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32: Plant Reproductive Development and Structure - Biology

32: Plant Reproductive Development and Structure - Biology


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32: Plant Reproductive Development and Structure

A typical flower has four main parts—or whorls—known as the calyx, corolla, androecium, and gynoecium (Figure). The outermost whorl of the flower has green, leafy structures known as sepals. The sepals, collectively called the calyx, help to protect the unopened bud. The second whorl is comprised of petals—usually, brightly colored—collectively called the corolla. The number of sepals and petals varies depending on whether the plant is a monocot or dicot. In monocots, petals usually number three or multiples of three in dicots, the number of petals is four or five, or multiples of four and five. Together, the calyx and corolla are known as the perianth . The third whorl contains the male reproductive structures and is known as the androecium. The androecium has stamens with anthers that contain the microsporangia. The innermost group of structures in the flower is the gynoecium , or the female reproductive component(s). The carpel is the individual unit of the gynoecium and has a stigma, style, and ovary. A flower may have one or multiple carpels.

Art Connection

The four main parts of the flower are the calyx, corolla, androecium, and gynoecium. The androecium is the sum of all the male reproductive organs, and the gynoecium is the sum of the female reproductive organs. (credit: modification of work by Mariana Ruiz Villareal)

If the anther is missing, what type of reproductive structure will the flower be unable to produce? What term is used to describe an incomplete flower lacking the androecium? What term describes an incomplete flower lacking a gynoecium?

If all four whorls (the calyx, corolla, androecium, and gynoecium) are present, the flower is described as complete. If any of the four parts is missing, the flower is known as incomplete. Flowers that contain both an androecium and a gynoecium are called perfect, androgynous or hermaphrodites. There are two types of incomplete flowers: staminate flowers contain only an androecium, and carpellate flowers have only a gynoecium (Figure).

The corn plant has both staminate (male) and carpellate (female) flowers. Staminate flowers, which are clustered in the tassel at the tip of the stem, produce pollen grains. Carpellate flowers are clustered in the immature ears. Each strand of silk is a stigma. The corn kernels are seeds that develop on the ear after fertilization. Also shown is the lower stem and root.

If both male and female flowers are borne on the same plant, the species is called monoecious (meaning “one home”): examples are corn and pea. Species with male and female flowers borne on separate plants are termed dioecious, or “two homes,” examples of which are C. papaya and Cannabis. The ovary, which may contain one or multiple ovules, may be placed above other flower parts, which is referred to as superior or, it may be placed below the other flower parts, referred to as inferior (Figure).

The (a) lily is a superior flower, which has the ovary above the other flower parts. (b) Fuchsia is an inferior flower, which has the ovary beneath other flower parts. (credit a photo: modification of work by Benjamin Zwittnig credit b photo: modification of work by "Koshy Koshy"/Flickr)


Sexual Reproduction in Gymnosperms

Gymnosperms produce both male and female gametophytes on separate cones and rely on wind for pollination.

Learning Objectives

Describe the process of sexual reproduction in gymnosperms

Key Takeaways

Key Points

  • In gymnosperms, a leafy green sporophyte generates cones containing male and female gametophytes female cones are bigger than male cones and are located higher up in the tree.
  • A male cone contains microsporophylls where male gametophytes ( pollen ) are produced and are later carried by wind to female gametophytes.
  • The megaspore mother cell in the female cone divides by meiosis to produce four haploid megaspores one of the megaspores divides to form the female gametophyte.
  • The male gametophyte lands on the female cone, forming a pollen tube through which the generative cell travels to meet the female gametophyte.
  • One of the two sperm cells released by the generative cell fuses with the egg, forming a diploid zygote that divides to form the embryo.
  • Unlike angiosperms, ovaries are absent in gymnosperms, double fertilization does not take place, male and female gametophytes are present on cones rather than flowers, and wind (not animals) drives pollination.

Key Terms

  • megasporophyll: bears megasporangium, which produces megaspores that divide into the female gametophyte
  • microsporophyll: a leaflike organ that bears microsporangium, which produces microspores that divide into the male gametophyte (pollen)

Sexual Reproduction in Gymnosperms

As with angiosperms, the life cycle of gymnosperms is also characterized by alternation of generations. In conifers such as pines, the green leafy part of the plant is the sporophyte the cones contain the male and female gametophytes. The female cones are larger than the male cones and are positioned towards the top of the tree the small, male cones are located in the lower region of the tree. Because the pollen is shed and blown by the wind, this arrangement makes it difficult for a gymnosperm to self-pollinate.

Conifer life cycle: This image shows the life cycle of a conifer. Pollen from male cones blows up into upper branches, where it fertilizes female cones. Examples are shown for female and male cones.

Male Gametophyte

A male cone has a central axis on which bracts, a type of modified leaf, are attached. The bracts, known as microsporophylls, are the sites where microspores will develop. The microspores develop inside the microsporangium. Within the microsporangium, cells known as microsporocytes divide by meiosis to produce four haploid microspores. Further mitosis of the microspore produces two nuclei: the generative nucleus and the tube nucleus. Upon maturity, the male gametophyte (pollen) is released from the male cones and is carried by the wind to land on female cones.

Male and female gametophytes: These series of micrographs shows male and female gymnosperm gametophytes. (a) This male cone, shown in cross section, has approximately 20 microsporophylls, each of which produces hundreds of male gametophytes (pollen grains). (b) Pollen grains are visible in this single microsporophyll. (c) This micrograph shows an individual pollen grain. (d) This cross section of a female cone shows portions of about 15 megasporophylls. (e) The ovule can be seen in this single megasporophyll. (f) Within this single ovule are the megaspore mother cell (MMC), micropyle, and a pollen grain.

Female Gametophyte

The female cone also has a central axis on which bracts known as megasporophylls are present. In the female cone, megaspore mother cells are present in the megasporangium. The megaspore mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form the multicellular female gametophyte, while the others divide to form the rest of the structure. The female gametophyte is contained within a structure called the archegonium.

Reproductive Process

Upon landing on the female cone, the tube cell of the pollen forms the pollen tube, through which the generative cell migrates towards the female gametophyte through the micropyle. It takes approximately one year for the pollen tube to grow and migrate towards the female gametophyte. The male gametophyte containing the generative cell splits into two sperm nuclei, one of which fuses with the egg, while the other degenerates. After fertilization of the egg, the diploid zygote is formed, which divides by mitosis to form the embryo. The scales of the cones are closed during development of the seed. The seed is covered by a seed coat, which is derived from the female sporophyte. Seed development takes another one to two years. Once the seed is ready to be dispersed, the bracts of the female cones open to allow the dispersal of seed no fruit formation takes place because gymnosperm seeds have no covering.

Angiosperms Versus Gymnosperms

Gymnosperm reproduction differs from that of angiosperms in several ways. In angiosperms, the female gametophyte in the ovule exists in an enclosed structure, the ovary in gymnosperms, the female gametophyte is present on exposed bracts of the female cone and is not enclosed in an ovary. Double fertilization is a key event in the life cycle of angiosperms, but is completely absent in gymnosperms. The male and female gametophyte structures are present on separate male and female cones in gymnosperms, whereas in angiosperms, they are a part of the flower. Finally, wind plays an important role in pollination in gymnosperms because pollen is blown by the wind to land on the female cones. Although many angiosperms are also wind-pollinated, animal pollination is more common.


32: Plant Reproductive Development and Structure - Biology

Plants have evolved different reproductive strategies for the continuation of their species. Some plants reproduce sexually, and others asexually, in contrast to animal species, which rely almost exclusively on sexual reproduction. Plant sexual reproduction usually depends on pollinating agents, while asexual reproduction is independent of these agents. Flowers are often the showiest or most strongly scented part of plants. With their bright colors, fragrances, and interesting shapes and sizes, flowers attract insects, birds, and animals to serve their pollination needs. Other plants pollinate via wind or water still others self-pollinate.

Figure 1. Plants that reproduce sexually often achieve fertilization with the help of pollinators such as (a) bees, (b) birds, and (c) butterflies. (credit a: modification of work by John Severns credit b: modification of work by Charles J. Sharp credit c: modification of work by “Galawebdesign”/Flickr)


Angiosperms versus Gymnosperms

Figure 10. (a) Angiosperms are flowering plants, and include grasses, herbs, shrubs and most deciduous trees, while (b) gymnosperms are conifers. Both produce seeds but have different reproductive strategies. (credit a: modification of work by Wendy Cutler credit b: modification of work by Lews Castle UHI)

Gymnosperm reproduction differs from that of angiosperms in several ways (Figure 10). In angiosperms, the female gametophyte exists in an enclosed structure—the ovule—which is within the ovary in gymnosperms, the female gametophyte is present on exposed bracts of the female cone. Double fertilization is a key event in the lifecycle of angiosperms, but is completely absent in gymnosperms. The male and female gametophyte structures are present on separate male and female cones in gymnosperms, whereas in angiosperms, they are a part of the flower. Lastly, wind plays an important role in pollination in gymnosperms because pollen is blown by the wind to land on the female cones. Although many angiosperms are also wind-pollinated, animal pollination is more common.

Link to Learning

Watch this video to see an animation of the double fertilization process of angiosperms.


Male Gametophyte

A male cone has a central axis on which bracts, a type of modified leaf, are attached. The bracts are known as microsporophylls (Figure) and are the sites where microspores will develop. The microspores develop inside the microsporangium. Within the microsporangium, cells known as microsporocytes divide by meiosis to produce four haploid microspores. Further mitosis of the microspore produces two nuclei: the generative nucleus, and the tube nucleus. Upon maturity, the male gametophyte (pollen) is released from the male cones and is carried by the wind to land on the female cone.

Link to Learning

Watch this video to see a cedar releasing its pollen in the wind.


Reproductive Development and Structure

The alternation of generations in angiosperms is depicted in this diagram. (credit: modification of work by Peter Coxhead)

The life cycle of higher plants is dominated by the sporophyte stage, with the gametophyte borne on the sporophyte. In ferns, the gametophyte is free-living and very distinct in structure from the diploid sporophyte. In bryophytes, such as mosses, the haploid gametophyte is more developed than the sporophyte.

During the vegetative phase of growth, plants increase in size and produce a shoot system and a root system. As they enter the reproductive phase, some of the branches start to bear flowers. Many flowers are borne singly, whereas some are borne in clusters. The flower is borne on a stalk known as a receptacle. Flower shape, color, and size are unique to each species, and are often used by taxonomists to classify plants.


Introduction

Plants have evolved different reproductive strategies for the continuation of their species. Some plants reproduce sexually, and others asexually, in contrast to animal species, which rely almost exclusively on sexual reproduction. Plant sexual reproduction usually depends on pollinating agents, while asexual reproduction is independent of these agents. Flowers are often the showiest or most strongly scented part of plants. With their bright colors, fragrances, and interesting shapes and sizes, flowers attract insects, birds, and animals to serve their pollination needs. Other plants pollinate via wind or water still others self-pollinate.

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    Pollination by Insects

    Bees are perhaps the most important pollinator of many native species, garden plants and most commercial fruit trees (Figure). Because bees cannot see the color red, bee-pollinated flowers usually have shades of blue, yellow, or other colors. Bees mutualistically collect energy-rich pollen or nectar for their survival and energy needs. Bee-pollinated flowers are open during the day, brightly colored, strongly scented, and tubular in shape. The pollen sticks to the bees’ fuzzy hair as it harvests nectar or pollen, and when the bee visits another flower, some of the pollen is transferred to the second flower. Note that the honeycomb picture in the rotating banner images on this website represents a diversity of bee pollen in the different cell colors in the comb (HHMI Biointerative Image of the Week).

    Insects, such as bees, are important agents of pollination. (credit: modification of work by Jon Sullivan)

    Recently, there have been many reports about the declining population of honeybees. Many flowers will remain unpollinated and not bear seed if honeybees disappear. The impact on commercial fruit growers could be devastating.

    Flies are an important pollinator in natural systems. Flies are attracted to flowers that have a decaying smell or an odor of rotting flesh, like the corpse flower or voodoo lily (Amorphophallus), dragon arum (Dracunculus), and carrion flower (Stapleia, Rafflesia). These flowers, which produce nectar, usually have dull colors, such as brown or purple. The nectar provides energy, whereas the pollen provides protein. Wasps are also important insect pollinators, and pollinate many species of figs.

    Butterflies, such as the monarch, pollinate many garden flowers and wildflowers, which usually occur in clusters. These flowers are brightly colored, have a strong fragrance, are open during the day, and have nectar guides to make access to nectar easier. The pollen is picked up and carried on the butterfly’s limbs. Moths, on the other hand, pollinate flowers during the late afternoon and night. The flowers pollinated by moths are pale or white and are flat, enabling the moths to land. One well-studied example of a moth-pollinated plant is the yucca plant, which is pollinated by the yucca moth. The shape of the flower and moth have adapted in such a way as to allow successful pollination. The moth deposits pollen on the sticky stigma for fertilization to occur later. The female moth also deposits eggs into the ovary. As the eggs develop into larvae, they obtain food from the flower and developing seeds. Thus, both the insect and flower benefit from each other in this symbiotic relationship. The corn earworm moth and Gaura plant have a similar relationship (Figure).

    A corn earworm sips nectar from a night-blooming Gaura plant. (credit: Juan Lopez, USDA ARS)


    Nephrolepis: Habitat, External Features and Reproduction

    Nephrolepis (Nephros, kidney lepis, the indusium kidney-shaped and scale like) is represented by about 30 species. These species are distributed in the tropics of the entire world.

    About five species [N.cordifolia (Fish bornfern, Narrow sword fern), N. exaltata (sword fern), N. volubilis, N. acuta and N. ramosa] are found in India. Majority of the species are terrestrial but a few species e.g., N. ramosa are found climbing on the trees. N. cordifolia is found throughout the Indian region upto 5000 feet elevation.

    External Features of Nephrolepis:

    The mature plant body is sporophytic and can be differentiated into rhizome, roots and leaves.

    It is short, slender, suberect (TV. cordifolia), erect (N. exaltata), or wide creeping (N. volubilis, goes upto 50 feet over trees). It bears a close tuft of leaves and long, slender lateral branches called runners. The runners spread for a considerable distance and bear roots (Fig. 1).

    Branched adventitious roots arise from the rhizome and runners in aeropetal succession.

    Leaves are tufted, long, narrow and simply pinnate. Steple (a leaf stalk) is 2.5 to 10 cm long in N. cordifolia and upto 20 cm long in N. acuta. Fronds (leaves Fig. 2 A, B) are 30 cm (N. ramosa) to 240 cm long (N. acuta), cm long (N. acuta) Pinnae are numerous, crowded, often imbricated, 3 cm long (N. cordifolia) to 20 cm long (N. acuta), slightly falcate and articulated at the base.

    Margins are entire or slightly crenate. The veins are free and there are present white line dots over the vein tips. Sori are present on the lower surface. Sori are half the way between the midrib and margin in a single row [(N. cordifolia), (Fig. 3 A, B)] or submarginal (N. exaltata) or near the margin (N. acuta).

    The indusia are usually round-reniform with a narrow sinus. Sometimes the sinus widens to a broad curved base. Young leaves are covered with multicellular hairs and scales and show circinate vernation.

    Internal Structure of Nephrolepis:

    1. Transverse Section (T.S.) of Rhozome:

    Internal structure of rhizome can be differentiated into three parts: epidermis, cortex and stele. Epidermis is outermost layer. It is made up single layer of narrower cells. Epidermis is followed by cortex. Stele is primitive type of dictyostele. Two curved strands facing each other are present in the centre. Both strands are separated from each other and are surrounded by sclerenchyma. Each strand has its own pericycle and endodermis (Fig. 4).

    2. Transverse Section of Runner:

    Internally it is also differentiated into epidermis, cortex and stele (Fig. 5). The outermost layer is epidermis. It has stomata at young stages.

    Epidermis is followed by a few layers of closely packed cells (intercellular spaces are absent). The remaining layers are made up of large, round or oval cells with abundant intercellular spaces. Cortex is followed by endodermis. Just inside the endodermis lies the pericycle.

    Stele consists Xylem cylinder in the form of fluted column, composed of tracheids and parenchyma. The protoxylem consists of 7-9 distinct exarch strands forming the ridges of the xylem cylinder. Phloem goes round the xylem. It is many cells thick in the bays between protoxylem ridges and also forms a wavy mantle over the protoxylem ridges.

    3. Transverse Section of Petiole:

    The petiole has an adaxial groove. The internal structure of petiole can be differentiated into three parts: epidermis, cortex and stele (Fig. 6). Epidermis is outermost protective layer. It is composed of small thick walled cells.

    Epidermis is followed by 3-4 layers of sclerenchymatous cortex and compact parenchyma. Usually 3-6 conducting strands are embedded in the parenchymatous cortex and are arranged in horse-shoe like shape.

    The xylem portion of each strand is crecentric. Protoxylem lies towards outside. Phloem completely surrounds xylem which in turn is surrounded by a layer of pericycle and endodermis.

    4. Transverse Section of Pinnule or Lamina:

    It is similar to Pteridium. The transverse section of the sporophyll passing through sori reveals that shield shaped indusium is attached to the receptacle. Club shaped stalked sporangia are attached, to the receptacleand are protected by indusium.

    5. Transverse Section of Root:

    It is circular is outline and can be differentiated into epidermis, cortex and stele. Epidermis is single layered. Cortex is wide, many layered and parenchymatous. The stele is usually diarch and exarch. Xylem is surrounded by pericycle and endodermis each.

    Reproduction in Nephrolepis:

    Nephrolepis reproduces by two methods:

    (i) Vegetative reproduction.

    (i) Vegetative reproduction:

    It takes place by the formation of buds. Runner produces buds at some distance from parent rhizome. These buds give rise to fronds and thus help in vegetative reproduction of the plants.

    (ii) Sexual Reproduction:

    Structure and Development of Sporangium:

    Structure and development of sporangium is similar to Pteridium. Mature spores are brown in colour, with thin exosporium, thick endosporium with small warts.

    Structure and Development of Prothallus:

    Development of prothallus is similar to Pteridium. A mature prothallus is formed in about fifty days. It is large, green and heart shaped (Fig. 7) and 0.3 cm x 0.5 cm in diameter. It has a large cushion on the ventral side. It is 4 – 6 cells thick in the region of cushion and one celled elsewhere.

    It has a deep notch and 4-7 meristematic cells lie in it. Prothallus is monoecious. At maturity antheridia are first to appear anteroposteriorly on the posterior side of the cushion. Development and structure of antheridium is similar to Pteridium.

    A mature antheridium is large and spherical. At maturity it consists of 32 spermatocytes. Structure and development of the archegonium is similar to Pteridium. Many archegonia are formed and lie in the anterior part of the cushion at maturity. Fertilization and development of sporophyte is also similar to Pteridium.

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