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  1. 1. 39 Reproduction in Flowering Plants For many people, pollen means sneezing and misery because pol- len grains of many plant species are potent allergens. However, pollen is not part of nature simply to annoy human beings. What is a pollen grain? It is a tiny, haploid male plant. To the stigma (the pollen “landing pad”) of a flower, pollen grains represent an opportunity for mate selection. That is, the stigma may allow some pollen grains to germinate, but not others. If a pollen grain survives the mate selection process and germinates, it may eventually deliver male gametes to a microscopic, haploid female plant em- bedded in the flower. Why do angiosperms expend energy and resources to produce flowers and that sometimes obnoxious pollen? The answer is simple: Flowers are sexual reproductive structures, and reproduction is the most important goal in a plant’s—or any organ- ism’s—life. In this chapter we will look at several aspects of plant reproduction, including Key Players in Sexual Reproduction some that are still not well understood. We will contrast sexual and asexual repro- Each species’ pollen has a characteristic duction, and we will consider sexual reproduction in detail. In doing so, we will look size, shape, and cell wall structure. These at angiosperm gametophytes, pollination, double fertilization, embryonic develop- structures are the male gametophytes and are essential for sexual reproduction in ment, and the roles of fruits in seed dispersal. The tran- seed plants. sition from the vegetative state to the flowering state is a key event in plant development, and we’ll see how changing seasons trigger flowering in some plants—and speculate on the existence of a flowering hormone. We will conclude the chapter with an examination of asex- ual reproduction in nature and in agriculture. Many Ways to Reproduce Plants have many ways of reproducing—and humans have developed even more ways of reproducing them. Flowers contain the sex organs of plants; it is thus no sur- prise that almost all angiosperms reproduce sexually. But some angiosperms reproduce asexually as well; some even reproduce asexually most of the time. What are the advantages and disadvantages of these two kinds of re- production? The answers to this question involve genetic recombination. As we have seen, sexual reproduction produces new combinations of genes and diverse phe- notypes. Asexual reproduction, in contrast, produces a clone of genetically identical individuals.
  2. 2. 750 CHAPTER THIRT Y-NINE Both sexual and asexual reproduction are important in porangium, within which a female gametophyte may de- agriculture. Many important annual crops are grown from velop. The stalk of the pistil is the style, and the end of that seeds, which are the products of sexual reproduction. Seed- stalk is the stigma. Each stamen is composed of a filament grown crops include the great grain crops, all of which are bearing a two-lobed anther, which consists of four microspo- grasses—wheat, rice, corn, sorghum, and millet—as well as rangia fused together. Male gametophytes begin their devel- plants in other families, such as soybeans and safflower. opment within the microsporangia. Other crops, such as strawberries, potatoes, and bananas, are The petals and sepals of many flowers are arranged in produced asexually. whorls (circles) or spirals around the carpels and stamens. To- Orange trees, which have been under cultivation for cen- gether, the petals constitute the corolla. Below them, the sepals turies, can be grown from seed—except for the navel orange, constitute the calyx. The petals are often colored, attracting which has no seeds. This plant apparently arose only once in pollinating animals; the sepals are often green and photosyn- history. Early in the nineteenth century, on a plantation on thetic. All the parts of the flower are borne on a stem tip, the the Brazilian coast, a single orange seed gave rise to one tree receptacle. Flower parts are very diverse in form, in contrast to that had aberrant flowers. Parts of the flowers aborted, and the microscopic gametophytes that develop within them. seedless fruits formed. Asexual reproduction is the only way of propagating this plant, and every navel orange in the world comes from a tree that has been derived asexually Flowering plants have microscopic gametophytes from that original Brazilian navel orange tree. Before reading this section, you may wish to review the sec- Unlike navel oranges, strawberries are capable of forming tion in Chapter 29 entitled “Life cycles of plants feature al- seeds and need not be propagated asexually. Nonetheless, ternation of generations” (pages 571–572). Central to under- asexual propagation of strawberries is common because vast standing plant reproduction is the concept of alternation of numbers of plants that are genetically and phenotypically generations, in which a multicellular diploid generation al- identical to a plant humans find particularly desirable can be ternates with a multicellular haploid generation. produced in this way. In angiosperms, the diploid sporophyte generation is the We will treat asexual reproduction in greater detail at the larger and more conspicuous one. The sporophyte generation end of this chapter. We will begin, however, by considering produces flowers. The flowers produce spores, which develop sexual reproduction. into tiny gametophytes that begin and, in the case of the megagametophyte, end their development enclosed by sporo- phyte tisue. Sexual Reproduction in Plants The haploid gametophytes—the gamete-producing gen- Sexual reproduction provides genetic diversity through re- eration—of flowering plants develop from haploid spores in combination (see Chapter 9). Meiosis and mating between sporangia within the flower (Figure 39.1): different plants shuffle genes into new combinations, giving Female gametophytes (megagametophytes), which are a population a variety of genotypes in each generation, some called embryo sacs, develop in megasporangia. of which may be superior to those of their parents. This ge- Male gametophytes (microgametophytes), which are netic diversity may serve the population well as the envi- called pollen grains, develop in microsporangia. ronment changes or as the population expands into new en- vironments. The adaptability resulting from genetic diversity Within the ovule, a megasporocyte—a cell within the megas- is the major advantage of sexual reproduction over asexual porangium—divides meiotically to produce four haploid reproduction, although sexual reproduction can also break megaspores. In most plants, all but one of these megaspores up well-adapted combinations of alleles through the same then degenerate. The surviving megaspore usually undergoes process of recombination. three mitotic divisions, producing eight haploid nuclei, all ini- tially contained within a single cell—three nuclei at one end, three at the other, and two in the middle. Subsequent cell wall The flower is an angiosperm’s device formation leads to an elliptical, seven-celled megagameto- for sexual reproduction phyte with a total of eight nuclei (see Figure 39.1): A complete flower consists of four groups of organs that are modified leaves: the carpels, stamens, petals, and sepals (see At one end of the elliptical megagametophyte are three Figure 30.7). The carpels and stamens are, respectively, the fe- tiny cells: the egg and two cells called synergids. The male and male sex organs. A pistil is a structure composed of egg is the female gamete, and the synergids participate one or more carpels. The base of the pistil, called the ovary, indirectly in fertilization by attracting and accepting the contains one or more ovules, each of which contains a megas- pollen tube.
  3. 3. REPRODUCTION IN FLOWERING PLANTS 751 Ovary Anther Ovule 4 The second sperm nucleus fuses with the two polar nuclei. Microsporocyte Endosperm Ovary nucleus (3n) Ovule Zygote (2n) 3 One sperm Megasporocyte (2n) nucleus fuses with the egg. Seed Megasporangium DIPLOID (2n) Double Fertilization Meiosis HAPLOID (n) Pollen grains (microgametophytes, n) Surviving megaspore (n) Pollen grain Mega- (microgametophyte) gametophyte (n) Antipodal Pollen cells (3) tube 1 In the ovule, three of the four meiotic Polar nuclei (2) products degenerate. Egg 4 The pollen tube grows toward the embryo Synergids (2) sac (see Figure 39.5). 2 The embryo sac is the female gametophyte. After three mitotic divisions, it contains 3 The pollen grain eight haploid nuclei. is transferred to the stigma. Sperm (2) Tube cell nucleus of pollen grain 39.1 Development of Gametophytes and Nuclear Fusion The embryo sac is the female gametophyte; the pollen grain is the male gametophyte. The male and female nuclei meet and fuse within the embryo sac. Most angiosperms have dou- ble fertilization, in which a zygote and an endosperm nucleus form from separate fusion events—the zygote from one sperm and the egg and the endosperm from the other sperm and two polar nuclei. At the opposite end of the megagametophyte are three The pollen grain (microgametophyte) consists of fewer antipodal cells, which eventually degenerate. cells and nuclei than the embryo sac. The development of a In the large central cell are two polar nuclei. pollen grain begins when a microsporocyte within the anther The embryo sac (megagametophyte) is the entire seven-cell, divides meiotically. Each resulting haploid microspore de- eight-nucleus structure. You can review the development of velops a spore wall, within which it normally undergoes one the embryo sac in Figure 39.1. mitotic division before the anthers open and release these
  4. 4. 752 CHAPTER THIRT Y-NINE Pollen two-celled pollen grains. The two cells are the tube cell Stigma and the generative cell. Further development of the Pollen pollen grain, which we will describe shortly, is delayed until the pollen arrives at a stigma. In angiosperms, the S3 S4 S2 S1 Pistil S3 S1 transfer of pollen from the anther to the stigma is re- Style ferred to as pollination. Pollen tube Ovary Pollination enables fertilization in the absence of liquid water S1S2 S1S2 S1S2 Gymnosperms and angiosperms do not require external wa- ter as a medium for gamete travel and fertilization—a free- dom not shared by other plant groups. The male gametes of gymnosperms and angiosperms travel within pollen grains. S3 and S4 pollen …but S1 and S2 pollen do are compatible not germinate. They are But how do angiosperm pollen grains travel from an anther with an S1S2 pistil… self-incompatible. to a stigma? Many different mechanisms have evolved for pollen trans- 39.3 Self-Incompatibility Pollen grains do not germinate normally port. In some plants, such as peas and their relatives, self-pol- if their S allele matches one of the S alleles of the stigma. Thus, the egg cannot be fertilized by a sperm from the same plant. lination is accomplished before the flower bud opens. Pollen is transferred by the direct contact of anther and stigma within the same flower, resulting in self-fertilization. Wind is the vehicle for pollen transport in many species. Some aquatic angiosperms are pollinated by water carrying Wind-pollinated flowers have sticky or featherlike stigmas, pollen grains from plant to plant. Animals, including insects, and they produce pollen grains in great numbers (Figure 39.2). birds, and bats, carry pollen among the flowers of many plants. Some plants practice “mate selection” In our discussion of Mendel’s work (see Chapter 10), we saw that some plants can reproduce sexually either by cross-pol- lination or by self-pollination. But not all plants have this flexibility. Many plants reject pollen from their own flowers. This phenomenon, known as self-incompatibility, promotes genetic variation. A single gene, the S gene, is responsible for self-incompati- bility in most plants. The S gene has dozens of alleles. A pollen grain is haploid and possesses a single S allele; the recipient pis- til is diploid. In self-incompatible plants, pollen fails to germi- nate, or the pollen tube fails to traverse the style, if the S allele of the pollen matches one of the two S alleles in the pistil (Figure 39.3). The stigma plays an important role in “mate selection” by flowering plants. The stigmas of most plants are exposed to the pollen of many other species as well as their own. Pollen from the same species binds strongly to the stigma due to cell–cell sig- naling between the stigma and the cell walls of the pollen grains. In contrast, foreign pollen falls off readily or fails to germinate. 39.2 Wind Pollination The numerous anthers on these A pollen tube delivers male cells to the embryo sac inflorescences (groups of flowers) of a hazelnut tree all point away from the stalk and stand free of the plant, promoting When a pollen grain lands on the stigma of a compatible pis- dispersal of the pollen by wind. til, it germinates. Germination, for a pollen grain, is the de-
  5. 5. REPRODUCTION IN FLOWERING PLANTS 753 Pollen grain Pollen tube Stigma Angiosperms perform double fertilization In most angiosperm species, the mature pollen grain con- sists of two cells, the tube cell and the generative cell. The larger tube cell encloses the much smaller generative cell. Guided by the tube cell nucleus, the pollen tube eventually grows through the megasporangial tissue and reaches the embryo sac. The generative cell meanwhile has undergone one mitotic division and cytokinesis to produce two hap- loid sperm cells. Both of the sperm cells enter the embryo sac, where they are released into the cytoplasm of one of the synergids. This synergid degenerates, releasing the sperm cells (Figure 39.5). 39.4 Pollen Tubes Begin to Grow These pollen grains have landed Each sperm cell then fuses with a different cell of the embryo on hairlike structures on the stigma of an Arabidopsis flower, and sac. One sperm cell fuses with the egg cell, producing the pollen tubes have penetrated the stigma. diploid zygote. The other fuses with the central cell, and that sperm cell nucleus and the two polar nuclei unite to form a triploid (3n) nucleus. While the zygote nucleus begins mitotic velopment of a pollen tube (Figure 39.4). The pollen tube ei- division to form the new sporophyte embryo, the triploid nu- ther traverses the spongy tissue of the style or, if the style is cleus undergoes rapid mitosis to form a specialized nutritive hollow, grows downward on the inner surface of this female tissue, the endosperm. The endosperm will later be digested organ until it reaches an ovule. The pollen tube may grow by the developing embryo, as we saw in the previous chap- millimeters or even centimeters in the process. ter. The antipodal cells and the remaining synergid eventu- The rapid growth of the pollen tube requires calcium ions, ally degenerate, as does the pollen tube nucleus. which are taken up by the growing tip of the tube, as well as This process is known as double fertilization because it cell adhesion proteins. The downward growth of the pollen involves two nuclear fusion events: tube is believed to be guided by a long-distance chemical sig- One sperm cell fuses with the egg cell. nal from the synergids within the ovule. If one synergid is The other sperm cell fuses with the two polar nuclei. destroyed, the ovule still attracts pollen tubes, but destruc- tion of both synergids renders the ovule unable to attract pollen tubes, and fertilization does not occur. 39.5 Sperm Nuclei and Double Fertilization The sperm nuclei contribute to the formation of the diploid zygote and the triploid endosperm. Double fertilization is a characteristic feature of angiosperm reproduction. 4 The synergid breaks down; one sperm nucleus unites with the two polar nuclei, Tube cell forming the first cell of the Three antipodal 3n endosperm generation. cells Generative cell Tube cell Polar nucleus nuclei Egg Synergids 1 Initially the pollen tube contains 2 The generative cell divides 3 The sperm cells enter the 5 The other sperm nucleus two haploid cells, the generative mitotically, producing two cytoplasm of a synergid. fertilizes the egg, forming cell and the tube cell. haploid sperm cells. the zygote, the first cell of the 2n sporophyte generation.
  6. 6. 39.6 Early Development of a Eudicot The embryo develops through intermediate stages, including a characteristic heart-shaped stage, to reach the torpedo stage. The zygote nucleus divides Torpedo-stage embryo mitotically, one daughter cell Heart-stage Endosperm giving rise to the embryo proper embryo and the other to the suspensor. Cotyledons Shoot apex Embryo Hypocotyl Endosperm Root apex nucleus Suspensor Embryo Suspensor Seed sac coat Zygote The tissues surrounding the embryo sac develop into the seed coat. The fusion of a sperm cell nucleus with the two polar nuclei to form endosperm takes place only in angiosperms. The fu- sion of these three nuclei, the possession of flowers, and the formation of fruit are the three most definitive characteristics the internal tissues begin to differentiate (see Figure 39.6). Be- shared by angiosperms. tween the cotyledons is the shoot apex; at the other end is the root apex. Between the shoot and root apices is the hypocotyl. Each of the apical regions contains an apical meristem whose Embryos develop within seeds dividing cells will give rise to the organs of the mature plant. Shortly after fertilization, highly coordinated growth and de- During seed formation, large amounts of nutrients are velopment of embryo, endosperm, integuments, and carpel moved in from other parts of the plant, and the endosperm ensues. The integuments—protective tissue layers immedi- accumulates starch, lipids, and proteins. In many species, the ately surrounding the megasporangium—develop into the cotyledons absorb the nutrient reserves from the surround- seed coat, and the carpel ultimately becomes the wall of the ing endosperm and grow very large in relation to the rest of fruit that encloses the seed. the embryo (Figure 39.7a). In others, the cotyledons remain The first step in the formation of the embryo is a mitotic thin (Figure 39.7b); they draw on the reserves in the en- division of the zygote that gives rise to two daughter cells. dosperm as needed when the seed germinates. These two cells face different fates. An asymmetrical (un- even) distribution of cytoplasm within the zygote causes one daughter cell to produce the embryo proper and the other daughter cell to produce a supporting structure, the suspen- In some eudicots, In other eudicots, the In monocots, the sor (Figure 39.6). The suspensor pushes the embryo against the cotyledons endosperm remains single cotyledon is or into the endosperm and provides one route by which nu- absorb much of the separate and the pressed against the endosperm and fill cotyledons remain thin. endosperm. trients pass from the endosperm into the embryo. most of the seed. With the asymmetrical division of the zygote, polarity has Seed coat Seed coat been established, as has the longitudinal axis of the new Cotyledon Cotyledon plant. A long, thin suspensor and a more spherical or globu- Shoot apex Shoot apex lar embryo are distinguishable after just four mitotic divi- sions. The suspensor soon ceases to elongate. However, cell Root apex Root apex divisions continue, the primary meristems form, and the first Cotyledon organs begin to form within the embryo. Endosperm Endosperm In eudicots (monocots are somewhat different), the initially (a) Kidney bean (b) Castor bean (c) Corn globular embryo takes on a characteristic heart stage form as the cotyledons (“seed leaves”) start to grow. Further elonga- 39.7 Variety in Angiosperm Seeds In some seeds, such as kidney beans (a), the nutrient reserves of the endosperm are absorbed by tion of the cotyledons and of the main axis of the embryo gives the cotyledons. In others, such as castor beans (b) and corn (c), the rise to what is called the torpedo stage, during which some of reserves in the endosperm will be drawn upon after germination.
  7. 7. REPRODUCTION IN FLOWERING PLANTS 755 In the late stages of embryonic development, the seed (a) 39.8 Dispersal of Fruits loses water—sometimes as much as 95 percent of its original (a) A samara is a winged fruit characteristic of the maple family. water content. In this desiccated state, the embryo is inca- (b) A coconut seed germinates pable of further development; it remains quiescent until in- where it washed ashore on a ternal and external conditions are right for germination. (Re- beach in the South Pacific. call from Chapter 38 that a necessary early step in seed germination is the massive imbibition of water.) In addition to embryo and endosperm development, the structures of the ovary are also undergoing developmental changes to form a seed and fruit. Some fruits assist in seed dispersal After fertilization, the ovary wall of a flowering plant—to- gether with its seeds—develops into a fruit. A fruit may con- sist of only the mature ovary and the seeds it contains, or it may include other parts of the flower or structures that are closely related to it. In some species, this process produces fleshy, edible fruits such as peaches and tomatoes, while in other species the fruits are dry or inedible. Some major vari- ations on this theme are illustrated in Figure 30.12, which shows only fleshy, edible fruits. Whatever its form, the fruit (b) serves to assure seed dispersal. Some fruits help disperse seeds over substantial distances, improving the chances that at least a few of the many seeds produced by a plant will find suitable conditions for germi- pany flowering or resume after flowering is completed. But nation and growth to sexual maturity. Various trees, includ- whatever the specific pattern, flowering always entails ma- ing ash, elm, maple, and tree of heaven, produce a dry, jor developmental changes. winged fruit that may be blown some distance from the par- ent tree by the wind (Figure 39.8a). Water disperses some fruits; coconuts have been spread in this way from island to Apical meristems can become inflorescence meristems island in the Pacific Ocean (Figure 39.8b). Still other fruits The first visible sign of the transition to the flowering state may travel by hitching rides with animals—either inside or out- be a change in one or more apical meristems in the shoot sys- side them. Fleshy fruits such as berries provide food for tem. During vegetative growth, an apical meristem continually mammals or birds; seeds that are swallowed whole travel produces leaves, lateral buds, and internodes (Figure 39.9a). safely through the animal’s digestive tract and are deposited This unrestricted growth is indeterminate (see Chapter 35). some distance from the parent plant. In some species, seeds Flowers may appear singly or in an orderly cluster that must pass through an animal to break dormancy. constitutes an inflorescence. If a vegetative apical meristem We have now traced the sexual life cycle of angiosperms becomes an inflorescence meristem, it ceases production of from the flower to the fruit to the dispersal of seeds. Seed ger- leaves, lateral buds, and internodes and produces other struc- mination and the vegetative development of the seedling tures: smaller leafy structures called bracts, as well as new were presented in Chapter 38. Now let’s complete the cycle meristems in the angles between the bracts and the intern- by considering the transition from the vegetative to the flow- odes (Figure 39.9b). These new meristems may also be inflo- ering state, and how this transition is regulated. rescence meristems, or they may be floral meristems, each of which gives rise to a flower. Each floral meristem typically produces four consecutive The Transition to the Flowering State whorls or spirals of organs—the sepals, petals, stamens, and If we view a plant as something produced by a seed for the carpels—separated by very short internodes, keeping the purpose of bearing more seeds, then the act of flowering is flower compact (Figure 39.9c). In contrast to vegetative api- one of the supreme events in a plant’s life. The transition to cal meristems and some inflorescence meristems, floral the flowering state marks the end of vegetative growth for meristems are responsible for determinate growth—the lim- some plants. In other plants, vegetative growth may accom- ited growth of the flower to a particular size and form.
  8. 8. 756 CHAPTER THIRT Y-NINE (a) Having seen how flowering occurs, we will now Vegetative apical consider how the transition from the vegetative to the meristem A vegetatively growing flowering state is initiated. apical meristem Leaf continues to produce leaves and internodes. Internode Photoperiodic Control of Flowering Environmental cues trigger the transition to the flow- ering state in many cases, but such environmental con- Floral trol is also subject to genetic modification. The life cy- (b) meristem cles of flowering plants fall into three categories: Inflorescence annual, biennial, and perennial. Annuals, such as meristem Inflorescence A bract is a modified, many food crops, complete their life cycle (seed to meristems give usually reduced rise to floral flower) in one growing season. Biennials, such as car- leaflike structure. meristems, rots and cabbage, grow vegetatively for all or part of bracts, and more one growing season and live on into a second growing inflorescence meristems. season, during which they flower, form seeds, and die. Perennials, such as oak trees, live for a few to many growing seasons, during which both vegetative growth Carpel and flowering occur. What control systems give rise to (c) Floral meristem these and other differences in flowering behavior? Stamen Modified A floral meristem leaflike In 1920, W. W. Garner and H. A. Allard of the U.S. gives rise to structures Department of Agriculture studied the behavior of a Petal a flower. of the flower Sepal newly discovered mutant tobacco plant. The mutant, named ‘Maryland Mammoth,’ had large leaves and ex- ceptional height. When the other plants in the field 39.9 Flowering and the Apical Meristem A vegetative apical meristem (a) grows without producing flowers. Once the transition flowered, ‘Maryland Mammoth’ plants continued to grow. to the flowering state is made, inflorescence meristems (b) give rise Garner and Allard took cuttings of ‘Maryland Mammoth’ into to bracts and to floral meristems (c), which become the flowers. their greenhouse, and the plants that grew from those cuttings finally flowered in December. Garner and Allard guessed that this flowering pattern had something to do with the mutant’s response to some envi- A cascade of gene expression leads to flowering ronmental cue. They tested several likely environmental vari- How do apical meristems become inflorescence meristems, ables, such as temperature, but the key variable proved to be and how do inflorescence meristems give rise to floral meris- day length. By moving plants between light and dark rooms tems? How does a floral meristem give rise, in short order, to at different times to vary the day length artificially, they were four different floral organs? How does each flower come to able to establish a direct link between flowering and day have the correct number of each of the floral organs? Nu- length. We now know that the key variable is the length of merous genes collaborate to produce these results. We’ll re- the night, rather than the day, but Garner and Allard did not fer here to some of the genes whose actions have been most make that distinction. thoroughly studied in Arabidopsis and snapdragons. The ‘Maryland Mammoth’ plants did not flower if the light period they were exposed to was longer than 14 hours per day, Expression of a group of meristem identity genes initiates but flowering commenced after the days became shorter than a cascade of further gene expression. 14 hours. Thus, the critical day length for ‘Maryland Mam- This cascade begins with cadastral genes, which partici- moth’ tobacco is 14 hours (Figure 39.10). The phenomenon of pate in pattern formation—the spatial organization of the control by the length of day or night is called photoperiodism. whorls of organs. Cadastral genes trigger the expression of floral organ iden- tity genes, which work in concert to specify the successive There are short-day, long-day, and day-neutral plants whorls (see Figure 19.12) Plants that flower in response to photoperiodic stimuli fall Floral organ identity genes are homeotic genes, and their into several classes. Poinsettias, chrysanthemums, and ‘Mary- products are transcription factors that mediate the expression land Mammoth’ tobacco are short-day plants (SDPs), which of still other genes. flower only when the day is shorter than a critical maximum.
  9. 9. REPRODUCTION IN FLOWERING PLANTS 757 ‘Maryland Mammoth’ tobacco Henbane flowers only when The flowering of some angiosperms, such as corn, roses, flowers only when days are days are longer than 14 hours, and tomatoes, is not photoperiodic. In fact, there are more of shorter than 14 hours, its critical its critical day length. these day-neutral plants than there are short-day and long- day length. day plants. Some plants are photoperiodically sensitive only 14 hours 14 hours when young and become day-neutral as they grow older. Light Dark Light Dark Others require specific combinations of day length and other factors—especially temperature—to flower. The length of the night determines whether a photoperiodic plant will flower ‘Maryland Henbane, The terms “short-day plant” and “long-day plant” became Mammoth’ tobacco Hyoscyamus niger entrenched before scientists learned that photoperiodically (short-day plant) (long-day plant) sensitive plants actually measure the length of the night, or of a period of darkness, rather than the length of the day. This fact was demonstrated by Karl Hamner of the University of California at Los Angeles and James Bonner of the California Institute of Technology (Figure 39.11). Working with cocklebur, an SDP, Hamner and Bonner ran a series of experiments using two sets of conditions: Long days; Short days; Long days; Short days; plant remains plant flowers plant flowers plant remains vegetative vegetative EXPERIMENT 39.10 Day Length and Flowering By artificially varying the length Question: Do short-day plants measure day length or night length? of the day, Garner and Allard showed that the flowering of ‘Maryland Mammoth’ tobacco is initiated when the days become shorter than a METHOD critical length. ‘Maryland Mammoth’ tobacco is thus called a short- day plant. Henbane, a long-day plant, shows an inverse pattern of Plants were moved between light and dark rooms flowering. for specified numbers of hours. RESULTS Light constant/Darkness varied 16 6 Spinach and clover are examples of long-day plants (LDPs), which flower only when the day is longer than a critical min- 16 7 No flowering imum. Generally, LDPs are triggered to flower in midsummer 16 8 and SDPs in late summer, fall, or sometimes in the spring. Be- 16 9 Only plants given cause short days occur both before and after midsummer, 9 or more hours 16 10 there is a degree of ambiguity in this signal. Could there be of dark flowered. a more precise way for plants to regulate flowering? 16 11 Some plants require photoperiodic signals that are more Light varied/8 or 10 hours of darkness complex than just short or long days. One group, the short- 8 10 Only plants given long-day plants, must experience first short days and then long 10 10 10 hours of dark ones in order to flower. Accordingly, white clover and other flowered. 12 10 short-long-day plants flower during the long days before midsummer. Another group, the long-short-day plants, cannot 8 8 flower until the long days of summer have been followed by 10 8 No flowering shorter ones, so they bloom only in the fall. Kalanchoe, seen 12 8 in Figure 39.16b, is a long-short-day plant. Time (hours) Other processes besides flowering are also under pho- toperiodic control. We have learned, for example, that short Conclusion: Short-day plants measure the length of the night and could more accurately be called long-night plants. days trigger the onset of winter dormancy in plants. (Ani- mals, too, show a variety of photoperiodic behaviors, as we’ll 39.11 Night Length and Flowering The length of the dark period, see in Chapter 52.) not the length of the light period, determines flowering.
  10. 10. 758 CHAPTER THIRT Y-NINE For one group of plants, the light period was kept con- EXPERIMENT A Question: How does interrupting a long day or night affect flowering? stant—either shorter or longer than the critical day length—and the dark period was varied. Short-day Long-day For another group of plants, the dark period was kept plants Experimental conditions plants constant and the light period was varied. No flowering Flowering No flowering Flowering The plants flowered under all treatments in which the dark Flowering No flowering period exceeded 9 hours, regardless of the length of the light No flowering Flowering period. Thus, Hamner and Bonner concluded that it is the length of the night that matters; for cocklebur, the critical night Conclusion: Photoperiodic plants measure the length of the night, not length is about 9 hours. Thus, it would be more accurate to the day. Interrupting a long night with a brief period of light inhibits call cocklebur a “long-night plant” than a short-day plant. flowering in short-day plants. Long-day plants flower when the night In cocklebur, a single long night is sufficient photoperiodic is short, but interrupting their long day has no effect. stimulus to trigger full flowering some days later, even if the intervening nights are short ones. Most plants are less sensi- EXPERIMENT B tive than cocklebur and require from two to several nights of Question: Does phytochrome participate in the photoperiodic timing mechanism? appropriate length to induce flowering. For some plants, a single shorter night in a series of long ones, even one day be- Short-day Long-day fore flowering would have commenced, inhibits flowering. plants plants By means of other experiments, Hamner and Bonner Flowering No flowering gained some insight into how plants measure night length. They grew SDPs and LDPs under a variety of light condi- tions. In some experiments, the dark period was interrupted No flowering R Flowering by a brief exposure to light; in others, the light period was in- Flowering FR No flowering terrupted briefly by darkness. Interruptions of the light pe- Flowering R FR No flowering riod by darkness had no effect on the flowering of either No flowering R FR R Flowering short-day or long-day plants. Even a brief interruption of the Flowering R FR R FR No flowering dark period by light, however, completely nullified the effect Conclusion: When plants are exposed to red (R) and far-red (FR) light of a long night (Figure 39.12a). An SDP flowered only if the in alternation, the final treatment determines the effect of the light long nights were uninterrupted. An LDP experiencing long interruption, suggesting that phytochrome participates in nights flowered if those nights were interrupted by exposure photoperiodic responses. to light. Thus, the investigators concluded, these plants must 39.12 The Effect of Interrupted Days and Nights have a timing mechanism that measures the length of a con- (a) Experiments suggest that plants are able to measure the tinuous dark period. length of a continuous dark period and use this information The nature of this timing mechanism has been partially to trigger flowering. (b) Phytochromes seem to be involved in the photoperiodic timing mechanism. revealed, beginning with the determination of the effective wavelengths of light and the identity of the photoreceptors. In the interrupted-night experiments, the most effective wavelengths of light were in the red range (Figure 39.12b), this suggestion is inconsistent with many experimental ob- and the effect of a red-light interruption of the night could be servations, such as the fact that when a plant is subjected to a fully reversed by a subsequent exposure to far-red light, in- dark period several days in duration, the plant’s sensitivity to dicating that a phytochrome is the photoreceptor. Phy- a light flash during the long night varies on a roughly 24-hour tochromes and blue-light receptors, which affect several as- cycle. Such data suggest instead that the phytochrome is only pects of plant development (see Chapter 38), also participate a photoreceptor, and that the timekeeping role is played by a in the photoperiodic timing mechanism. biological clock that is linked to the phytochrome (which sets What might that mechanism consist of? It was once hy- the clock) and also to the production of flowers. pothesized that the timing mechanism might simply be the slow conversion of a phytochrome during the night from the Pfr form—produced during the light hours—to the Pr form. Circadian rhythms are maintained by a biological clock Such phytochrome conversion would function as an “hour- It is clear that organisms have some way of measuring time, glass,” and the effect of a night would depend simply upon and that they are well adapted to the 24-hour day–night cy- whether all the phytochrome had been converted. However, cle of our planet. A biological clock resides within the cells of
  11. 11. REPRODUCTION IN FLOWERING PLANTS 759 all eukaryotes and some prokaryotes. The major outward placed in light on a day–night cycle totaling exactly 24 hours, manifestations of this clock are known as circadian rhythms it would express a rhythm with a period of exactly 24 hours. (from the Latin circa, “about,” and dies, “day”). However, if an experimenter used a day–night cycle of, say, We can characterize circadian rhythms, as well as other 22 hours, then over time the rhythm would change—it regular biological cycles, in two ways: The period is the would be entrained to a 22-hour period. length of one cycle, and the amplitude is the magnitude of If an organism is maintained under constant darkness, it the change over the course of a cycle (Figure 39.13). will express a circadian rhythm with an approximately 24- The circadian rhythms of cyanobacteria, protists, animals, hour period. However, a brief exposure to light under these fungi, and plants have been found to share some important circumstances can cause a phase shift—that is, it can make the characteristics: next peak of activity appear either later or earlier than ex- pected, depending on when the exposure is given. Moreover, The period is remarkably insensitive to temperature, al- the organism does not then return to its old schedule if it re- though lowering the temperature may drastically reduce mains in darkness. If the first peak is delayed by 6 hours, the the amplitude of the rhythmic effect. subsequent peaks are all 6 hours late. Such phase shifts are per- Circadian rhythms are highly persistent; they may contin- manent—until the organism receives more exposures to light. ue for days even in an environment in which there are no environmental cues, such as light–dark periods. Circadian rhythms can be entrained, within limits, by Photoreceptors set the biological clock light–dark cycles that differ from 24 hours. That is, the Phytochromes and blue-light receptors are known to affect period an organism expresses can be made to coincide the period of the biological clock, with the different pigments with that of the light–dark cycle to which it is exposed. reporting on different wavelengths and intensities of light. A brief exposure to light can shift the peak of the cycle— This diversity of photoreceptors could be an adaptation to it can cause a phase shift. the changes in the light environment that a plant experiences Plants provide innumerable examples of circadian rhy- in the course of a day or a season. How do these photore- thms. The leaflets of plants such as clover normally hang ceptors interact with a plant’s biological clock? down and fold at night and rise and unfold during the day. The biological clock of Arabidopsis is based on the activi- The flowers of many plants show similar “sleep movements,” ties of at least three “clock genes.” The clock genes encode closing at night and opening during the day. They continue regulatory proteins that interact to produce a circadian oscil- to open and close on an approximately 24-hour cycle even lation. How does this oscillating clock interact with photore- when the light and dark periods are experimentally modified. ceptors and the environment? The period of circadian rhythms in nature is approxi- Arabidopsis is an LDP. Its clock controls the activity of mately 24 hours. If a clover plant, for example, were to be CONSTANS (a gene that is not part of the clock mechanism) in such a way that the CONSTANS product, CO protein, ac- cumulates in one phase of the clock’s cycle—the phase in which night falls. Under long nights (short days), CO protein is found at night. Under short nights (long days), CO is also Circadian rhythms are characterized on the basis of time, measured in periods of about 24 hours… relatively abundant at dawn and dusk. When CO protein levels are high, light absorbed by phytochrome A and the Period (about 24 hours) blue-light receptor cryptochrome 2 leads to flowering (Fig- ure 39.14). Thus, Arabidopsis flowering results from the coin- cidence of light (detected by the two photoreceptors) with a Effect clock-determined phase of the circadian oscillation. Amplitude Where is this coincidence-based photoperiodic mechanism located in relation to where flowering occurs? Is the timing device for flowering located in a particular plant part, or are all parts able to sense the length of the night? This question Time was resolved by “blindfold” experiments, as described next. …and on the basis of the magnitude of the rhythmic effect, measured by the cycle‘s amplitude. Is there a flowering hormone? 39.13 Features of Circadian Rhythms Circadian rhythms, like all biological rhythms, can be characterized in two ways: by period and It quickly became apparent that each leaf is capable of tim- by amplitude. ing the night. If a cocklebur plant—an SDP—is kept under a
  12. 12. 760 CHAPTER THIRT Y-NINE Under short days, CO protein level Under long days, CO protein levels are high remains low throughout the light period, enough at both dawn and dusk so that light and the plant does not flower. absorption by pigments leads to flowering. 39.14 Photoreceptors and the Biological Clock Interact in Light Dark Light Dark Photoperiodic Plants One of Relative concentration of CO Relative concentration of CO the genes regulated by the circa- 1.0 1.0 dian clock in Arabidopsis encodes the CO protein. Flowering depends on enough CO being present when photoreceptors have light available to them. 0.5 0.5 No flowering Flowering 40 40 0 4 8 12 16 18 24 0 4 8 12 16 18 24 Time (hours) Time (hours) plant does not flower. If, however, the induced leaf re- regime of short nights and long days, but a leaf is covered so mains attached to the plant for several hours, the plant as to give it the needed long nights, the plant will flower (ex- flowers. This result suggests that something must be syn- periment A in Figure 39.15). This type of experiment works thesized in the leaf in response to the inductive dark peri- best if only one leaf is left on the plant. If one leaf is given a od and then move out of the leaf to induce flowering. photoperiodic treatment conducive to flowering—called an If two cocklebur plants are grafted together, and if one inductive treatment—other leaves kept under noninductive plant is exposed to inductive long nights and its graft conditions will tend to inhibit flowering. partner exposed to noninductive short nights, both Although it is the leaves that sense an inductive photope- plants flower (experiment B in Figure 39.15). riod, the flowers form elsewhere on the plant. Thus, some In at least one species, if an induced leaf from one plant kind of signal must be sent from the leaf to the site of flower is grafted onto another, noninduced plant, the host plant formation. Three lines of evidence suggest that this signal is flowers. a chemical substance—a flowering hormone. If a photoperiodically induced leaf is immediately re- moved from a plant after the inductive dark period, the 39.15 Evidence for a Flowering Hormone If even a single leaf is exposed to inductive conditions, a signal travels to the entire plant (and even to other plants, in grafting experiments), inducing it to flower. EXPERIMENT A EXPERIMENT B Question: What part of the plant measures the dark period? Question: How stable is the flowering hormone? Cocklebur, a short-day If even one leaf is masked for 1 Five cocklebur plants are grafted together plant, will not flower if part of the day—thus shifting and kept under long days and short nights, kept under long days that leaf to short days and long with most leaves removed. and short nights. nights—the plant will flower; note the burrs. 3 If a leaf on a plant at one end of the chain is subjected to long nights, all of the plants will flower. Graft Burrs 2 A leaf is induced by long nights/ short days. Masked leaf Masked leaf Hypothetical flowering hormone Conclusion: The leaves measure the dark period. Therefore, some signal must move from the induced leaf to the flowering Conclusion: The very stable flowering signal can even travel across multiple parts of the plant. grafts.
  13. 13. REPRODUCTION IN FLOWERING PLANTS 761 Jan A. D. Zeevaart, a plant physiologist at Michigan State The implications of this finding were of great agricultural University, performed this last experiment. He exposed a sin- interest in Russia because winter wheat is a better producer gle leaf of the SDP Perilla to a short-day/long-night regime, than spring wheat, but it cannot be grown in some parts of inducing the plant to flower. Then he detached this leaf and Russia because the winters are too cold for its survival. Sev- grafted it onto another, noninduced, Perilla plant—which re- eral studies performed in Russia during the early 1900s sponded by flowering. The same leaf grafted onto successive demonstrated that if seeds of winter wheat were premoist- hosts caused each of them to flower in turn. As long as 3 ened and prechilled, they could be sown in the spring and months after the leaf was exposed to the short-day/long- would develop and flower normally the same year. Thus, night regime, it could still cause plants to flower. high-yielding winter wheat could be grown even in previ- Experiments such as Zeevaart’s led to the conclusion that ously hostile regions. the photoperiodic induction of a leaf causes a more or less This induction of flowering by low temperatures is called permanent change in the leaf, causing it to start and continue vernalization. Vernalization may require as many as 50 days producing a flowering hormone that is transported to other of low temperatures (in the range from about –2° to +12°C). parts of the plant, where the hormone initiates the develop- Some plant species require both vernalization and long days ment of reproductive structures. Biologists have named this to flower. There is a long wait from the cold days of winter hypothetical hormone florigen, even though, after decades to the long days of summer, but because the vernalized state of active searching, it has not been isolated or characterized. easily lasts at least 200 days, these plants do flower when An elegant experiment suggested that the florigen of SDPs they experience the appropriate night length. is identical to that of LDPs, even though SDPs produce it only under long nights and LDPs only under short nights. An SDP and an LDP were grafted together, and both flow- Asexual Reproduction ered, as long as the photoperiodic conditions were inductive Although sexual reproduction takes up most of the space in for one of the partners. Either the SDP or the LDP could be this chapter, asexual reproduction is responsible for many of the one induced, but both would always flower. These results the new plant individuals appearing on Earth. This fact sug- suggest that a flowering hormone—the elusive florigen—was gests that in some circumstances, asexual reproduction must being transferred from one plant to the other. be advantageous. The direct demonstration of florigen activity remains a At the beginning of this chapter, we saw that one of the cherished goal of plant physiologists. For a long time it was advantages of sexual reproduction is genetic recombination. thought that florigen could be neither a protein nor an RNA Self-fertilization is a form of sexual reproduction, but when because those molecules were too large to pass from one liv- a plant self-fertilizes, there are fewer opportunities for genetic ing plant cell to another. However, we now know that such recombination than there are with cross-fertilization. A macromolecules can be transferred by way of plasmodes- diploid, self-fertilizing plant that is heterozygous for a cer- mata, and biologists are reexamining the possibility that an tain locus can produce both kinds of homozygotes for that RNA or a protein is the long-sought florigen. locus plus the heterozygote among its progeny, but it cannot We have considered the photoperiodic regulation of flow- produce any progeny that carry alleles that it does not itself ering, from photoreceptors in a leaf to the biological clock to possess. Yet many plants continue to be self-compatible, un- the need for a signal that travels from the induced leaf to the dergo self-fertilization, and produce viable offspring. sites of flower formation. However, light is not the only en- Asexual reproduction goes even further than self-fertiliza- vironmental variable that affects flowering. In some plants, tion: It eliminates genetic recombination altogether. When a low temperatures are an essential cue that eventually triggers plant reproduces asexually, it produces a clone of progeny that flowering. are genetically identical to the parent. If a plant is well adapted to its environment, asexual reproduction may spread its geno- type throughout that environment. This ability to exploit a par- Vernalization and Flowering ticular environment is an advantage of asexual reproduction. Certain cereal grains serve as classic examples of the control of flowering by temperature. In both wheat and rye, we dis- tinguish two categories of flowering behavior. Spring wheat, There are many forms of asexual reproduction for example, is sown in the spring and flowers in the same We call stems, leaves, and roots vegetative organs to distin- year. It is an annual plant. Winter wheat is biennial and must guish them from flowers, the reproductive parts of the plant. be sown in the fall; it flowers in the following summer. If The modification of a vegetative organ is what makes vege- winter wheat is not exposed to cold after its first year, it will tative reproduction—asexual reproduction in plants—pos- not flower normally the next year. sible. In many cases, the stem is the organ that is modified.
  14. 14. 762 CHAPTER THIRT Y-NINE Strawberries and some grasses, for example, produce hori- with stolons or rhizomes, such as beach grasses, rushes, and zontal stems, called stolons or runners, that grow along the sand verbena, are common pioneers on coastal sand dunes. soil surface, form roots at intervals, and establish potentially Rapid vegetative reproduction enables these plants, once in- independent plants (see Figure 35.4b). Tip layers are upright troduced, not only to multiply but also to survive burial by branches whose tips sag to the ground and develop roots, as the shifting sand; in addition, the dunes are stabilized by the in blackberry and forsythia. extensive network of rhizomes or stolons that develops. Veg- Some plants, such as potatoes, form enlarged fleshy tips etative reproduction is also common in some deserts, where of underground stems, called tubers (see Figure 35.4a). Rhi- the environment is not often suitable for seed germination zomes are horizontal underground stems that can give rise to and the establishment of seedlings. new shoots. Bamboo is a striking example of a plant that re- Dandelions, citrus trees, and some other plants reproduce produces vegetatively by means of rhizomes. A single bam- by the asexual production of seeds, called apomixis. As we boo plant can give rise to a stand—even a forest—of plants have seen, meiosis reduces the number of chromosomes in constituting a single, physically connected entity. gametes, and fertilization restores the sporophytic number Whereas stolons and rhizomes are horizontal stems, bulbs of chromosomes in the zygote. Some plants can skip over and corms are short, vertical, underground stems. Lilies and both meiosis and fertilization and still produce seeds. onions form bulbs (Figure 39.16a), short stems with many Apomixis produces seeds within the ovary without the min- fleshy, highly modified leaves that store nutrients. These stor- gling and segregation of chromosomes and without the age leaves make up most of the bulb. Bulbs are thus large un- union of gametes. The ovule simply develops into a seed, derground buds. They can give rise to new plants by divid- and the ovary wall develops into a fruit. An apomictic em- ing or by producing new bulbs from lateral buds. Crocuses, bryo has the sporophytic number (2n) of chromosomes. The gladioli, and many other plants produce corms, underground result of apomixis is a fruit with seeds that are genetically stems that function very much as bulbs do. Corms are identical to the parent plant. disclike and consist primarily of stem tissue; they lack the Apomixis sometimes requires pollination. In some fleshy modified leaves that are characteristic of bulbs. apomictic species, a sperm nucleus must combine with the Not all vegetative organs modified for reproduction are polar nuclei in order for the endosperm to form. In other stems. Leaves may also be the source of new plantlets, as in the apomictic species, the pollen provides the signals for embryo succulent plants of the genus Kalanchoe (Figure 39.16b). Many and endosperm formation, although neither sperm nucleus kinds of angiosperms, ranging from grasses to trees such as as- participates in fertilization. This observation emphasizes that pens and poplars, form interconnected, genetically homoge- pollination and fertilization are not the same thing. neous populations by means of suckers—shoots produced by roots. What appears to be a whole stand of aspen trees, for ex- ample, may be a clone derived from a single tree by suckers. Asexual reproduction is important in agriculture Plants that reproduce vegetatively often grow in physi- Farmers and gardeners take advantage of some natural cally unstable environments, such as eroding hillsides. Plants forms of vegetative reproduction. They have also developed (a) (b) Storage leaves grow from the stem of this onion. The plantlets forming on the margin of this Kalanchoe leaf will fall to the ground and start independent lives. The short stem is visible at the bottom of the bulb. 39.16 Vegetative Organs Modified for Reproduction (a) Bulbs are short stems with large buds that store nutrients and can give rise to new Allium sp. plants. (b) In Kalanchoe, new plantlets can form on leaves.
  15. 15. REPRODUCTION IN FLOWERING PLANTS 763 Recombinant DNA techniques applied to tissue cultures can provide plants with increased resistance to pests or in- Scion creased nutritive value to humans. There is also interest in making certain valuable, sexually reproducing plants capable In grafting, the scion is aligned so that its vascular cambium is adjacent to the of apomixis. By causing cells of different types to fuse, one can vascular cambium in the stock. obtain plants with exciting new combinations of properties. Stock Chapter Summary Many Ways to Reproduce Almost all flowering plants reproduce sexually, and many 39.17 Grafting Grafting—attaching a piece of a plant to the root or also reproduce asexually. Both sexual and asexual reproduction root-bearing stem of another plant—is a common horticultural tech- are important in agriculture. nique.The “host” root or stem is the stock; the upper grafted piece is the scion. Sexual Reproduction in Plants Sexual reproduction promotes genetic diversity in a popula- tion, which may give the population an advantage under chang- new types of asexual reproduction by manipulating plants. ing environmental conditions or in exploiting new territory. One of the oldest methods of vegetative reproduction used The flower is an angiosperm’s device for sexual reproduction. in agriculture consists of simply making cuttings of stems, Flowering plants have microscopic gametophytes that devel- inserting them in soil, and waiting for them to form roots and op within the flowers of the sporophytes. The megagameto- phyte is the embryo sac, which typically contains eight nuclei in thus become autonomous plants. The cuttings are usually en- a total of seven cells. The microgametophyte is the pollen grain, couraged to root by treatment with a plant hormone, auxin, which usually contains two cells. Review Figure 39.1. See as described in Chapter 38. Web/CD Tutorial 39.1 Horticulturists reproduce many woody plants by graft- Pollination enables fertilization in the absence of external water. ing—attaching a bud or a piece of stem from one plant to the In self-incompatible species, the stigma or style rejects pollen from the same plant. Review Figure 39.3 root or root-bearing stem of another plant. The part of the re- The pollen grain delivers sperm cells to the embryo sac by sulting plant that comes from the root-bearing “host” is means of a pollen tube. called the stock; the part grafted on is the scion (Figure 39.17). Most angiosperms perform double fertilization: One sperm In order for a graft to succeed, the vascular cambium of the nucleus fertilizes the egg, forming a zygote, and the other scion must become associated with that of the stock. By cell sperm nucleus unites with the two polar nuclei to form a triploid endosperm. Review Figure 39.5 division, both cambia form masses of wound tissue. If the two The zygote develops into an embryo (with an attached sus- masses meet and fuse, the resulting continuous cambium can pensor), which remains quiescent in the seed until conditions produce xylem and phloem, allowing transport of water and are right for germination. The endosperm supplies the nutritive minerals to the scion and of photosynthate to the stock. Grafts reserve upon which the embryo depends at germination. Review Figures 39.6, 39.7. See Web/CD Activity 39.1 are most often successful when the stock and scion belong to Flowers develop into seed-bearing fruits, which often play the same or closely related species. Most fruit grown for mar- important roles in the dispersal of the species. ket in the United States is produced on grafted trees. Scientists in universities and industrial laboratories have The Transition to the Flowering State For a vegetatively growing plant to flower, an apical meri- been developing new ways to produce useful plants via tis- stem in the shoot system must become an inflorescence meri- sue culture. Because many plant cells are totipotent (see Fig- stem, which gives rise to bracts as well as more meristems. The ure 19.3), cultures of undifferentiated tissue can give rise to meristems it produces may become floral meristems or addi- tional inflorescence meristems. Review Figure 39.9 entire plants, as can small pieces of tissue cut directly from a Flowering results from a cascade of gene expression. Floral parent plant. Tissue cultures are used commercially to pro- organ identity genes are expressed in floral meristems that give duce numerous new plants rapidly without resorting to seeds. rise to sepals, petals, stamens, and carpels. Culturing tiny bits of apical meristem can produce plants Photoperiodic Control of Flowering free of viruses. Because apical meristems lack developed vas- Photoperiodic plants regulate their flowering by measuring cular tissues, viruses tend not to enter them. Treatment with the length of light and dark periods. hormones causes a single apical meristem to give rise to 20 Short-day plants flower when the days are shorter than a or more shoots; thus, a single plant can give rise to millions species-specific critical day length; long-day plants flower when of genetically identical plants within a year by repeated the days are longer than a critical day length. Review Figure 39.10 Some angiosperms have more complex photoperiodic meristem culturing. Using this approach, strawberry and po- requirements than short-day or long-day plants have, but most tato producers are able to start each year’s crop from virus- are day-neutral. free plants.