1 Flowering Processes In Plants Plant growth originates within the buds in regions known as meristems. In the meristems, cell division and elongation occur, and these processes produce tissues that soon develop into specific plant parts. Vegetative meristems give rise to parts such as stems, leaves, and roots, while reproductive meristems give rise to floral organs that ultimately produce fruits and seeds. Within every meristem are minute primordia that resemble knobby outgrowths or ribbed inverted cones. Although hardly distinguishable to the naked eye, the configurations of the primordia become visible when the bud scales are removed and examined under magnification. As growth proceeds, the configurations enlarge and differentiate into recognizable plant organs. FLORAL INDUCTION The ability to support reproductive processes requires tremendous energy. Often, many crops do not begin to form flowers, and eventually seeds, until after substantial vegetative growth has occurred. In some cases, as with most annuals, this is at the end of the life cycle. In other cases, the plant may not become reproductive until after several growing seasons as with many fruit trees. During this phase, in which the plant is unable to form flowers because it does not possess sufficient vegetative structure, it is said to be in ajuvenile state. However, at some point, enough vegetative growth occurs and plants reach sexual maturity and are able to flower. After that stage, certain external (or internal) stimuli can trigger floral induction, a physiological change that permits the development of reproductive primordia. This change may precede actual flowering by several days, weeks, or even months. Temperature Stimuli For floral induction to occur, many plants require exposure to low temperatures. This process has been called vernalization. In its narrowest sense, vernalization means the promotion L. O. Copeland et al., Principles of Seed Science and Technology © Kluwer Academic Publishers 2001 2 Flowering Processes in Plants of flowering in some winter cereals by cold treatment ofthe moistened or germinating seeds. In a broader sense, vernalization means the induction of flowering in any winter annual, biennial, or even perennial species through exposure to low temperatures. For example, rye (Secale cereale), a winter annual, and perennial ryegrass (Lolium perenne) both must undergo prolonged exposure to low temperatures before they can produce flowers. Sugar beets and carrots are examples of biennial species that grow vegetatively the fIrst year, after which they are vernalized by exposure to winter temperatures. The optimum temperature for vernalization is between loe and 7°e (Figure 1.1). These temperatures must be experienced by the vegetative meristems for periods of between 10 and 100 days before a reproductive meristem is initiated when the crop is returned to warm temperatures. In chrysanthemum and tomato, floral induction is accomplished by repeated exposure to low night temperatures, separated by periods of higher temperature. This phenomenon occurs in many plants and has been called thermoperiodism. Day-Length Stimuli In many species, floral induction occurs in response to day length, or photoperiod. Thus, plant species have been categorized according to their day-length requirements as short-day, long-day, intermediate-day, or day-neutral; however, it is really the length ofthe night, or dark period, that is the critical factor that influences flowering. Table 1.1 provides examples of crops which require photoperiod and vernalization to induce flowering. The photoperiod requirements for flowering may be qualitative or quantitative. Some shortday plants such as the Biloxi variety of soybean and cocklebur are unable to flower except under short-day treatments; in other short-day species, such as sunflower, flowering is hastened by the appropriate short-day conditions, although it can eventually occur without them. 1.0 5 .~ .~ 0.8 ";i c: \" CII > 0.6 CII > .~ 1'0 "ii 0.4 ex: 0.2 -10 -5 o 5 10 15 20 Temperature during vernalization (OC) Figure 1.1. Vernalization response offlowering in winter cereals (based on data for "Petkus" rye (from Salisbury 1963). Flowering processes in plants 3 Table 1.1. Photoperiodic and Vernalization Responses of Some Agricultural Species. Obligate photoperiodic response Short-Day Plants Day-Neutral Plants soybean rice dry bean soybean cotton potato rice sunflower tobacco maize coffee Facultative photoperiodic response soybean cotton sugarcane rice potato sunflower Positive vernalization requirement onion Long-Day Plants oat annual ryegrass canary grass red clover timothy grass spinach radish cabbage spring barley spring wheat spring rye potato sunflower red clover onion carrot broadbean winter oat winter barley perennial ryegrass winterwheat sugarbeet Since the original discovery of photoperiod control of flowering by Garner and Allard in 1920 and the discovery of temperature or thermal induction by a Russian scientist, Lysenko (1932), there has been a widespread search for the existence of a universal flowering hormone, jlorigen, in plants. However, it now appears that flowering is controlled not by one, but by several different hormone-like substances. Phytochrome. With plant responses other than flowering-for example, seed germination, bud dormancy, stem elongation, and petiole development-research has shown almost identical responses to light in different plant parts, suggesting that plant reactions are controlled by the same light-receptive substance. In 1959, this substance was fmally isolated, identified, and named phytochrome. Two photoreversible forms of phytochrome exist in plants. P R phytochrome is receptive to red light [600-680 nanometers (nrn)] and inhibits flowering while P FR phytochrome is receptive to far-red light (700-760 nrn) and induces flowering. The conversion from P FR phytochrome to P R phytochrome takes place in the dark, but at a much slower rate than that induced by far-red light. This is the basis for the "day-length," or photoperiodic light response, as well as the response to light quality (color, or wavelength) in the control of flowering. By successive exposures to red and far-red light, flowering of light-sensitive plants can be repeatedly induced or inhibited. Chemical Stimuli Certain natural and synthetic chemical substances can cause floral induction. Some are auxinlike compounds-for example, indoleacetic acid, naphthaleneacetic acid, or the common herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D). At certain concentrations, gibberellic acid Flowering Processes in Plants 4 may also cause floral induction. It promotes flowering of long-day plants held under short-day conditions; however, it inhibits flowering of short-day plants under the same conditions. It has been demonstrated that the gibberellin content increased markedly during floral induction of Hyoscyamus niger; this is consistent with the effects of gibberellic acid in promoting floral induction. Other substances known to cause flowering or to increase flower production include cytokinins, ethylene, acetylene, ethylene chlorohydrin, and 2,3,5-triiodobenzoic acid. In contrast, maleic hydrazide inhibits flowering. With our growing knowledge about plant flowering responses and increasing capability for producing synthetic hormones, it is often convenient and commercially feasible to manipulate flowering and fruit development in the commercial production of certain crops. Nutritional Status In floral induction, the nutritional status of a plant is also important, since construction of the flowering parts is dependent on food availability and translocation. The carbon-nitrogen ratio is particularly influential; in species, such as holly, that bear male and female flowers on separate plants, a high nitrogen-to-carbon ratio favors pistillate rather than staminate flowers. In tomatoes, carbohydrate deficiencies cause microspore degeneration, leading to pollen sterility; however, a nitrogen deficiency has no such effect. FLORAL INITIATION Following floral induction, which may be triggered by external stimuli,floral initiation is the morphological expression of the induced state and usually occurs more or less deeply within the meristems of a plant. In monocotyledonous species, or flowering plants in which a single embryonic seed leaf appears at germination, floral initiation begins in specialized meristems called dermatogens, which also give rise to the epidermis. In dicotyledonous species, or flowering plants in which a pair of embryonic seed leaves appear at germination, floral initiation occurs in the lateral, terminal, or axillary buds. Early in their development, reproductive meristems are similar to vegetative meristems, appearing as knobby or ribbed configurations on an inverted cone or pedestal. As development procedes, these configurations develop into recognizable flower parts. The structure, development, and closure of the carpels to form the ovary can be traced in Figures 1.2 and 1.3. FLORAL MORPHOLOGY The typical flower of an angiosperm, or plant whose seeds are enclosed in an ovary, is composed of petals, sepals, stamens, and a pistil. The petals, often the most conspicuous, collectively are called the corolla. Sepals, usually (but not always) less conspicuous, are known collectively as the calyx. The stamens are the male pollen-bearing organs, and each consists of an anther and filament. The pistil, sometimes called the gynoecium, is the female part of the flower and consists of the stigma, which receives the pollen, the style, and the ovary. The ovary may be composed of one or more carpels, which may be considered as highly modified leaflike structures. When only one carpel forms the ovary, it is termed simple and usually contains only one locule, or cavity. A compound ovary is made up of two or more united carpels and may 5 Flowering processes in plants ovary wall (pericarp) A B Figure 1.2. Arrangement offruit into locules: (A) afruit arrangement with three locules, (B) other arrangements. A B c Figure 1.3. (A) a simple carpel with one locule, or cavity, (8) a compound carpel with one locule, (C) a compound carpel with two locules. Flowering Processes in Plants 6 contain one or more locules, depending on their arrangement (Figure 1.3). The outermost wall ofthe ovary is called the pericarp. The manner in which the seeds are attached to the placenta within the ovary locules is called placentation. Placentation occurs in one of three basic forms (Figure 1.4). Parietal placentation occurs when the seeds are attached to the ovary wall, usually to both sides ofthe seam where the carpels fuse to form the ovary. Axile placentation occurs in flowers with ovaries divided by partitions, called septa, in which the placental attachment arises along the central axis of the ovary. When no septa are present in the ovary and the seeds are attached along the central axis, the placentation is termed free central; modifications of this occur in the case of basal or apical placentation. Flowers having pistils, stamens, petals, and sepals are termed complete. Incomplete flowers lack any of these four parts. Flowers containing both stamens (male) and pistils (female) are termed perfect; unisexual flowers, which are either pistillate or staminate, are called imperfect. Species such as corn, that have both male and female flowers on the same plant, are known as monoecious; those that have unisexual flowers on different plants such as holly are dioecious. MEGASPOROGENESIS The seeds of angiosperms originate from meristematic tissue of the ovary wall called ovule primordia. In species with simple ovaries, these primordia are usually located near the suture of the ovary wall where the carpel is fused. In species with more than one carpel, or with polycarpellate ovaries, the seeds form at the fusion of the carpels or along the septa, or central carpel axes, depending on the type of placentation (Figure 1.4). In some fruits (e.g., tomato), a well-developed placenta arises from which many ovule primordia develop. Within the nucellus, or specialized tissue ofthe carpel, one cell, known as the archesporial cell, develops special characteristics that distinguish it from adjacent cells. As this cell increases in size, its nucleus becomes larger and its cytoplasm grows more dense in preparation for cell division. The first division results in a megaspore mother cell and a parietal cell. Usually the parietal cell remains undivided and soon deteriorates; however, in some species, it undergoes further division and contributes to seed formation. The megaspore mother cell is diploid (2N), having the same number of chromosomes as the parent plant. However, it soon undergoes a two-step cell division known as meiosis (Figure 1.5). Parietal Figure 1.4. Types o/placentation. Axile Free Central (basal) Flowering processes in plants 7 This process gives rise to four megaspores, each having one-half the chromosome complement of the mother plant; these are thus haploid (l N) cells. Normally, only one megaspore is functional, while the other three degenerate. MEGAGAMETOGENESIS The development ofthe female gametophyte, or embryo sac, from the functional megaspore is known as megagametogenesis, which is a process of successive nuclear divisions within an enlarging cell that becomes the embryo sac. Three successive free nuclear divisions (mitosis) occur (Figure 1.6), culminating in eight haploid (IN) nuclei. Soon these nuclei arrange themselves within the enlarging embryo sac and cell walls form, resulting in three antipodal cells at one end, two polar nuclei (without cell walls) near the center, and the egg apparatus (composed of the egg between two synergid cells) at the other end. After the two polar nuclei fuse to form a diploid (2N) nucleus, the resulting seven-celled structure is known as the mature female gametophyte (embryo sac), or megagametophyte, which is ready to receive the mature male gametophyte. This describes the normal embryo sac development as it occurs in most species. Variations to this pattern occur in certain species, especially in the polar nuclei and antipodal development. With few exceptions, the egg apparatus development is as described. stigma archesporial cell (2N) ovule primordia / -style Oval}' wall (pericarp) ovary B A ovule primordia (:J 8·.·~· ---",~:.;....-tetrad of CD megaspores (I N) MEIOSIS I MEIOSIS II 8.· c o Figure 1.5. Megasporogenesis: (A) location of ovule development, (B) cutaway section ofthe lower region of the ovary wall (pericarp), showing origin ofthe archesporial cell; note that it is larger than surrounding cells, having a larger nucleus and denser cytoplasm, (C) cell division during megasporogenesis, (D) cutaway section of lower part of the ovary, showing location of the four megaspores, three of which normally degenerate. Flowering Processes in Plants 8 functional megaspore .' '.: "':: ,: .. , .' .. filA VsifJJ ' .. '.'. ~ . 2 3 A polar--+..a.....:....,... nuclei initial arrangement B Figure 1.6. Megagametogenesis: (A) three normal mitotic nuclear divisions leading to one large cell enclosing eight nuclei. Later, cell walls enclose the nuclei and the entire structure becomes the female gametophyte, or embryo sac. (B) mature female gametophyte. The egg cell comprises most of the egg apparatus. It is a complete cell containing a haploid (IN) nucleus with surrounding cytoplasm enclosed in a thin wall, or fellicle. The egg cell is positioned near the small opening (micropyle) of the ovule formed by the surrounding integuments. A small vacuole may be present near the point of attachment away from the micropyle. THE DEVELOPING OVULE Ovule development (Figure 1.7) occurs within the ovary, which provides a location for the nurture and development of the female gametophyte, its sexual fusion with the male gametophyte, and embryo development, survival, and eventual regrowth. Ovule growth begins as a small outgrowth within the nucellus. As megasporogenesis and megagametogenesis continue, the region of the nucellus that is to become the ovule enlarges and differentiates into definite morphological characteristics. Secondary outgrowths, or collars (integuments), soon appear around the periphery of the nucellar outgrowths and envelop it. These usually consist of the inner and outer integuments and ultimately become the testa (seed coat) of the mature ovule. The developing ovule is commonly attached to the placenta by thefuniculus. The scar on the ovule where the funiculus detaches at maturity is known as the hilum. The point where the integuments meet at the nucellar apex is the micropyle, and the region of integumentary origin and attachment, usually opposite the micropyle, is the chalaza. Between the chalaza and the hilum of many species is an area known as the raphe. The raphe may be visible on the seed coat of some species. The Nucellus The nucellus provides tissue for the origin and nurture of the female gametophyte, from the archesporia I cell to the mature megagametophyte. It originates from ovary tissue and provides the site of archesporial cell origin. Subsequently, part of it becomes trapped within the 9 Flowering processes in plants integument ovarian locule I nucellus with a fUniculus tetrad of megaspores A ovarian tocule outer integument funiculus inner integument' megagametophyte, B or embryo sac petal (corolla) anther } stamen ftlament ovule ~"-:--+- '/ placenta C stigma} style pistil ovary petal (corolla) egg D Figure 1. 7. Ovule development and its location in the flower: (A) longitudinal section through the ovary showing the developing ovule, (B) a later stage, (C) a still later stage showing the mature female gametophyte, (D) a generalized diagram of a complete flower showing the location of the ovule. integuments as the ovule continues to develop. Normally, no further growth occurs, and the nuceUus is at least partially consumed, since it supplies nutritive support to the developing embryo sac. However, in some species it undergoes considerable development and contributes substantially to the storage tissue as the perisperm. Examples of species with well-developed perisperm are sugar beets (Beta vulgaris) and leafY spurge (Euphorbia esula). Integuments The nature and thickness of the integuments vary considerably among species, depending on their role in contributing to embryo sac and ovule development. In Apiaceae, the inner integument is completely absorbed and only two or three cellular layers of the outer integument persist. In Asteraceae, most cells of both integuments are absorbed, leaving only a thin layer of crushed integumentary tissue on the inner side of the pericarp. Practically no integumentary 10 Flowering Processes in Plants tissue remains in the fully developed corn caryopsis, and in Symplocarpus, both integuments and endosperm are completely consumed by the developing embryo, leaving it naked inside the pericarp. Integumentary Outgrowths. Two types of integumentary outgrowths may occur in certain species, giving rise to special structures not found in most sOOds. A third integument, or aril, may either arise from the base of the nucellus or split off from the outer integument. Elymus, for example, has a well-developed aril. Another type of integumentary outgrowth, a caruncle, arises as a proliferation of the outer integument in the region of the micropyle. Seeds of Euphorbia esula have a well-developed but fragile caruncle that extends back over the seed and appears to have no function. Still another type of appendage arises from the seed coat over the area of the raphe in some species (e.g., Stylophorum and Trillium) and is known as the strophiole. Integumentary Tapetum. In some species, the cells of the inner integument serve as nutritional support for the developing embryo sac and later harden and act as a protective layer for the ovule. In Lobelia, the cells of the inner integument take on a pronounced radial elongation and become binucleate before becoming hardened as the integumentary tapetum. Micropyle The micropyle is an integumentary pore or opening in the ovule through which the pollen tube grows to fertilize the egg cell of the female gametophyte. The micropyle may assume one of several configurations, depending on the closure of the inner and outer integuments (Figure 1.8). Epistase Epistase is the development of well-defmed nucellar or integumentary tissue in the micropylar region of the seed of certain species. In Castalia (water lily) and Costus (spiral flag) species, epidermal cells of the nucellus proliferate and form a plug beneath the micropyle, which remains after the rest of the nucellar tissue is gone. Cells adjacent to the micropyle may show a marked radial elongation. Another type of epistase, an operculum, develops when cells of the integument proliferate and form a tightly compacted micropylar plug, as in species of Lemna and Acorus. Figure 1.8. Types of micropyle arrangements showing different closure of the inner and outer integuments. 11 Flowering processes in plants Mature ovules are classified into five different types based on their arrangement within the ovary (F igure 1.9). The difference in arrangement begins at the time of archesporial development and becomes well defmed by the time of embryo sac maturity. Ovule types have been determined for most well-known plant species and serve as a means of plant classification. The effect of the ovule arrangement is often visible externally. For example, the relative position ofthe hilum (funicular detachment scar), chalaza, and micropyle of many legumes can be easily seen. MICROSPOROGENESIS AND MICROGAMETOGENESIS The period of flower development when the stigma is ready to receive the pollen is known as anthesis. Pollen is usually produced in four sacs, or microsporangia (Figure 1.10), of the anther, although occasionally fewer sporangia may occur. Within the sporangia, certain cells become the microspore mother cells and undergo a two-step reduction division (meiosis), or microsporogenesis, to yield four microspores, each of which is haploid (1 N). Each ofthe four microspores is normally functional and undergoes two divisions, known as microgametogenesis, giving rise to a microgametophyte, or mature pollen grain. FRUIT DEVELOPMENT To understand seeds and seed formation, one must have a basic knowledge of fruit development and morphology. The botanical definition of fruit is much broader than that conveyed by popular usage of the term. Actually, a fruit is a mature or ripened ovary that usually contains one or more ovules that develop into true seeds. Legume pods, peppers, and cereal grains are fruits, as are apples, oranges, and peaches. The pericarp, or ovary wall of angiosperm fruits, is composed of three different layers which are more or less distinct in various species: the exocarp, or outer layer; the mesocarp, or middle layer; and the endocarp or inner layer. The relative development of each in various species contributes to the overall fruit structure and morphology. A B c D E Figure 1.9. Types of ovules as seen in vertical longitUdinal section: (A) atropous (or orthotropous)-nucellar apex points away from the funiculus as in Polygonaceae, (B) anatropous- ovule completely inverted so that nucellar apex is turned toward the funiculus as in Sympetalae, (C) campylotropous-ovule is slightly curved, with the nucellar apex andfunicular end pointed slightly downward as in Fabaceae, (D) hemianatropous-ovule is straight with axis lying perpendicular to the funiculus, as in Ranunculaceae, (£) amphitropous-ovule has a pronounced curve, with the nucellar apex near the funiculus, as in Botomaceae (From P. Maheshwari 1950). Flowering Processes in Plants 12 micros pore mother ~1--~.eIl CROSS SECTION, YOUNG ANTHER pollen grain microsporangium mlCrospores tube cell GERMINA TION OF POLLEN Figure 1.10. The anther and the pollen grain. Each microspore mother cell within a microsporangium divides to form a tetrad of microspores that soon separate. The nucleus of each microspore then divides, and a tube cell and generative cell are formed within the wall of the microspore, which subsequently develops into a pol/en grain. Following pollination, the pollen grain germinates, producing a pollen tube, and the generative cell gives rise to two male gametes (From Wilson and Loomis 1952). FRUIT TYPES Pseudocarpic fruit consists of one or more ripened ovaries attached or fused to modified bracts or other non floral structures. Examples: burdock, sandbur. Multiple fruit is composed of the ovaries of more than one flower. Each unit of these fruits may be berries, drupes, or nutlets. Examples: fig, mulberry, pineapple. Aggregatefruit is composed of several ovaries ofa single flower. Each unit of these fruits may be a berry, drupe, or nutlet. Examples: strawberry, raspberry, blackberry. Simple fruit is derived from a single pistal. A. Fleshy fruits have a fleshy or leathery pericarp. 1. Berry has a fleshy pericarp. Examples: grape, tomato, gooseberry, huckleberry. 2. Pepo has a hard rind but no internal separations, or septa. Examples: watermelon, cantaloupe, squash, cucumber. 3. Pome has a floral cup that forms a thick outer fleshy layer and a papery inner pericarp (endocarp) forming a multiseeded core. Examples: apple, pear, quince. Flowering processes in plants 13 4. Drupe is also called stone fruit, and has a stony endocarp, a thick, leathery, or fleshy mesocarp, and a thin exocarp. The pit is usually one~seeded, but occasionally several one~seeded pits are present. Examples: cherry, coconut, walnut, peach, plum, olive. 5. Hesperidia are berrylike fruits with papery internal separations, or septa, and a leathery, separable rind. Examples: orange, lemon, lime, grapefruit. B. Dry fruit has a thin pericarp that is dry at maturity. 1. Dehiscent fruits split open at maturity and releases mature seed. a. Legume has a simple (single) pistil that splits open at maturity along two sutures. Examples: bean, pea, soybean, black locust. b. Follicle has a simple (single) pistil that splits open at maturity along one suture. Examples: milkweed, larkspur, spirea. c. Capsule has a compound pistil that splits open at maturity in one of four ways: Loculicidal-splitting open through the midrib of the carpel into the locules. Examples: iris, tulip. Circumscissle-splitting open at the middle so that the top comes off like a lid (also called pyris). Examples: plantain, portulaca. Septicidal-splitting along the septa. Examples: yucca, azalea. Poricidal-splitting open at pores near the top, releasing mature seeds. Example: poppy. d. Silique and Silicle are characteristic of the mustard family, with two valves which at maturity split away from a persistent central partition. A fruit that is several times longer than wide is termed silique, while a silicle is broad and short. 2. lndehiscent fruits do not open at maturity to release the seeds. a. Achene is a small one-seeded fruit in which the seed is attached to the pericarp at only one point and may be rather loose inside the pericarp. Examples: dandelion, buttercup, sunflower, dock. b. Utricle is similar to an achene except that it has an inflated papery pericarp. Example: Russian thistle. c. Caryopsis is similar to an achene except that the entire seed coat is tightly fused with the pericarp. Example: grasses. d. Samara is similar to an achene except that the pericarp develops a thin, flat, winglike appendage. This is a characteristic of some woody species. Examples: ash, elm, tree of heaven. Double samaras occur in the fruit of maple. e. Nut is a dry on~seeded fruit from a compound pistil that has a very hard and tough pericarp that is usually wholly or partially enclosed in an involucre. Examples: acorn, hazel, filbert, chestnut. f. Nutlet is a small, dry fruit composed of on~half a carpel, enclosing single seed. It is developed by folding and splitting of the carpels into a compound pistil. Examples: members ofLamiaceae (mint family) and Boraginaceae (forget-me-not family). g. Schizocarp has two fused carpels separating at maturity to form one-seeded mericarps. Example: members of Apiaceae (carrot family). Flowering Processes in Plants 14 FLORAL TAXONOMY The arrangement of the floral axis determines the type of inflorescence (structure of a flower), and is a stable species characteristic. The main stalk of the inflorescence is the peduncle. Lateral stalks supporting the individual flowers are called pedicels. Inflorescences may be determinate or indeterminate. Determinate inflorescences are those in which the axis terminates as a flower. Indeterminate inflorescences terminate in a vegetative bud, which continues to grow and produce flowers throughout the growing season, resulting in flowers of different maturity within the same inflorescences (see Figure 1.11). Determinate Flowers Solitary flower-The simplest expression of a determinate inflorescence. Example: com cockle. Simple cymt>-The simplest branched determinate inflorescence where the lateral flowers develop later than the terminal flower. Example: chickweed. Compound cymt>-A determinate inflorescence where there is secondary branching and each lateral unit becomes a simple cyme. Example: bouncing bet. Scorpioid cymt>-A determinate inflorescence in which the lateral buds on one side are suppressed during growth, resulting in a curved or coiled arrangement. Example: Heliotropium curassavicum. Glomeruit>-A very compact compound cyme. Example: saxifrage. Indeterminate Flowers Racemt>-The basic type of inflorescence in which pedicels arise laterally on a long central peduncle. Examples: pennycress and field or garden bean. Paniclt>-An inflorescence in which the lateral branches arising from the peduncle produce flower-bearing branches instead of single flowers. Example: oats. Spikt>-An inflorescence in which the flowers arising along the peduncle are essentially sessile, or stalkless, and are attached to the peduncle. Example: wheat. Catkin-A modified type of spike with a single unisexual flower arising from the peduncle. Example: red alder Spadix-A special kind of spike covered by a spathe. Example skunk cabbage. Corymb-An inflorescence in which the lower pedicels arising from the peduncle are successively longer than the upper ones, giving a round or flat-topped appearance. Example: bitter cherry. Umbel-An inflorescence similar to a corymb except that the lateral branches arising from the peduncle originate from the same location. Example (simple umbel): pennywart. A compound umbel is similar except that each pedicel is branched, bearing multibranched individual flowers. Example: wild carrot. Head-An inflorescence where the peduncle and the pedicels are tightly clustered, surrounded by a group offlowerlike bracts called involucre. Example: sunflower. 15 Flowering processes in plants Detenninate Inflorescences Simple Cyme Compound Cyme Scorpioid Cyme Indeterminate Inflorescences Panicle Catkin Raceme Corymb Spike Simple Umbel Compound Umbel Head Spadix Figure 1.11. Types of determinate and indeterminate iriflorescences (From Dennis 1967). Flowering Processes in Plants 16 Questions 1. 2. 3. 4. 5. 6. 7. What is the difference between floral induction and floral initiation? Do you think that phytochrome and florigen are the same? What is the difference between a complete flower and a perfect flower? Can a dioecious plant have complete flowers? What is the relationship between a peduncle and a pedicel? What is the difference between a caryopsis and an achene? What is the difference between a schizocarp and a mericarp? General References Boke, N. H. 1947. Development of the adult shoot apex and floral initiation in Vinca rosea. American Journal ofBotany 34:433-439. _ _ . 1948. Development of the perianth in Vinca rosea L. American Journal of Botany. 35:413-423. _ _ . 1949. Development of the stamens and carpels in Vinca rosea L. American Journal of Botany 36:535-547. Bonnett, O. T. 1935. The development of the barley spike. Journal ofAgricultural Research 51:451-457. _ _. 1936. The development ofthe wheat spike. Journal ofAgricultural Research 53:445451. _ _ . 1937. The development of the oat spike. Journal ofAgricultural Research 54 :92 7-931. Dennis, L. 1. 1967. Manual of Introductory Taxonomy and Field Botany. Corvallis, OR.: Oregon State University Bookstores. Gamer, W. W. and H. A. Allard. 1920. Effect of the relative length of day and night and other factors of the environment on growth and reproduction in plants. Journal of Agricultural Research 18:553-606. Lysenko, T. D., 1932. Fundamental results of research on vernalization of agricultural plants. Bull. Jarovizacci, No.4, 1-57. Quoted by Maximow, 1934. Maheshwari, P. 1950. An Introduction to the Embryology of the Angiosperms. New York: McGraw-Hill Book Company. Maximow, N. A. 1934. The theoretical significance of vernalization. Imperial Bulletin of Genenetics. Aberystwyth (Wales) Herbage Publication Series Bulletin 16. Salisbury, F. B. 1961. Photoperiodism and the flowering process. Annual Review of Plant Physiology 2:293-326. _ _. 1963. The Flowering Process. New York: Pergamon Press. Searle, N. E. 1965. Physiology of flowering. Annual Review of Plant Physiology 16:97-118. Siegelman, W. and W. L. Butler. 1965. Properties of phytochrome. Annual Review of Plant Physiology 16:383-392. Stratford, G. A. 1965. Plant hormones II: Florigen and gibberellins. Essentials of Plant Physiology. London: Heineman Educational Books, Ltd.
