Alternation of Generations - k12resources.nelson.comk12resources.nelson.com/.../documents/life_cycle_of_plants.pdf · The diploid organism is called the sporophyte and has two sets

Unit 30 C Cell Division, Genetics, and Molecular Biology

Much of the Earth’s surface is covered with some type of vegetation. There are over 260,000 known species of plants. In Canada alone, there are about 5,000 species of plants. Each species of plant has adapted to certain surroundings and environments that are favourable for its growth and survival. For a plant species to continue and produce offspring, it must go through a reproductive life cycle. However, all plants do not go through the same reproductive life cycle. Figure 1 shows the major divisions of the kingdom Plantae. In this activity, you will compare the life cycles of these different classes of plants.

Figure 1 The major divisions of Kingdom Plantae.

Alternation of Generations The life cycle of a multi-cellular organism involves two types of cell division, mitosis and meiosis. During the life cycle of animals, diploid cells divide by mitosis to produce more diploid cells. These are the somatic cells that make up the body of the organism. During meiosis, on the other hand, a specialized diploid parent cell gives rise to four haploid cells. These haploid cells are gametes (egg or sperm cells) that normally do not undergo further cell division. Each subsequent generation of animals will also be composed of diploid multicellular organisms.

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The reproductive life cycle of all plants involves two generations of multicellular organisms – a haploid generation and a diploid generation. This means that during the life cycle there are both haploid and diploid generations (phases). The haploid organism is composed of haploid cells, and the diploid organism is composed of diploid cells. The life cycle of a plant is therefore said to involve alternation of generations. The diploid organism is called the sporophyte and has two sets of chromosomes. During the diploid generation, the sporophyte produces haploid spores (reproductive cells) through meiosis. The haploid organism is called the gametophyte, and has only one set of chromosomes. Unlike during an animal’s life cycle, the gametophyte produces additional haploid gametes through mitosis. The way in which the alternation of generations occurs in a plant’s life cycle depends on the type of plant. Plant life cycles may seem complex, but they are all essentially the same. As you read about the life cycle of plants, follow the names and the direction of the arrows in Figure 2. Pay particular attention to whether the part is haploid or diploid. Since each stage leads to the next, there is no real start or end in a life cycle. For the purpose of this discussion, we will start with the spores.

Figure 2 Generalized life cycle with alternation of generations: In reality, the two generations are not usually the same size nor do they last the same length of time. Since it is a cycle, there is no specific beginning or end. The labels here will apply to all other plant life cycles in this activity. A spore is a haploid (1n) reproductive cell, and is the first cell of the gametophyte generation. This is the stage in a plant’s life cycle in which all cells have haploid nuclei. The spore divides by mitosis, eventually producing all the haploid cells that make up the body of the gametophyte plant. Specialized parts of the gametophyte plant divide and produce haploid gametes (egg and sperm cells), also by mitosis. An egg cell and a sperm cell fuse and form a diploid (2n) zygote during fertilization. The zygote is the first cell of the sporophyte generation. This is the stage in a plant’s life cycle in which



cells have diploid nuclei. The zygote grows by mitosis to form the embryo and eventually the mature diploid sporophyte. Eventually, the mature sporophyte will produce spore mother cells by mitosis. These cells therefore are also diploid. The spore mother cells undergo meiosis to produce haploid spores. The spore mother cells are the last cells of the sporophyte generation. During the life cycle of plants, fertilization and meiosis change the chromosome number between 1n and 2n, and enable the switch from the gametophyte generation to the sporophyte generation. When the union of the male and female gametes takes place during fertilization, more is happening than simply restoring the diploid chromosome number. Sexual reproduction is also taking place. The genetic material from both parents is combined in the zygote, giving rise to offspring with different genotypes than the parent plants. This genetic diversity increases the ability of plants to adapt to changes in environmental conditions. Look closely at Figure 2 again and identify where sexual reproduction occurs in the plant life cycle. Bryophytes Bryophytes are the group of plants that spend the main part of their life cycle in the haploid generation. In bryophytes, the haploid gametophyte structure makes up the main part of the plant. The gametophyte produces both male and female haploid gametes through mitosis. These gametes join during fertilization to form a diploid zygote, thus beginning the diploid generation. The zygote undergoes mitosis to develop into a sporophyte. The diploid sporophyte produces haploid spores by meiosis. The spores undergo mitosis to develop into new haploid gametophyte plants and the cycle continues. The plants in the bryophytes group are simple nonvascular plants. They include mosses, liverworts, and hornworts. Since they have no vascular tissue to move liquid through their bodies, they must absorb all their water and nutrients through their plant surface. Water is also required for bryophyte fertilization. Therefore, bryophytes grow mostly in moist, shady environments where the water is easily accessed. However, some bryophytes can grow in more arid environments. To survive in these environments, these plants remain in a desiccated state, with decreased nutrient requirements. The plants go dormant until water comes into the environment. The simple structure of bryophytes provides a defence mechanism for survival. Tracheophytes Tracheophytes are the group of plants that spend the main part of their life cycle in the diploid stage. In tracheophytes, the diploid sporophyte structure is dominant and the haploid gametophyte structure is very small. The sporophyte structure produces both male and female haploid spores, which give rise to single-sex gametophytes. These gametophytes produce gametes that join together to form a diploid zygote that develops into a seed. The seed develops into a new sporophyte and the cycle begins again. The plants in the tracheophytes group are vascular plants. They include conifers and flowering plants. The vascular tissue in tracheophytes allows the transport of water and nutrients to all parts of the plant. Tracheophytes have also developed reproductive cycles that are independent of water. These characteristics allow tracheophytes to survive and flourish in a variety of environments.



Life Cycle of Mosses The full life cycles of mosses and other bryophytes involve an alternation of generations as shown in Figure 3. Follow Figures 3 and 4 carefully as you read about the life cycle.

Figure 3 Alternation of generations of a moss. Starting with the spores at the right side of the diagram, follow the cycle clockwise and note the labels carefully as you read the detailed description.



Figure 4 Alternation of generations of a moss. Starting with the spores at the right side of the diagram, follow the cycle clockwise and note the labels carefully. Compare these details with the text and with the simpler diagram, Figure 3. The tiny haploid spores that have been produced by meiosis are the first cells of the gametophyte generation. As they are released, they are usually carried by the wind or sometimes by water. If a spore lands in an environment suitable for its growth, the protective covering splits and the spore germinates. The single cell divides rapidly by mitosis. The resulting haploid plant is called a protonema and resembles a filamentous green alga. This tiny gametophyte plant continues to grow. Rhizoids are produced from its lower surface to help anchor it. In time, the protonema produces little buds that grow into larger gametophyte plants, which may grow upright or along the surface of the soil or rock. As these plants mature, sex organs are formed. In the case of upright plants, these structures are surrounded by the leaflets at the top of the stalks. The male sex organ, called an antheridium (plural: antheridia) is tiny and shaped somewhat like an elongated balloon. Several of these organs are clustered at the top of one plant. Inside



these organs, male gametes (sperm cells) are produced by mitosis. The female sex organ, called an archegonium (plural: archegonia), is also tiny and shaped like a bowling pin. Inside each of these organs, a female gamete (egg) is produced by mitosis. These sex cells are haploid, as are all the cells of this gametophyte generation. When the sperm are mature, they are released from the antheridia. The mature eggs remain in the archegonia, which now produce a very sticky material. Transfer of the sperm from the male plants to the female plants can only occur if there is water. However, the plants are usually very close together and the water required can be as little as dew drops. Once in the vicinity of the archegonia, the sperm cells are attracted to the sticky material and swim down the neck of the archegonia. Only one sperm fuses with the waiting egg. This fertilization marks the beginning of the new sporophyte generation. The first cell of this new sporophyte generation is the diploid zygote that grows rapidly by mitosis to form the diploid embryo—still in the archegonium. The embryo continues to grow into the new sporophyte plant, remaining embedded in the archegonium, which supplies nearly all the nutrients for the growing sporophyte plant. As the embryo grows, it becomes visible as a thin, brown stalk rising out of the top of the female gametophyte plant. Sometimes the brown stalk seems to be wearing a “hat.” This “hat” is the top of the old archegonium, which was torn off and rides up as the stalk grows. The stalk’s total height is often equal to the height of the gametophyte plant supporting it. Gradually the top of the stalk, under the “hat,” enlarges into a sporangium, inside of which are many diploid spore mother cells. Each spore mother cell undergoes meiosis to form four haploid spores, which are the first cells of the next gametophyte generation. The sporangia open and eject the spores, which are carried away by air currents. The stalk height increases the efficiency of spore dispersal. Life Cycle of Ferns The full life cycle of ferns involves an alternation of generations. Follow Figures 6 and 7 carefully as you read the details that follow.

Figure 6 Alternation of generations of a fern. Starting with the spores at the right side of the diagram, follow the cycle clockwise and note the labels carefully as you read the detailed description in the main text.



Figure 7 Alternation of generations of a fern. Starting with the spores at the right side of the diagram, follow the cycle clockwise and note the labels carefully as you read the detailed description in the main text and review the simpler diagram in Figure 6. The tiny haploid spores that have been produced by meiosis are the first cells of the gametophyte generation. As they are released, they are usually carried by the wind. If a spore lands in an environment suitable for its growth, the protective covering splits and the spore germinates. The single cell divides rapidly by mitosis. The resulting haploid plant is called a prothallus (or prothallium). This thin, green, heart-shaped gametophyte is about the size of the fingernail on your baby finger. A cluster of rhizoids grow from the underside of the prothallus. Also on the underside of the prothallus are found spherical antheridia, in which sperm are produced by



mitosis, or flask-shaped archegonia, in which eggs are produced by mitosis. Whether the male and female sex organs are on the same or separate gametophyte plants depends on the fern species. Like mosses, mature fern sperm are released from the antheridia but require moisture to help transfer them to the female sex organs, the archegonia. Following fertilization, the diploid zygote, the first cell of the sporophyte generation, grows by mitosis into an embryo that continues to grow. The tiny immature sporophyte plant produces small roots to absorb water and minerals and a tiny frond that can photosynthesize even before it has fully unfurled and reached its full size. A rhizome is also produced, which grows laterally and produces more fronds and roots. The prothallus withers and dies. The mature sporophyte frond bears clusters of sporangia on its lower surface. One of these clusters is called a sorus (plural: sori). In some fern species, the sporangia develop on special separate fronds with a distinctive form and colour. Many species of fern can be identified by the distinctive patterns formed by the sori. Diploid spore mother cells are produced by mitosis inside each sporangium. Each of these undergoes meiosis to form four haploid spores, the first cells of the next gametophyte generation. When the spores are mature, the sporangia use a variety of mechanisms to eject the spores. One frond alone can release many thousands of spores, which will be carried away in the wind. Life Cycle of Gymnosperms Gymnosperms produce unprotected, or “naked,” seeds in conelike structures and are often referred to as conifers. The pine tree will provide a general example of a gymnosperm’s life cycle (Figure 8). A pine tree is the diploid sporophyte plant. In the spring, each tree produces two types of cones, neither of which looks like the woody, brown cones you have seen. The male cones, sometimes called pollen cones, are quite small and delicate and are found in clusters. Each male cone consists of many scales, each one with two sacs. In each sac, diploid microspore mother cells undergo meiosis to form four haploid microspores. Each of these develops into a haploid pollen grain, which is the male gametophyte. The female cones, sometimes called seed cones, are also quite small and somewhat sticky. They are often a pinkish-purple colour and are found singly or in groups of two or three. Each cone consists of many scales. On the upper side of each scale are two ovules. In each ovule, the megaspore mother cell undergoes meiosis but only one survives as a haploid megaspore, the female gametophyte.



Figure 8 Alternation of generations of a pine tree. Starting with the mature diploid sporophyte tree at the top of the diagram, follow the cycle clockwise and note the labels carefully as you read the detailed description in the main text. Remember that even though the gametophyte generation is small, it is very important for maintaining diversity within the species. When the pollen grains are mature, the tiny sacs of the male cones disintegrate and millions of dry pollen grains are released. The pollen grains have little flaps or wings that allow them to be carried easily by the wind. A parked car under a pine tree during this pollen release will accumulate a layer of yellow dust. The remnants of the male cones gradually dry up and fall off the tree. The female cones are held by the tree such that the tip is pointing upward and the scales angle downward. When ripe, airborne pollen lands on the female cones of the same or a different pine tree, and the sticky sap and angles of the scales ensure that the pollen moves toward the ovules. After pollination, the female cones become greenish, quickly increase in size, and reorient themselves so that the tips are pointing downward. In some gymnosperms, fertilization may occur right away, but in pines, it usually takes a year before fertilization occurs.



Fertilization, the union of the microspore and megaspore nuclei, produces the diploid zygote, which is the first cell of the next sporophyte generation. The zygote grows by mitosis to produce the diploid embryo, which remains inside the ovule. Now that there is an embryo, the ovule becomes a seed. It develops a seed coat which protects the embryo until there are suitable conditions for germination. Some gymnosperm seeds may develop within a few months, but most species take two to four years to mature. During this seed development, the female cones become brown and take on a woody texture. As they dry, the scales separate and the seeds fall out. Eventually the empty female cones fall to the ground. If conditions are suitable, the seeds germinate. After germination, the tiny plant is called a seedling. Gymnosperms usually have to grow for many years before they produce male and female cones. Although the gymnosperm gametophyte generation is extremely tiny, both in size and duration, it still creates variety in the next generation of plants to ensure survival under many different circumstances. Life Cycle of Angiosperms Angiosperms produce seeds that are enclosed and protected inside a fruit, which is formed by various flower parts. Many can reproduce asexually by a broad variety of mechanisms, but they all reproduce by alternation of generations. The sexual phase of alternation of generations allows genetic material to be recombined as a result of fertilization. Even the young produced by the same two parents can be very different from each other. The entire angiosperm plant, including the roots, stem, leaves, and flowers, belongs to the diploid sporophyte generation. In the appropriate season, the flower bud opens and the petals unfurl. The reproductive parts are revealed and, in a few days, they mature (Figure 9). The filament of the stamen elongates and the anther enlarges. Each anther consists of several chambers in which diploid microspore mother cells are located. Each of these undergoes meiosis to form four haploid microspores, or male gametophytes. Each will develop into a mature pollen grain. When the pollen grains reach maturity, the anther chambers split, and as they curl inside out, the pollen grains appear to be coating the outside of the anthers. The pollen of some species is quite sticky, while in others it is like dry powder. The pollen of some plants has tiny wings, while in others the surface has distinctive ridges and grooves. During this stamen development, the style of the pistil also elongates and the stigma enlarges slightly and secretes a sticky, sometimes scented, substance that covers its surface. At the bottom of the pistil, the ovary also enlarges. Inside are one or more ovules. Within each ovule, the diploid megaspore mother cell undergoes meiosis and forms four haploid megaspores, but only one survives as the female gametophyte.



Figure 9 Alternation of generations of a typical flowering plant. Starting with the mature diploid sporophyte flower at the left of the diagram, follow the cycle clockwise and note the labels carefully. Remember that even though the gametophyte generation is small, it is very important for maintaining diversity within the species. Pollination is usually carried out by wind or insects, but for some angiosperms, pollination is aided by birds or bats. The transfer of pollen from the anther to the stigma on the same flower or another flower on the same plant is called self-pollination. When pollen is transferred to a flower on a different plant, it is called cross-pollination. The pollen grains tend to adhere to the sticky stigmas. Part of the pollen makes its way down through the style tissue and the sperm eventually reaches the egg in the ovule.



Fertilization, the fusion of microspore and megaspore nuclei, produces the diploid zygote, which is the first cell of the next sporophyte generation. The zygote grows by mitosis to form an embryo, which remains inside the ovule. Now that there is an embryo, the ovule is called a seed and has its own protective seed coat. Besides the embryo, the seed also contains some special tissue that will provide nourishment to the developing embryo during seed germination and to the seedling until photosynthetic leaves become functional. While these changes are taking place inside the ovule, the ovary and perhaps some surrounding tissue are developing into a fruit. The fruit may be fleshy or quite hard and dry. The fruit provides protection for the seeds and often helps secure dispersal of the seeds. During seed and fruit development, the other flower parts often become dry and blow away or may stay attached to the fruit as withered bits of tissue. After some time elapses, the mature fruit falls or is carried away by animals. Under suitable conditions, the fruit decomposes, the seed coat splits, and germination occurs. The embryo grows very quickly and is now called a seedling. In some angiosperms, the seedling will grow big enough to produce its own flowers and seeds within a few months. In other angiosperms, the plant has to grow many years before it produces its own flowers. There are many angiosperms that also reproduce asexually. The methods vary widely. Humans have also intervened in plant reproduction by devising methods of reproducing angiosperms by asexual or vegetative means that the plants could not actually do themselves. Humans also intervene in the pollination process, either by manually transferring the pollen from one flower to another or by preventing pollen from reaching the stigmas of the flowers. Case Study Questions 1. Why do you think the sporophyte (diploid) generation is regarded as “genetically safer” than

the gametophyte (haploid) generation in plants?

2. Which group of plants (bryophytes or tracheophytes) do you think are better adapted for

terrestrial life? (Be sure to support your choice.)

3. Why have tracheophytes evolved into more complex plant forms than bryophytes?

4. (a) In which generation of mosses are the cells haploid?

(b) In which are the cells diploid?

(c) Where in the life cycle does the reduction in chromosome number occur and by what

process?

(d) Where is the diploid condition restored and by what process?

5. Although most ferns are terrestrial, during what part of their life cycle do they depend on

water?

6. (a) What generation of ferns do we normally see growing in the woods?

(b) What specific part of that generation is the most noticeable?

7. Where exactly in the fern life cycle does the diploid number get restored?

8. Describe a fern prothallus.

9. To what generation do most of the spermatophyte plant parts belong?

10. Describe the male and female sporophyte parts of gymnosperms and angiosperms.

11. Compare the seeds of gymnosperms and angiosperms.

12. Compare pollination in gymnosperms and angiosperms.

13. Compare fertilization in gymnosperms and angiosperms.


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Alternation of Generations - k12resources.nelson.comk12resources.nelson.com/.../documents/life_cycle_of_plants.pdf · The diploid organism is called the sporophyte and has two sets