalk. When it matures, the capsule splits open into four equal quarters, releasing the spores into the air. Liverwort sporophytes are not often seen because they shrivel up and disappear shortly after releasing their spores. Liverworts can also reproduce asexually by means of tiny pieces of tissue that become detached from the thallus. Hornworts (Anthocerophytes) Anthocerophytes are commonly known as hornworts. Their gametophytes are broad, flat, usually less than 2 cm in diameter, and have a blue-green colour. Figure 13.11 shows the mature SKILL FOCUS 1 3 • B Performing and recording Alternation of Generations in Mosses In mosses, both the spore-bearing sporophyte and the gamete-bearing gametophyte are visible to the naked eye. In this investigation, you will study the structure of both generations. Analyzing and interpreting Communicating results Conducting research Pre-lab Questions Is the gametophyte haploid or diploid? In which generation does meiosis occur? Problem What are the characteristics of the alternating generations in mosses? Prediction Predict the importance of identifying the reproductive structures of both generations of mosses. CAUTION: The scalpel blade is sharp. Always cut away from your body. Materials microscope microscope slides single-edged razor forceps blade or scalpel dropper water moss plants with male and female gametophytes and sporophytes paper towels 472 MHR • Diversity of Living Things Procedure 1. Obtain moss gametophytes with both male and female reproductive structures. You may use Figure 13.8 on page 470 for reference. 2. Using forceps, remove all leaves from the upper part of one stem, being careful not to damage the reproductive structure at the tip of the stem. Repeat with a stem of the opposite sex. sporophyte, which look like miniature green cattle horns thus giving the group its common name. The sporophyte continues to grow throughout its life. As in other bryophytes, it remains attached to its parent. Eventually the sporophyte splits into two halves lengthwise, releasing the maturing spores. Although hornworts and liverworts are similar in appearance, they can be distinguished from each other. The best way to differentiate them is to look at their cells under a low-power microscope. Hornworts usually have one large chloroplast per cell, while liverworts typically have many small chloroplasts per cell. sporophyte with sporangium (2n) gametophyte (n) Figure 13.11 Hornwort sporophytes and gametophytes. 3. With a sharp blade, cut off the top 1 cm of each stem you have prepared. 4. Place each stem tip at opposite ends of a clean slide. Add several drops of water, and place a second glass slide over the first. 5. Gently press down with the eraser end of a pencil to slightly squash the reproductive structures. 6. Place the sandwiched slides on a microscope stage, using the stage clips to keep them from slipping apart. Observe the reproductive structures under low-power magnification only. 7. Draw both types of reproductive structures. Using Figure 13.8 as a guide, label the male and female gametophytes, antheridium, and archegonium. 8. Obtain a sample of moss with sporophytes. 9. Using forceps, carefully remove a small capsule from the tip of a stalk, and mount it in several drops of water on a microscope slide. 10. Add a second slide on top of the first and carefully squash the capsule as in the procedure above. 11. Observe the squashed capsule under lowpower magnification only. Draw and label the sporophyte, capsule, and spores. Wash your hands thoroughly at the end of this procedure. Post-lab Questions 1. Which structure is responsible for a) sexual reproduction? b) asexual reproduction? 2. What type of reproductive cell is formed by the a) archegonium? b) antheridium? c) sporophyte? 3. Which cells are a) haploid? b) diploid? Conclude and Apply 4. Diagram the life cycle of a moss, using your own observations. Use different colours to indicate the haploid and diploid stages. Mark where fertilization and meiosis occur. 5. Are eggs or sperm cells produced in greater numbers? Suggest an explanation for your answer. Exploring Further 6. The most important bryophytes to humans are probably peat mosses. Prepare a brief report describing these plants and some of the ways in which they are useful to people. Plants and Animals • MHR 473 Seedless Vascular Plants (Ferns and Their Relatives) Non-vascular plants represent the pioneers of plant life on land, but about 300 million years ago they were literally overshadowed by the first vascular plants. If you could travel back to this time in Earth’s history, you would find vast swampy forests. You would notice insects and some amphibians, but no birds, mammals, or even dinosaurs. The plants you would see growing in forests around you would contain vascular tissues, roots, stems, and leaves. These first forests are the source of some of our non-renewable fossil fuels. Whisk Ferns (Psilotophytes) Looking like small green whisk brooms, psilotophytes or whisk ferns are among the simplest of living vascular plants (Figure 13.13). They do not have leaves or roots, but grow to about 30 cm in height from short rhizomes, which are horizontal underground stems. Photosynthesis takes place in the outer cells of the branching stems. PLAY To view videos on the alternation of generations and on fern development, turn to your Electronic Learning Partner. These early seedless vascular plants were different from the non-vascular plants in several ways. Not only had they developed the vascular tissue that allowed them to grow tall, but they had the sporophyte generation as the dominant stage in their life cycle. Their gametophytes were reduced to tiny, short-lived structures that still depended on moisture to carry out sexual reproduction. Nearly all the trees that covered the land in that era are now extinct. There are a few remaining groups of those early vascular plants that live on today, however. They include the whisk ferns, club mosses, horsetails, and ferns like the one shown in Figure 13.12. Figure 13.13 The small yellowish clusters on this whisk fern are sporophytes. The tips of the stems of a whisk fern produce yellow sporangia. After the spores are released, they disperse and germinate into tiny, colourless gametophytes beneath the soil surface. Only a few millimetres long, the subterranean gametophytes obtain their nutrition by absorbing dissolved substances from the surrounding soil. At sexual maturity, they develop both egg and sperm cells. Fertilization produces the next generation of sporophytes, which begin to grow on their parent gametophyte. Eventually, the sporophyte grows into a mature whisk fern. BIO Figure 13.12 Giant relatives of ferns like these helped form Earth’s forests millions of years before dinosaurs evolved. 474 MHR • Diversity of Living Things FACT Young fern sporophytes first appear as tightly coiled fiddleheads that later unroll to form a mature frond. Today, fiddleheads are eaten as a seasonal delicacy. At one time, the fine silky hairs that cover the fiddleheads of some larger tropical tree ferns were stripped off and used to stuff pillows and mattresses. During the late 1800s, over 1900 metric tons of this fern hair were shipped from Hawaii to North America. Club Mosses (Lycopodophytes) The lycopodophytes, or club mosses, that we see today are mostly small evergreen plants that grow in dense mats on moist temperate and tropical forest floors. Extinct club mosses, however, were prominent members of Earth’s forests for 40 million years. They formed trees more than 35 m tall and up to 2 m in diameter. Despite their name and appearance, club mosses are not related to true mosses. The sporophytes of all club mosses have true stems and roots. Tiny leaves grow from their stems in spirals, whorls, or pairs. Each narrow leaf has a single, unbranched vein of vascular tissue along its centre, whereas most plant leaves have multiple, branching veins. At the end of a stem, specialized spore-bearing leaves form compact clusters called strobili (singular strobilus), which protect the reproductive cells. Strobili resemble pine cones, and because of this club mosses are sometimes called ground pines (Figure 13.14). A lycopodophyte spore germinates to form a tiny, independent gametophyte called a prothallus. Some species of lycopodophytes produce two different types of spores: large megaspores and small microspores. The megaspore germinates to form a female prothallus and the microspore produces a male prothallus. The prothallus lives on the soil and produces either sperm or eggs. In wet conditions, sperm swim to an egg and fertilize it. The resulting zygote grows into the larger, dominant sporophyte. We b LINK The vast swampy forests found on early Earth about 300 million years ago are the source of most of our nonrenewable fossil fuels. Find out more about the extinct species of vascular plants that made up these forests. To conduct your research, go to Science Resources, then go to BIOLOGY 11 and follow the links to find out where to go next. What was the geographical distribution of these forests? Did the species of plant vary from place to place? Think about where fossil fuel reserves are located in Canada. Is this correlated to these forests? www.school.mcgrawhill.ca/resources/ Figure 13.14 This club moss, Lycopodium, is sometimes called a ground pine because it is evergreen and looks similar to a tiny pine tree. Horsetails (Sphenophytes) Sphenophytes, or horsetails, are another group of seedless vascular plant that once included treesized members but now contain plants that only grow to about a metre in height. These plants are often found in damp areas or along roadsides. Their hollow stems have a ribbed appearance, with whorls of tiny, scale-like leaves growing from each joint or node along the stem (Figure 13.15 on the next page). These plants are also known as scouring rushes, because they have a rough texture that can be used for scrubbing pots or polishing wood. The roughness is due to granules of silica stored within the plant cells. The life cycle of horsetails is similar to that of club mosses. In spring, the tips of the stems produce spore-bearing strobili. The spores germinate to form male and female gametophytes. Each female gametophyte has several eggs and may develop more than one sporophyte after fertilization. Plants and Animals • MHR 475 divided into small leaflets called pinnae. The conspicuous fern plant is the sporophyte, and you can often see small spore-producing structures called sori (singular sorus) clustered on the underside of the pinnae (Figure 13.16). FAST FORWARD Turn to Chapter 14, Section 14.3 to learn more about the processes and structures involved in gas exchange in plants. Figure 13.15 Horsetails have small leaves, and their photosynthesis occurs mainly in the stems. A Horsetails have traditionally been used by North American Aboriginal people and Asians for various medicinal purposes. Some Aboriginal groups burned the stems and used the ashes to treat burns or sore mouths; some ate the strobili to cure diarrhea; and others boiled horsetail stems in water to make a shampoo for controlling head lice. Ferns (Pteridophytes) Pteridophytes, commonly known as ferns, dominated the forests during the Carboniferous Period (315 million to 280 million years ago). Although their numbers have declined since their peak, pteridophytes remain the most familiar and successful of seedless vascular plants. Ferns evolved several adaptations to life on land that are also found in seed-bearing plants. In addition to roots and vascular tissue, ferns have a waxy, thickened outer epidermis that reduces water loss by evaporation. Small openings in the epidermis, called stomata, allow gases to enter and leave for the processes of photosynthesis and cellular respiration. Ferns have an enormous range of form and structure, with the greatest diversity being found in the tropics. It is possible to find tiny ferns less than 1 cm in diameter as well as giant tree ferns up to 25 m tall. The fronds, or fern leaves, grow up from a thick, underground rhizome and are commonly 476 MHR • Diversity of Living Things B Figure 13.16 The small round structures often seen on the underside of a fern frond (A) are called sori. They consist of clusters of sporangia (B). Ferns produce millions or even billions of spores during their lifetime, but very few ever land in a spot suitable for growth. A germinating spore does not produce a new fern plant with fronds; rather, it grows into a small, heart-shaped prothallus. The gametophyte generation is unnoticed by most people, because the prothallus is usually just over 1 cm across and lies flat on the ground. It is an independent structure with its own rhizoid system to provide it with nutrients and water. The prothallus contains female organs, called archegonia, and male organs called antheridia. You can observe the structure of a fern prothallus for yourself in the next mini lab. Gymnosperms (Conifers and Their Relatives) There are two groups of plants that disperse by means of a seed, gymnosperms and angiosperms. Seeds allow plants to reproduce sexually without needing water, and also provide protection against harsh environmental conditions. As seeds, plants can survive without water for many years. They can be carried by different means to disperse across continents and begin growing in new areas. The first small seed-bearing plants appeared about 280 million years ago, among forests of ferns, club mosses, and horsetails. At that time, the global climate had begun to grow cooler and drier. Most of the large spore-producing plants could not survive long periods of drought and freezing, and they became extinct. Gymnosperms have seeds that are exposed on the surface of cone scales. The word gymnosperm actually means “naked seed.” This group includes the cone-bearing trees (conifers) such as pines, firs, yew, spruce, cedars, redwood, and many other large trees (Figure 13.17). As well as the conifers, gymnosperms include three other small groups: cycadophytes, gnetophytes, and ginkgophytes shown in Figure 13.19 on page 479. Reproduction in Gymnosperms Recall that club mosses and horsetails bear spores in strobili — structures made of compact clusters of specialized leaves. Cones are simply large, woody strobili. Female cones develop ovules in archegonia that grow on the upper surface of each cone scale. Male cones produce microspores that develop into pollen grains. Each pollen grain is a four-celled male gametophyte enclosed in a hard, water- MINI resistant coat. Many hundreds of thousands of dust-like pollen grains are produced in each male cone and dispersed to female cones by wind. When a pollen grain lands next to an archegonium, it produces a pollen tube, which grows into the tissue of the female gametophyte. Sperm pass along this tube to the egg. After fertilization, the zygote develops into an embryo in the female cone. The embryo, together with a small supply of stored food, is covered by a tough, waterproof coat to form a seed. Female cones remain on the tree until the seeds have matured, which may take from several months to two years. The evolution of pollen, pollen tubes, and seeds allowed plants to reproduce sexually without the need of moisture. This ability helped them survive cold, dry conditions. Figure 13.17 This conifer is one example of a gymnosperm. LAB Investigating the Fern Prothallus Most people have seen fern plants, but few have observed the gametophyte generation, which produces the gametes. In this lab, you will study the structure of a fern prothallus. Obtain a mature fern prothallus from your teacher and prepare a wet mount. Observe all standard safety procedures, especially when using electricity and glass. Examine the prothallus under the low power of a microscope. Make a labelled sketch indicating its general shape and features. Use Figure 13.18 on the next page as a reference if you need to. Identify the female organs, located near the notch of the heart-shaped plant, and the male organs, located near the pointed end of the prothallus. Using the eraser end of a pencil, press gently on the cover slip over the area bearing the male organs. Study your specimen under medium power to see if any sperm were released. If so, observe their structure and behaviour. Analyze 1. What is the name of the structure that produces eggs? that produces sperm? 2. Is the prothallus haploid or diploid? 3. What type of cell division produces the gametes? 4. Sketch and label a diagram showing the stages in the fern life cycle between the gametophyte generation and the sporophyte generation. Plants and Animals • MHR 477 A A sporangium produces haploid spores that germinate to form a gametophyte called a prothallus. gametophyte spore rhizoids B The prothallus produces antheridia (male organs) and archegonia (female organs). prothallus meiosis egg sperm sporangium F Sori develop on the pinnae. Spores are formed in the sori by meiosis. antheridium archegonium egg sori fertilization C Sperm from the antheridia swim via a droplet of water to an egg produced by the archegonium. sperm zygote mitosis pinna fronds new sporophyte gametophyte sporophytes rhizome roots E The sporophyte matures, and roots and fronds develop out of the growing rhizome. Figure 13.18 The life cycle of a fern. 478 MHR • Diversity of Living Things D The fertilized egg begins to grow into a sporophyte. Conifers Conifers are the largest group of gymnosperms, and they form vast forests in cold regions of the world. As well as being able to reproduce without water, they have other adaptations to cold, dry habitats. Their protective covering of bark helps protect the stem and reduce water loss. The pyramidal shape of many conifers, such as the one shown previously in Figure 13.17, together with their flexible branches, helps snow and ice slide off the tree. This reduces the risk of a heavy buildup of snow that could break branches. The needle-like leaves of conifers have a thick, waxy cuticle and sunken stomata, which reduce the rate of evaporation (Figure 13.20 on the next page). Pine needles are usually semicircular or round in cross section, resembling a cylindrical leaf. The shape reduces surface area, which in turn reduces water loss by evaporation. Most conifers are evergreen, so they continually lose and replace their needle-like leaves all year long rather than losing all of their leaves at once like a deciduous tree. A Conifers bear seeds in cones like the ones on this Eastern Hemlock. B Welwitschia (Gnetophyta) is an odd-looking plant found only in deserts in southern Africa, where the average rainfall is less than 2.5 cm per year. Its short stem produces two broad leaves that continue growing throughout the life of the plant. Welwitschia may live 100 years, and the older ends of its leaves become tattered and weathered. C Cycads (Cycadophyta) were common trees during the Mesozoic era when dinosaurs shared the forests with the first birds and mammals. About 100 species exist today, mainly in the tropics. They are short, palmlike trees with scaly trunks, but they are not closely related to palms. Male and female cones grow on separate trees. The cones of some cycads may grow to a metre in length. D Ginkgo biloba (Ginkgophyta) is the only living species of a group of plants common during the Jurassic period 200 million years ago. It has distinctive lobed leaves. Ginkgos were cultivated in Asian temple gardens for thousands of years, which may have helped protect them from extinction. Figure 13.19 Gymnosperms bear their seeds in cones. This group includes conifers, gnetophytes, cycads, and ginkgos. Plants and Animals • MHR 479 By keeping leaves through winter, they are able to start photosynthesis early in the spring, as soon as the weather grows warmer. This is a great advantage at high latitudes where the growing season is short. Conifers are also better able to grow in nutrient-poor soils, because they do not need to grow a complete new set of leaves all at once. stomata Angiosperms (Flowering Plants) Each spring, brightly coloured packages of plant seeds, like those shown in Figure 13.21, are displayed in gardening stores. On the outsides of the packages are pictures of flowers or vegetables. Inside each package is a number of small, dry, hard seeds. These tiny waterproof capsules contain plant embryos, ready to grow as soon as you add water. vascular tissues Figure 13.20 A cross section of a pine needle Figure 13.21 The dry hard seeds of angiosperms are often sold in packages in order to be planted in a garden. DESIGN YOUR OWN Investigation SKILL FOCUS 1 3 • C Initiating and planning Identifying Conifers Performing and recording How can you tell one coniferous tree from another? In this investigation, you will study the characteristics of several different species of coniferous plants and develop a key to identify them. Analyzing and interpreting Communicating results Problem Identify different species of conifers found in your area. Hypothesis Make a hypothesis about what specific characteristics will best distinguish among several coniferous species. Materials twigs, needles, and cones collected from several different conifers in your area a field guide to trees 480 MHR • Diversity of Living Things Experimental Plan 1. Make a list of characteristics in which species of conifers might differ in order to test your hypothesis. From the characteristics in your list, decide which ones would be most useful in a key. These plants grow and develop seeds that are enclosed within a fruit. Plants that protect their seeds within the body of a fruit are called angiosperms or flowering plants. The first flowering plants appeared on Earth about 150 million years ago. More than three quarters of all species of living plants are seedbearing, flowering plants. Angiosperms include trees, shrubs, herbs, grasses, vines, and water plants, which may have inconspicous flowers, and they grow almost everywhere on land from the tropics to the tundra. Angiosperms are divided into two large classes, based on the number of seed leaves, or cotyledons, on the embryo within the seed. Angiosperms with one seed leaf are called monocots, and those with two seed leaves are dicots. Figure 13.22 on the next page shows examples of these different flowering plants. PLAY To see a video of different types of seed dispersal, use your Electronic Learning Partner. 2. Decide in which order the characteristics should appear in the key. 3. Collect samples of different local conifers. Checking the Plan 1. Traits are described in a key as sets of two alternative choices. For example, if you decide that the arrangement of needles is a useful characteristic for identification, one choice in your key might be: needles grouped in bundles, or needles growing singly 2. When writing your key, remember that it must be clear enough to be used by any person not familiar with conifer identification. Data and Observations Create your key. Examine your conifer samples and list the characteristics you observe. Identify the samples of conifers that you have brought into class using your key. FAST FORWARD Turn to Chapter 14, Section 14.1, for more information on monocots and dicots. Reproduction in Angiosperms What are flowers, and what advantages do they provide to plants? One clue is that colourful and scented flowers are not only attractive to people but also to some other animals. The flower contains the structures used for sexual reproduction in the plant, and when animals visit flowers to get pollen or nectar, they help flowering plants carry out the process of reproduction. Recall that the pollen grains of gymnosperms are dispersed in vast numbers by wind. This method of transport is quite imprecise, and most pollen grains do not land on an appropriate female cone. Some angiosperms also scatter their pollen on the wind, but the majority of flowering plants have more specific methods of transferring pollen from the male to the female of the same species. This is known as pollination, and flowers play an active role in this process. Analyze 1. Was your hypothesis supported? Explain your answer. 2. Exchange your key with students from a different group. Using the new key, identify your conifer specimens. Does the key work? Are you able to correctly identify your specimens? Conclude and Apply 3. Is there only one correct way to design a key to identify conifers? Explain your answer. Exploring Further 4. Mount and label your specimens. Display them in your classroom along with other students’ specimens. Try to provide examples of all the conifers in your area. Be sure to highlight the identifiable characteristics of each sample. Plants and Animals • MHR 481 A B C D Figure 13.22 Monocots include (A) lilies and (B) grasses while dicots include (C) daisies and (D) deciduous trees. Many different organisms depend on flowers for food, because most flowers produce nectar — a concentrated liquid mixture of proteins and sugars. When an insect, a bird, or even a bat lands on a flower to collect nectar, it becomes brushed with pollen from the anthers (Figure 13.23). Then, as the organism flies on to the next flower, the pollen grains are brushed onto the stigma, where they can fertilize the ovum. BIO FACT Civilization as we know it would not have been possible without members of the grass family. Nine species in this family, including wheat, barley, rice, oats, and corn, provide over 75% of the food eaten by people worldwide today. The development of agriculture and the cultivation of cereal crops thousands of years ago allowed people to produce surplus food, settle in large populations, and create cities for the first time. PAUSE RECORD You have learned about the different ways that plants reproduce. What are the similarities in how all plants reproduce? What are the main differences among the four groups of plants? Create a short checklist or diagram to organize these similarities and differences. Figure 13.23 As a bee lands on the lower petal of this flower, the anther deposits pollen on the bee’s back. 482 MHR • Diversity of Living Things Adaptations for Pollination Most methods of pollination in flowering plants involve close relationships between flowers and certain species of insects, birds, and bats. Flowering plants and the animals that pollinate them evolved together. For example, flowers pollinated by bats are open at night (Figure 13.24). Flowers pollinated by butterflies usually store their nectar at the base of a long corolla, where it can only be reached by moths or butterflies with their longer specialized mouth parts. Specialization is an advantage to both the plant and the pollinator. The distinct colour, shape, and scent of a flower helps the pollinator easily recognize it as a reliable source of food (Figure 13.25). Differences between the flowers of different species increase the chance that a pollinator will search for nectar from only one species at a time. Transporting rose pollen to a daisy, or vice versa, would not help either plant become pollinated and complete its life cycle (which is shown in Figure 13.27 on the next page). After a pollinator deposits pollen on a stigma, each pollen grain grows a pollen tube to reach the ovule. Only the first tube to reach the ovule will penetrate and fertilize it. This process leads to pollen-tube competition. The fastest growing pollen tube usually carries the best genes and results in more vigorous offspring. Because a single plant often produces many flowers, there is a risk that the plant may selfpollinate (that is, that pollen may be transferred from one flower to another flower on the same plant). Although self-pollination results in the development of seeds, it does not involve the exchange and recombination of genetic material that helps create diversity within a population. Self-pollination produces offspring with the same genome as the parent. Flowering plants have evolved several strategies to favour cross-pollination over self-pollination. Some plants have separate male and female flowers that mature at different times. This ensures that male flowers have already dispersed their pollen before female flowers on the same plant are ready to be fertilized. Other species have chemical barriers that prevent fertilization by pollen from the same plant. In flowers with both male and female parts, the stigma usually extends much farther than the anthers, keeping the two parts well separated so they do not self-pollinate accidentally. Figure 13.25 The markings on this flower are only visible to the human eye when illuminated with ultraviolet light. These same markings are easily seen by pollinating insects. REWIND Turn to Chapter 4, Section 4.2, to review cross- and self-fertilization. Seed Development and Dispersal Flowering plants are diploid sporophytes. Unlike the earlier plant groups you have studied, however, angiosperms do not produce spores. The pollen grains and ovum cells are all that remain of the gametophyte generation in flowering plants. Fertilization produces a diploid sporophyte embryo, which is enclosed in a hard case to form a seed. Angiosperm seeds are further protected by the fleshy walls of the ovary. As the ovary matures, it swells to form a fruit or seed pod like the one shown in Figure 13.26. Fruits also help in seed dispersal. When the fruit is eaten by an animal, seeds are carried away from the parent plant in the animal’s digestive tract. The seeds resist digestion and pass out with the animal’s feces to germinate in a new spot. Angiosperm fruit seeds sepals Figure 13.26 A fruit such as an apple consists of the fleshy Figure 13.24 Flowers pollinated by bats must be large enough to support the weight of the bat. walls of the ovary enclosing the seeds. You can see the remains of the flower sepals at the bottom of the apple, opposite the stalk that attaches the fruit to a stem. Plants and Animals • MHR 483 anther pollen grain haploid microspores in fours ovary male gametophyte sperm ovule haploid megaspores female gametophyte meiosis Gametophyte generation n egg pollen tube fertilization flowers pollen tube young seedling Sporophyte generation 2n zygote germinating seed adult sporophyte plant seed fruit (containing seed) Figure 13.27 The life cycle of a flowering plant 484 MHR • Diversity of Living Things Biology Magazine TECHNOLOGY • SOCIETY • ENVIRONMENT Adjusting the Balance Animal and plant species change over time, sometimes evolving into different forms and sometimes going extinct. As the human population grows, it disturbs more and more land mass, driving many plants and animals into extinction. For example, the vast forests in the Mediterranean region disappeared by Roman times. Some scientists believe humans may have caused the extinction of large mammals like the North American mammut (a kind of mastodon) more than 10 000 years ago. The list of species that have been negatively affected by humans keeps growing. The loss of plant and animal species affects all other animal and plant species with which the lost species interact. It is only recently that biologists have begun to study such interactions and to understand the farreaching impact of mismanaging our ecosystems. As our understanding of the importance of interactions grows, so does the need to protect the plant and animal species threatened by human expansion. forests. However, logging continued in and around the sanctuaries, and more and more of the monarchs’ winter home was converted into oat and corn crops. To make matters worse, as the trees were cut, the remaining trees became subject to strong winds, drying the normally moist habitat and making the standing trees more prone to disease. Some conservationists rallied to the cause of the monarch butterflies, calling for stronger policing of the Oyamel sanctuaries. However, they ignored the economic needs of the farmers. The competing needs of the butterflies and the people created a dilemma — how to find a solution that would take into account the needs of both the monarchs and the people who live in the area. Forests for People and Forests for Monarchs Maintaining a keystone species in a community — a plant or animal that is crucial to the overall health of the ecosystem — may have a direct economic benefit for humans. But conservation efforts sometimes have indirect economic benefits that seem surprising. The work being done by conservationists to protect monarch butterflies and the forests they live in is an excellent example of this. Robert L. Small of Oakland, California and Jose Luis Alvarez of El Rosario, Mexico started a project in 1997 that may satisfy the needs of both the monarchs and the farmers. With the help of donations from conservationists in the United States, Small has been able to send fir seedlings to Alvarez, who distributes them to willing farmers. The goal is to use the donated trees to create a new forest that can be planted, harvested, and replanted by the farmers, while at the same time allowing the monarchs their current habitat. Five farmers planted 7000 donated trees in 1997, 20 farmers planted 40 000 trees in 1998, and the project continues to grow. The reforestation project promises so much success that the Mexican government is considering similar projects of its own. The Monarchs and the Oyamel Forests Follow-up The Oyamel fir forests in central Mexico are the winter home of monarch butterflies, whose yearly migration from Canada and the eastern United States has captured the imaginations of people worldwide. Unfortunately, the indigenous farmers (ejidatarios) who live in the area depend on logging these forests for their livelihood. For the past 20 years, the Oyamel forests have been slowly Monarch butterflies migrate shrinking, thus threatening the monarchs’ winter habitat. by the thousands to these forests in Mexico. In 1986, the Mexican government created monarch sanctuaries by banning logging in parts of the Oyamel 1. The Oyamel reforestation project shows how even the conservation of species that do not affect the human economy can have a positive impact on whole communities, including human populations. Briefly write down your thoughts on conservation that is not driven directly by economic need. 2. A new potential threat to monarch butterflies has surfaced. Genetically modified corn crops, whose pollen kills certain corn pests, may be affecting the monarch caterpillars in the United States. Studies of the corn/butterfly connection have brought attention to the controversy surrounding genetically modified plants (and animals) and how they affect whole communities. Write a paragraph outlining possible steps that could be taken to avoid further damage to the monarch butterfly populations. Which do you think is more important: developing a better food source for hungry people, or conserving the monarch butterfly? Explain your answer. Plants and Animals • MHR 485 Asexual Reproduction and Multiple Chromosomes Nearly half (47%) of all angiosperms are polyploid — that is, they have more than two (2n) sets of chromosomes. They may have 3n, 4n, or more. This situation is caused by irregularities during meiosis. It results in sterile offspring that can still reproduce asexually, and has been an important mechanism in the evolution of plants. For example, polyploidy can produce larger cells, thicker and fleshier leaves, and larger flowers and fruits. The original South American ancestor of the potato is tetraploid (4n), and many common food plants such as strawberries and apples are polyploid. A species of wheat commonly grown for bread (T. aestivum) is hexaploid (6n = 42 chromosomes). SECTION K/U Which three characteristics do plants share with green algae? 2. K/U Name two characteristics that adapt plants to life on land. 3. K/U Name the four broad groups into which the plant kingdom is divided. 4. K/U What are the products of meiosis in the four different groups of plants? 5. K/U What are bryophytes? How do they differ from the other groups of plants? 7. 8. K/U C Sketch the life cycle of a fern, clearly showing the sporophyte and gametophyte generations. Indicate which is haploid and which diploid. To most people, it is fairly clear that carrots are a type of vegetable, and apples are a type of fruit. Some people argue that tomatoes can be classified as either a fruit or a vegetable. What are the differences? To botanists, a fruit is a mature plant ovary that contains seeds. Thus, tomatoes, string beans, cucumbers, and squashes are all fruits. Make up your own definition of a vegetable. Do all vegetables fit your definition? 9. I Suppose you find yourself in a wilderness area in a part of the world you have never been before. How might a knowledge of plant classification help you determine the typical climate of the area? Describe the steps you would take to do this. 10. I Copy and complete the following table about the growth patterns of certain conifers. Use the graph shown on the left below for your information. Give your table a title. Maximum height Age at maximum height Height at 45 years How does the graph below show that these conifers grow according to their own growth pattern? spruce larch pine I fir spruce 33 Height (m) LINK fir Which two plant groups produce seeds? 39 larch pine 27 21 9 3 0 10 11. MC Why do some angiosperms have showy flowers while others have inconspicuous flowers? What does flower type indicate about the plant? If you wanted to farm a plant with showy flowers, name one challenge you would face and explain how you would deal with it. UNIT ISSUE PREP 15 486 Wo rd REVIEW 1. 6. Odd-number polyploids are sterile because they cannot segregate chromosomes evenly into gametes during meiosis (odd numbers are not divisible by two). Sterility caused by triploidy produces such results as seedless bananas and less bitter cucumbers. 20 30 40 50 60 70 Age in years MHR • Diversity of Living Things 80 90 100 If you have decided to investigate the status of a plant species in your Unit 4 Issue Analysis, make sure you have noted the characteristics of the phylum to which your species belongs. These characteristics may have an impact on how your species can be protected.