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Chapter 10 – Reproduction in Plants CHAPTER 10 – REPRODUCTION IN PLANTS Many plants are able to reproduce both asexually and sexually. Sexually reproducing plants have alternating haploid and diploid stages in their life cycles, and this alternation of generations is clearly visible in the life cycle of a liverwort or a fern. In flowering plants, the haploid stage of the life cycle is much reduced. The flowers of flowering plants are organs specialised for sexual reproduction on land, where mechanisms are needed which allow the transfer of male to female gametes without any danger of them drying out. ASEXUAL REPRODUCTION IN PLANTS 10.1 Vegetative reproduction Asexual reproduction is much more common in plants than in animals. Many species of plants are able to reproduce both sexually and asexually. Growth in a plant which does not involve the development of structures involved in sexual reproduction is sometimes known as vegetative growth. So the growth of new individuals by asexual reproduction is known as vegetative reproduction. Fig. 10.1 shows asexual reproduction in a strawberry plant. Here, stems called runners grow horizontally from the parent plant, and put out roots at a node (a place on the stem from which a leaf stalk grows, and where there is a bud). The roots grow downwards into the soil, while the bud grows upwards and forms a young plant. Eventually, the connection between the original parent plant and the daughter plant withers. By this time, the daughter plant will probably have begun to grow runners of its own. Virtually every part of one plant or another can be involved in asexual reproduction. You may like to try to think of some examples of plants which grow new individuals from their roots or from their leaves. The cells in the new individuals produced by asexual reproduction are formed by mitosis and so, of course, are genetically identical to each other and to their parent. They form a clone. Horticulturists often wish to produce large quantities of genetically identical plants, as these will all grow in a similar way and produce a similar crop. This makes them easier to tend, harvest and market. Such methods as grafting and taking cuttings have been used for hundreds of years, and still are used very widely. However, in recent years the technique of tissue culture has been developed which allows very large numbers of genetically identical new plants to be produced from a very small number of original cells taken from a parent. It involves taking a small piece of tissue from the parent plant, and placing it on sterile agar containing nutrients and plant growth substances which stimulate the cells to divide by mitosis. They are kept in controlled conditions of light and temperature, and quickly grow into complete little plants which can later be removed from the agar and planted into soil. Asexual reproduction can be advantageous to plants growing in natural conditions, 137 Chapter 10 – Reproduction in Plants as it can enable rapid coverage of the ground in a suitable environment. However, the genetic uniformity of the offspring can be a problem, for example if a parasite or other pest is able to infect these plants. If one plant has no defence against the pest, then this is true of all of them and the whole population may be killed. This is also a problem for plant breeders, as genetically uniform crop plants can become vulnerable to pests and diseases which may sweep through an entire crop. Genetic variation can help to lessen the probability that such widespread infection will occur, as at least some of the plants may have a gene which confers resistance to the pest. In the wild, almost all species of plants are able to reproduce sexually, so ensuring genetic variation in their offspring. In horticulture and agriculture, plant breeders maintain stocks of many different varieties of crop plants which can be used in breeding programmes to introduce new genes into commercially available varieties. Fig10. 1 Asexual reproduction in a strawberry plant REPRODUCTION IN LIVERWORTS AND FERNS Almost all plants, like animals, reproduce sexually - that is, they produce haploid gametes which fuse to form a diploid zygote which then grows to form an adult plant. Most of this chapter concentrates on reproduction in flowering plants, but firstly we will look at sexual reproduction in two less familiar types of plant liverworts and ferns. 10.2 The life cycle of a liverwort Liverworts belong to the phylum Bryophyta. They are not very big plants, and you may never have noticed a liverwort growing. However, once you know what to look for, they are easy to find. Liverworts grow in damp places, often where it is shady. You may find them on the banks of streams, on bricks or stones at the base of damp walls, or on the soil on top of flowerpots. Fig. 10.2 shows the structure of a common liverwort in Britain called Pellia epiphylla. The plant which you see growing is haploid - that is, each cell contains a 138 Chapter 10 – Reproduction in Plants single set of chromosomes rather than the two complete sets which are present in the diploid cells of almost all adult animals and flowering plants. The structure of this haploid plant is very simple. Liverworts have no true roots or stems, just a simple flat body called a thallus. The thallus lies close to the ground, to which it is attached by single-celled structures called rhizoids. The rhizoids do take up water and mineral salts from the soil, but the whole thallus can also absorb these substances when raindrops splash onto it. Most of the cells in the thallus contain chloroplasts and photosynthesise. Unlike the leaf of a flowering plant the thallus has no waxy cuticle covering its surface, and so it easily loses water. This is one reason why liverworts usually grow in damp places. There are no xylem vessels or phloem tubes, so water cannot easily be transported to different parts of the plant, and the only support is given by cellulose cell walls and turgor. This, combined with the problems of water loss, prevents liverworts from growing very big. The thallus reproduces by producing haploid gametes, and so it is called a gametophyte ('gamete-producing plant'). On the upper surface of the thallus, male gametes develop in small depressions called antheridia. As the cells of the thallus are already haploid, these gametes are produced by mitosis. The male gametes are long, thin cells with two very long flagella. They are called antherozoids, or sperm. At the edges of the thallus, the female gametes, called oospheres or eggs, are formed by mitosis inside archegonia. Whereas large numbers of sperm are made in each antheridium, only one egg is made in each archegonium. Fertilisation happens when the sperm swim to the egg while it is still in the archegonium. They can only do this when there is a film of water on the surface of the thallus, so that they have something to swim through and so that they do not dry out. Fertilisation therefore happens in wet weather. Splashing of raindrops may also be important in transferring the sperm to the eggs, especially from one thallus to another. The sperm are attracted to the egg by chemicals, especially sugars, which it secretes. Fertilisation takes place inside the archegonium, producing a diploid zygote. The zygote divides by mitosis, eventually producing a diploid structure called a sporogonium. This stays attached to the thallus which provides it with food. At the tip of the sporogonium a spherical structure called a spore capsule develops. Inside this some of the diploid cells, called spore mother cells, divide by meiosis to produce haploid spores. (A spore is one or more cells, often surrounded by a protective waterproof covering, which can be dispersed from where it was made and grow into a new organism.) Because the sporogonium produces spores, it is called a sporophyte. After a while, the spore capsule bursts, flinging out the spores. If a spore lands in a suitable moist place, it germinates. Mitosis takes place, forming a new haploid thallus. So you can see that there are two alternating stages or 'generations' in the life cycle of a liverwort - a haploid gametophyte which produces gametes by mitosis and a diploid sporophyte which produces spores by meiosis. This kind of life cycle is said to show Alternation of generations. The life cycle of pellia is shown in Fig. 10.3. 139 Chapter 10 – Reproduction in Plants Fig-10.2-Reproduction in a liverwort 140 Chapter 10 – Reproduction in Plants Fig 10.3 Life cycle of a liverwort 10.3 The life cycle of a fern Ferns belong to the phylum Filicinophyta. Fig. 10.4 shows the structure of the male fern, Dryopteris filix-mas. This fern is very common, and you can find it growing in woodland and other shady places all over Britain. Ferns, like liverworts, have a clearly marked alternation of generations in their life cycle. The large fern plant which you see growing is the diploid sporophyte. It is much larger, and structurally much more complex, than either the thallus or the sporogonium of a liverwort. Ferns have true roots which can penetrate quite deeply into the soil. The roots absorb water and minerals as well as anchor the plant. The male fern has large leaves, called fronds, with a large surface area for the absorption of sunlight and carbon dioxide for photosynthesis. The fronds are divided into 'leaflets' called pinnae. The surface of the fronds is covered with a waterproof cuticle, and there are stomata on the upper and lower epidermis which can be closed by guard cells to conserve water. Ferns have xylem vessels and phloem tubes; the xylem vessels transport water to all parts of the plant and also help to support it. Thus, ferns can grow in drier places than liverworts, and they can grow larger. The spores are produced on the undersides of the fronds, in structures called sporangia. (This name is very similar to, but not the same as, the 'sporogonium' in which spores are formed in a liverwort - make sure that you do not confuse them!) The sporangia are found in clusters called sori, and each sorus is covered by a protective 'hood' called an indusium. Inside the sporangia, diploid spore mother cells divide by meiosis to produce haploid spores. The spores are released in dry conditions as a strip of cells called the annulus, in the wall of the sporangium, curls back and rupture the sporangium. Fig. 10.4 explains how this happens. The spores are carried away on air currents. They germinate in moist conditions, dividing by mitosis to form a haploid structure which is quite similar to the thallus of a liverwort; it is called a prothallus. It is about one centimetre across and very thin, so it is unlikely that you will ever have noticed one. The prothallus has no cuticle and no supportive or specialised conducting tissue, so it can only grow in moist conditions. The prothallus is the gametophyte generation in the fern's life cycle. Male gametes and female gametes are produced by mitosis inside antheridia and 141 Chapter 10 – Reproduction in Plants archegonia respectively, both on the underside of the pro thallus. Fertilisation takes place in a very similar way to liverworts to form a diploid zygote. As in the liverwort, this remains inside the archegonium as it divides repeatedly by mitosis. It develops into a new sporophyte. At first, the embryo sporophyte obtains its nutrients from the prothallus, but once it has formed its first leaves it can photosynthesise for itself. The gametophyte then dies, and the sporophyte continues to grow to form a new fern plant. You can see that the life cycle of Dryopteris (Fig. 10.5) is in many ways similar to that of Pellia. Both of them show alternation of generations, with a haploid gameteforming gametophyte alternating with a diploid sporeforming sporophyte. However, whereas in the liverwort the gametophyte is the 'dominant' stage, in the fern this role is taken by the sporophyte. Fig 10.4.1 142 Chapter 10 – Reproduction in Plants Fig 10.4 Reproduction in a fern 143 Chapter 10 – Reproduction in Plants Fig 10.5 Life cycle of a fern SEXUAL REPRODUCTION IN FLOWERING PLANTS 10.4 The structure of a flower Flowering plants belong to the phylum Angiospermophyta. They differ from all other kinds of plants in having true flowers. Fig. 10.6 shows the structure of a generalised flower and also a flower belonging to the Papilionaceae. The different parts of the flower are arranged in rings, sometimes called whorls, attached to the top of the flower stalk or receptacle. These parts are always arranged in the same order, as shown in Fig. 10.6. The outer whorl is called the calyx and is made up of sepals. In many flowers these are dull in colour, and their main function is to protect all the other parts of the flower while it is still a bud. The next whorl is the corolla, made up of petals. In insect-pollinated flowers the petals are often brightly coloured and scented, as their function is to attract insects to the flower. The petals advertise the presence of the flower from a distance, and the colour and scent of the petals may determine the kind of insect or other animal which is attracted. In tropical countries, for example, many flowers are pollinated by birds and these often have bright red flowers. The dark reddish-brown, strangely marked flowers of Stapelia smell like rotten meat, and attract flies. In many flowers, nectaries secrete a sugar-rich fluid called nectar, again in order to attract insects for pollination. The next whorl is made up of the male parts of the flower and is called the androecium. It contains several often very many - stamens, each of which has a stalk called a filament supporting an anther. Inside the anthers, the male gametes are formed inside pollen grains. Finally, in the centre of the flower, the female parts or carpels are found. These make up the gynaecium. There are one or more ovaries, each containing one or more ovules, inside which the female gametes develop inside an embryo sac. At the top of each ovary is a style which supports a stigma. The stigma has the 144 Chapter 10 – Reproduction in Plants function of capturing pollen grains, one of the first stages in the series of events which will bring the male gametes to the female gametes. Fig 10.6 –The structure of a flower 10.5 Formation of pollen grains Male gametes are produced inside pollen grains. Pollen grains are formed inside the anthers. Each anther contains four compartments called pollen sacs. The wall of each pollen sac contains several layers of cells. One of the outer layers is made up of cells with thickened walls and is called the fibrous layer; this helps to liberate the pollen grains when they are ripe. The innermost layer is called the tapetum. The cells in this layer help to provide nutrients to the developing pollen grains. In the centre of each pollen sac, diploid pollen mother cells divide by meiosis, each producing four haploid cells. In some species, these stay together in a group of four called a tetrad, but in others they separate. Each of the haploid cells develops a tough protective wall around itself, becoming a pollen grain. The wall is made up of two layers, an outer very tough, waterproof exine and an inner intine. In place, the exine is absent, leaving a thin area in the wall called a pit. The form and structure of the exine varies from species to species, and it is possible to identify a plant just by looking at its pollen grains. They often have spikes or knobs to help them to stick to the bodies of insects. 145 Chapter 10 – Reproduction in Plants The haploid nucleus inside each pollen grain divides by mitosis, forming two haploid cells separated by a very thin cell wall. One of these haploid nuclei is called the generative nucleus, and the other is the tube nucleus. When the pollen grains are fully formed, the anthers spilt open in a process called dehiscence. They split along a line between the two pollen sac on either side (Fig.10.8), exposing the pollen grains on the surface. Before following the pollen grains further, we will look at how the female gamete- forming structures develop to a similar stage. 146 Chapter 10 – Reproduction in Plants Fig10.8- the production of pollen in an anther 147 Chapter 10 – Reproduction in Plants 10.6 Formation of embryo sac The female gametes are produced inside structures called embryo sacs which develop inside the ovules (Fig 10.9) Ovules are formed inside ovaries. Each ovule is connected to the ovary by a stalk called the funicle. The ovule has an outer covering, or integuments, surrounding a tissue made up of relatively undifferentiated cells called the nucellus. At one end of the ovule, the integuments do not quite meet, leaving an opening called the micropyle. The other end of the ovule, furthest from the micropyle and nearest to the funicle, is called the chalaza. Inside each ovule, a large, diploid, spore mother cell develops. This cell divides by meiosis to produce four haploid cells. All but one of these degenerates and the one surviving haploid cell then develops into an embryo sac. The embryo sac absorbs nutrients from the nucleus and grows larger. Its nucleus divides by mitosis three times, forming eight haploid nuclei. Two of these nuclei are found near the centre of the embryo sac, with no cell membranes around them. The other six arrange themselves at the ends of the embryo sac, three at one end and three at the other, and usually develop cell membranes. The three haploid cells at the end nearest the chalaza are called antipodal cells. One of the haploid cells at the end nearest to the micropyle is a little larger than the other two. This is the female gamete, the egg cell. The other two cells at this end are called synergids. The two nuclei in the middle may fused together to form a single diploid nucleus called the primary endosperm nucleus. Thus, a mature embryo sac usually contains six haploid nuclei and one diploid nucleus. 10.7 Pollination Pollination is the transfer of pollen from the anther, where it was made, to a stigma. Many species of plants have mechanisms which ensure that the pollen is transferred to a different individual of the same species; this is called cross pollination. In other species, such as the garden pea, it is usual for pollen to be transferred to the stigma of the same flower; this is called self pollination. As neither plants nor pollen grains can move actively from place to place, other agents are used to transfer the pollen grains. These include animals, such as insects, birds, bats and other small mammals such as mice, and the wind. A few aquatic plants make use of water currents to transfer pollen. We will look at how pollination happens in an insect-pollinated flower and a windpollinated flower. Insect-pollinated flowers attract insects by providing a 'reward' for them when they visit the flower. This reward is often either carbohydrate-rich nectar or proteinrich pollen, or sometimes both. The flower advertises the presence of these foods with brightly coloured petals and/or a scent. The petals are frequently arranged to provide a landing platform for insects. When the insect arrives at the flower, it brushes against the anthers as it collects its 'reward'. Some of the pollen grains stick to its body. The insect flies away, and will often go straight to another similar flower. Here, some of the pollen grains may brush off its body onto the stigma. 148 Chapter 10 – Reproduction in Plants 149 Chapter 10 – Reproduction in Plants Fig 10.9 – The development of the embryo sac It is clear that both the plant and the insect benefit from this arrangement. Insects and flowers have evolved together over millions of years, and this has resulted in some very elaborate pollination mechanisms. Some flowers, such as orchids, have such complex pollination mechanisms that they can only be pollinated by one species of insect. 150 Chapter 10 – Reproduction in Plants Wind-pollinated flowers have no need to attract insects, so they do not waste resources in producing large, brightly coloured petals or nectar. They are usually relatively small flowers, and are often held on long stalks so that they can easily catch the wind. Fig. 10.11 shows the structure of a typical grass flower. The filaments are long and dangle freely out of the flower; they are very flexible, and move easily in the wind, shaking the pollen free from the anthers. Huge quantities of tiny, light pollen grains are produced, and you can often see clouds of this pollen floating up from flowering grasses if you brush against them. (Grass pollen is one of the main culprits causing hay fever.) The stigmas are long and feathery, and protrude from the flower. Their large exposed surface increases their chance of catching pollen grains floating on the wind. Fig10. 11- Structure of a wind-pollinated flower 10.8 Fertilisation Pollination results in the arrival of pollen grains on to the stigma. The pollen grains stick to the surface of the stigma, absorb water and begin to germinate. This normally only happens if the pollen grain is on the stigma of the same species of flower, and - in some species - if it is on the stigma of a different flower of the same species. The contents of the pollen grain push out through one of the pits in the wall, forming a pollen tube (Fig. 10.12). The tube grows down through the style towards an ovule. The tube nucleus remains close to the tip of the tube as it makes its way through the style. Digestive enzymes are secreted from the tip of the tube, which is probably directed towards the ovule by chemicals which the ovule secretes. As the tube grows, the generative nucleus divides by mitosis, forming two haploid male gametes. In most plants, the pollen tube enters the ovule through the micropyle, although in a few species it may digest its way in through the chalaza. Once, 151 Chapter 10 – Reproduction in Plants the tube has penetrated the ovule, the tube nucleus degenerates as its role is completed. Behind it, the two male gametes make their way into the embryo sac. As you would expect, one of the male gametes fuses with the egg cell, forming a diploid zygote. But, in plants, a double fertilisation takes place inside the embryo sac. The other male gamete fuses with the diploid nucleus in the centre of the embryo sac, forming a triploid nucleus (that is, possessing three sets ofchromosomes). This triploid nucleus is called the endosperm nucleus. Wind 1. 2. 3. – pollinated flower Generally small Petals green or dull coloured Do not produce nectar 4. Flower hangs down for easy shaking 5. Stamens and stigma hang out of the ring of petals 6. Large number of pollen grains produced 7. Pollen grains very light with smooth surface 8. Stigma has feathery branches for catching pollen Insect- pollinated flowers Generally larger Petals often brightly coloured Petals have nectarines which produce nectar Flower faces upwards Stamens and stigma inside the ring of petals Smaller number of pollen grains produced Pollen grains heavier with spikes for sticking to insect Stigma is like pinhead and lacks branches. 152 Chapter 10 – Reproduction in Plants Fig 10.12 - Fertilization 153 Chapter 10 – Reproduction in Plants Fig10. 14- The development of the seed Development of a seed Once fertilisation has taken place, the ovule is called a seed. It remains attached to the parent plant and continues to receive nutrients from it as it develops. Fig. 10.14 154 Chapter 10 – Reproduction in Plants shows how a seed develops. The diploid zygote divides by mitosis to form an embryo plant, attached to the wall of the ovule by a large basal cell and a little column of cells called the suspensor. The embryo develops a radicle or embryo root, a plumule or embryo shoot, and two cotyledons. In some species, the cotyledons will become the first leaves, the 'seed leaves', of the young plant when the seed germinates, while in others the plumule is the first part to reach the light and photosynthesise. In many species, the cells of the cotyledons build up large stores of food within the seed, such as starch, which will be used by the embryo in the early stages of germination. The triploid endosperm nucleus also divides by mitosis, forming a tissue called the endosperm which surrounds the developing embryo. The function of the endosperm is to provide nourishment for the embryo. In some seeds, the endosperm has completed its function within a few days of fertilisation and disappears. In others, such as cereal grains it remains as the main storage tissue to provide nutrients during germination. While all this is happening, the integuments of the ovule are developing into the testa of the seed. This involves thickening and toughening, as waterproof substances such as lignin are laid down in the cell walls. The small gap in the integuments, the micropyle, remains as a tiny hole in the testa The wall of the ovary also undergoes changes after fertilisation. The ovary becomes a fruit, and its wall becomes the pericarp of the fruit. The seeds are, of course, contained within the fruit, and the fruit is often adapted to disperse the seeds away from the parent plant. The pericarp may become fleshy and sweettasting to attract animals, or it may develop hooks and spines to stick to their hair. Wind-catching projections may form, or the fruit may become dry and hard, later splitting forcefully and throwing the seeds in all directions. In many species, other parts of the flower are involved in these developments; in dandelions, for example, the calyx becomes the 'parachute' of the fruit. In general, however, the various parts of the flower have completed their role by now, and they wither and fall off as the seeds develop inside the fruits. 10.10 Seed dormancy and germination In some species of plants, such as the common garden weed groundsel, the seeds are ready to germinate as soon as they leave the parent plant. More commonly, the seeds are in a state of suspended growth, called dormancy, when they are shed. Towards the end of their development, while still attached to the parent plant, water is withdrawn from them until their water content is as little as 5% of that of other plant tissues. This dryness helps seeds to survive for many years in very hostile conditions, such as drought, or extreme heat or cold. The relative concentrations of hormones, such as ABA, gibberellin and cytokinin, are also thought to keep the seed in a state of dormancy. The exact conditions required for a seed to break dormancy and begin to germinate differ from species to species. Some require particular types of light; almost all require exposure to a particular range of temperature, an adequate supply of oxygen, and water. As germination begins, water is absorbed into the seed and hydrolytic enzymes, such as amylases, break down the food stores into smaller soluble molecules. These are transported to the developing embryo in which the nutrients are used to supply materials and energy to allow the cells to divide and lengthen. 155 Chapter 10 – Reproduction in Plants First the ridicule and then the plumule - or the cotyledons followed by the plumule - emerge from the seed. The radicle is positively geotropic and grows downwards. It can then absorb more water from the soil, as well as inorganic ions, allowing the rest of the seedling to grow even more rapidly. The plumule is negatively geotropic and positively phototropic, and grows up above the soil. Up until this point, all of the growth of the seedling depended on the use of nutrients such as starch and protein which were stored in either the cotyledons or endosperm. Some of these nutrients are used to form new molecules and structures such as cellulose in cell walls. Some of them are respired, at first anaerobically (because oxygen cannot easily penetrate the seed) and then aerobically, so that some of the carbon atoms from them are released as carbon dioxide. The dry mass of a germinating seedling therefore drops during these early stages of germination. Once the plumule gets above ground, however, photosynthesis can begin. Now carbon atoms from carbon dioxide in the air are fixed and become incorporated into new molecules in the plant, so its dry mass begins to increase (Fig. 10.15). Fig10.15 changes in dry mass during germination of a seed 156