Angiosperms

Angiosperm Flowers

Flowers are modified leaves containing the reproductive organs of angiospems; their pollination is usually accomplished by animals or wind.

Learning Objectives

Describe the main parts of a flower and their purposes

Key Takeaways

Key Points

  • Sepals, petals, carpels, and stamens are structures found in all flowers.
  • To attract pollinators, petals usually exhibit vibrant colors; however, plants that depend on wind pollination contain flowers that are small and light.
  • Carpels protect the female gametophytes and megaspores.
  • The stigma is the structure where pollen is deposited and is connected to the ovary through the style.
  • The anther, which comprises the stamen, is the site of microspore production and their development into pollen.

Key Terms

  • sepal: a part of an angiosperm, and one of the component parts of the calyx; collectively the sepals are called the calyx (plural calyces), the outermost whorl of parts that form a flower
  • corolla: an outermost-but-one whorl of a flower, composed of petals, when it is not the same in appearance as the outermost whorl (the calyx); it usually comprises the petal, which may be fused
  • stamen: in flowering plants, the structure in a flower that produces pollen, typically consisting of an anther and a filament
  • carpel: one of the individual female reproductive organs in a flower composed of an ovary, a style, and a stigma; also known as the gynoecium

Flowers

Flowers are modified leaves, or sporophylls, organized around a central stalk. Although they vary greatly in appearance, all flowers contain the same structures: sepals, petals, carpels, and stamens. The peduncle attaches the flower to the plant. A whorl of sepals (collectively called the calyx) is located at the base of the peduncle and encloses the unopened floral bud. Sepals are usually photosynthetic organs, although there are some exceptions. For example, the corolla in lilies and tulips consists of three sepals and three petals that look virtually identical. Petals, collectively the corolla, are located inside the whorl of sepals and often display vivid colors to attract pollinators. Flowers pollinated by wind are usually small, feathery, and visually inconspicuous. Sepals and petals together form the perianth. The sexual organs (carpels and stamens) are located at the center of the flower.

Styles, stigmas, and ovules constitute the female organ: the gynoecium or carpel. Flower structure is very diverse. Carpels may be singular, multiple, or fused. Multiple fused carpels comprise a pistil. The megaspores and the female gametophytes are produced and protected by the thick tissues of the carpel. A long, thin structure called a style leads from the sticky stigma, where pollen is deposited, to the ovary, enclosed in the carpel. The ovary houses one or more ovules, each of which will develop into a seed upon fertilization. The male reproductive organs, the stamens (collectively called the androecium), surround the central carpel. Stamens are composed of a thin stalk called a filament and a sac-like structure called the anther. The filament supports the anther, where the microspores are produced by meiosis and develop into pollen grains.

image

Structure of flowers: This image depicts the structure of a flower. Perfect flowers produce both male and female floral organs. The flower shown has only one carpel, but some flowers have a cluster of carpels. Together, all the carpels make up the gynoecium.

Angsiosperm Fruit

A fertilized, fully grown, and ripened ovary containing a seed forms what we know as fruit, important seed dispersal agents for plants.

Learning Objectives

Recall the evolutionary advantage of fruits

Key Takeaways

Key Points

  • Scientists classify fruit in many different categories that include descriptions, such as mature, fleshy, and dry; only a few are actually classified as being fleshy and sweet.
  • Some fruit are developed from ovaries, while others develop from the pericarp, from clusters of flowers, or from separate ovaries in a single flower.
  • Fruit are vital dispersal agents for plants; their unique shapes and features evolved to take advantage of specific dispersal modes.
  • Dispersal methods of seeds within fruit include wind, water, herbivores, and animal fur.

Key Terms

  • fruit: the seed-bearing part of a plant, often edible, colorful, and fragrant, produced from a floral ovary after fertilization
  • pericarp: the outermost layer, or skin, of a ripe fruit or ovary
  • hypanthium: the bowl-shaped part of a flower on which the sepals, petals, and stamens are borne

Fruit

In botany, a fertilized, fully-grown, and ripened ovary is a fruit. As the seed develops, the walls of the ovary in which it forms thicken and form the fruit, enlarging as the seeds grow. Many foods commonly-called vegetables are actually fruit. Eggplants, zucchini, string beans, and bell peppers are all technically fruit because they contain seeds and are derived from the thick ovary tissue. Acorns are nuts and winged, maple whirligigs (whose botanical name is samara) are also fruit. Botanists classify fruit into more than two dozen different categories, only a few of which are actually fleshy and sweet.

Mature fruit can be fleshy or dry. Fleshy fruit include the familiar berries, peaches, apples, grapes, and tomatoes. Rice, wheat, and nuts are examples of dry fruit. Another distinction is that not all fruits are derived from the ovary. For instance, strawberries are derived from the receptacle, while apples are derived from the pericarp, or hypanthium. Some fruits are derived from separate ovaries in a single flower, such as the raspberry. Other fruits, such as the pineapple, form from clusters of flowers. Additionally, some fruits, like watermelon and oranges, have rinds.

Regardless of how they are formed, fruits are an agent of seed dispersal. The variety of shapes and characteristics reflect the mode of dispersal, whether it be wind, water, or animals. Wind carries the light dry fruit of trees and dandelions. Water transports floating coconuts. Some fruits attract herbivores with color or perfume, or as food. Once eaten, tough, undigested seeds are dispersed through the herbivore’s feces. Other fruits have burs and hooks to cling to fur and hitch rides on animals.

image

Wind dispersal: The winged shape of Alsomitra macrocarpa’s seeds allow them to use wind for dispersal. They can, therefore, glide for great distances.

image

Fruit dispersal: A fruit’s distinctive shape and specialized characteristics will determine its dispersal mechanism.

The Life Cycle of an Angiosperm

Angiosperms are seed-producing plants that generate male and female gametophytes, which allow them to carry out double fertilization.

Learning Objectives

Explain the life cycle of an angiosperm, including cross-pollination and the ways in which it takes place

Key Takeaways

Key Points

  • Microspores develop into pollen grains, which are the male gametophytes, while megaspores form an ovule that contains the female gametophytes.
  • In the ovule, the megasporocyte undergoes meiosis, generating four megaspores; three small and one large; only the large megaspore survives and produces the female gametophyte (embryo sac).
  • When the pollen grain reaches the stigma, it extends its pollen tube to enter the ovule and deposits two sperm cells in the embryo sac.
  • The two available sperm cells allow for double fertilization to occur, which results in a diploid zygote (the future embryo) and a triploid cell (the future endosperm), which acts as a food store.
  • Some species are hermaphroditic (stamens and pistils are contained on a single flower), some species are monoecious (stamens and pistils occur on separate flowers, but the same plant), and some are dioecious (staminate and pistillate flowers occur on separate plants).

Key Terms

  • cotyledon: the leaf of the embryo of a seed-bearing plant; after germination it becomes the first leaves of the seedling
  • heterosporous: producing both male and female gametophytes
  • synergid: either of two nucleated cells at the top of the embryo sac that aid in the production of the embryo; helper cells

The Life Cycle of an Angiosperm

The adult, or sporophyte, phase is the main phase of an angiosperm’s life cycle. As with gymnosperms, angiosperms are heterosporous. Therefore, they generate microspores, which will produce pollen grains as the male gametophytes, and megaspores, which will form an ovule that contains female gametophytes. Inside the anthers’ microsporangia, male gametophytes divide by meiosis to generate haploid microspores, which, in turn, undergo mitosis and give rise to pollen grains. Each pollen grain contains two cells: one generative cell that will divide into two sperm and a second cell that will become the pollen tube cell.

image

Life cycle of angiosperms: The life cycle of an angiosperm is shown. Anthers and carpels are structures that shelter the actual gametophytes: the pollen grain and embryo sac. Double fertilization is a process unique to angiosperms.

The ovule, sheltered within the ovary of the carpel, contains the megasporangium protected by two layers of integuments and the ovary wall. Within each megasporangium, a megasporocyte undergoes meiosis, generating four megaspores: three small and one large. Only the large megaspore survives; it produces the female gametophyte referred to as the embryo sac. The megaspore divides three times to form an eight-cell stage. Four of these cells migrate to each pole of the embryo sac; two come to the equator and will eventually fuse to form a 2n polar nucleus. The three cells away from the egg form antipodals while the two cells closest to the egg become the synergids.

The mature embryo sac contains one egg cell, two synergids (“helper” cells), three antipodal cells, and two polar nuclei in a central cell. When a pollen grain reaches the stigma, a pollen tube extends from the grain, grows down the style, and enters through the micropyle, an opening in the integuments of the ovule. The two sperm cells are deposited in the embryo sac.

A double fertilization event then occurs. One sperm and the egg combine, forming a diploid zygote, the future embryo. The other sperm fuses with the 2n polar nuclei, forming a triploid cell that will develop into the endosperm, which is tissue that serves as a food reserve. The zygote develops into an embryo with a radicle, or small root, and one ( monocot ) or two (dicot) leaf-like organs called cotyledons. This difference in the number of embryonic leaves is the basis for the two major groups of angiosperms: the monocots and the eudicots. Seed food reserves are stored outside the embryo in the form of complex carbohydrates, lipids, or proteins. The cotyledons serve as conduits to transmit the broken-down food reserves from their storage site inside the seed to the developing embryo. The seed consists of a toughened layer of integuments forming the coat, the endosperm with food reserves, and the well-protected embryo at the center.

image

The fruit of the Aesculus or Horse Chestnut tree: These seeds are enclosed a protective outer covering called the seed coat, usually with some stored food. After fertilization and some growth in the angiosperm, the ripened ovule is produced. The formation of the seed completes the process of reproduction in seed plants (started with the development of flowers and pollination), with the embryo developed from the zygote and the seed coat from the integuments of the ovule.

Some species of angiosperms are hermaphroditic (stamens and pistils are contained on a single flower), some species are monoecious (stamens and pistils occur on separate flowers, but the same plant), and some are dioecious (staminate and pistillate flowers occur on separate plants). Both anatomical and environmental barriers promote cross-pollination mediated by a physical agent (wind or water) or an animal, such as an insect or bird. Cross-pollination increases genetic diversity in a species.

Diversity of Angiosperms

Angiosperm diversity is divided into two main groups, monocot and dicots, based primarily on the number of cotyledons they possess.

Learning Objectives

Explain how angiosperm diversity is classified

Key Takeaways

Key Points

  • Angiosperm are flowering plants that are classified based on characteristics that include (but are not limited to) cotyledon structure, pollen grains, as well as flower and vascular tissue arrangement.
  • Basal angiosperms, classified separately, contain features found in both monocots and dicots, as they are believed to have originated before the separation of these two main groups.
  • Monocots contain a single cotyledon and have veins that run parallel to the length of their leaves; their flowers are arranged in three to six-fold symmetry.
  • Dicots have flowers arranged in whorls, two cotyledons, and a vein arrangement that forms networks within their leaves.
  • Monocots do not contain any true woody tissue while dicots can be herbacious or woody and have vascular tissue that forms a ring in the stem.

Key Terms

  • dicot: a plant whose seedlings have two cotyledons; a dicotyledon
  • angiosperm: a plant whose ovules are enclosed in an ovary
  • monocot: one of two major groups of flowering plants (or angiosperms) that are traditionally recognized; seedlings typically have one cotyledon (seed-leaf)
  • cotyledon: the leaf of the embryo of a seed-bearing plant; after germination it becomes the first leaves of the seedling
  • basal angiosperm: the first flowering plants to diverge from the ancestral angiosperm, including a single species of shrub from New Caledonia, water lilies and some other aquatic plants, and woody aromatic plants

Diversity of Angiosperms

Angiosperms are classified in a single phylum: the Anthophyta. Modern angiosperms appear to be a monophyletic group, which means that they originated from a single ancestor. Flowering plants are divided into two major groups according to the structure of the cotyledons and pollen grains, among others. Monocots include grasses and lilies while eudicots or dicots form a polyphyletic group. However, many species exhibit characteristics that belong to either group; as such, the classification of a plant as a monocot or a eudicot is not always clearly evident. Basal angiosperms are a group of plants that are believed to have branched off before the separation into monocots and eudicots because they exhibit traits from both groups. They are categorized separately in many classification schemes. The Magnoliidae (magnolia trees, laurels, and water lilies) and the Piperaceae (peppers) belong to the basal angiosperm group.

image

Examples of basal angiosperms: The (a) common spicebush belongs to the Laurales, the same family as cinnamon and bay laurel. The fruit of (b) the Piper nigrum plant is black pepper, the main product that was traded along spice routes. Notice the small, unobtrusive, clustered flowers. (c) Lotus flowers, Nelumbo nucifera, have been cultivated since ancient times for their ornamental value; the root of the lotus flower is eaten as a vegetable. The red seeds of (d) a magnolia tree, characteristic of the final stage, are just starting to appear.

Basal Angiosperms

Examples of basal angiosperms include the Magnoliidae, Laurales, Nymphaeales, and the Piperales. Members in these groups all share traits from both monocot and dicot groups. The Magnoliidae are represented by the magnolias: tall trees bearing large, fragrant flowers that have many parts and are considered archaic. Laurel trees produce fragrant leaves and small, inconspicuous flowers. The Laurales grow mostly in warmer climates and are small trees and shrubs. Familiar plants in this group include the bay laurel, cinnamon, spice bush, and avocado tree. The Nymphaeales are comprised of the water lilies, lotus, and similar plants; all species thrive in freshwater biomes and have leaves that float on the water surface or grow underwater. Water lilies are particularly prized by gardeners and have graced ponds and pools for thousands of years. The Piperales are a group of herbs, shrubs, and small trees that grow in the tropical climates. They have small flowers without petals that are tightly arranged in long spikes. Many species are the source of prized fragrance or spices; for example, the berries of Piper nigrum are the familiar black peppercorns that are used to flavor many dishes.

Monocots

Plants in the monocot group are primarily identified as such by the presence of a single cotyledon in the seedling. Other anatomical features shared by monocots include veins that run parallel to the length of the leaves and flower parts that are arranged in a three- or six-fold symmetry. True woody tissue is rarely found in monocots. In palm trees, vascular and parenchyma tissues produced by the primary and secondary thickening of meristems form the trunk. The pollen from the first angiosperms was monosulcate, containing a single furrow or pore through the outer layer. This feature is still seen in the modern monocots. Vascular tissue of the stem is not arranged in any particular pattern. The root system is mostly adventitious and unusually positioned, with no major tap root. The monocots include familiar plants such as the true lilies (which are the origin of their alternate name: Liliopsida), orchids, grasses, and palms. Many important crops are monocots, such as rice and other cereals, corn, sugar cane, and tropical fruits like bananas and pineapples.

image

Monocots and Dicots: major crops of the world: The world’s major crops are flowering plants. (a) Rice, (b) wheat, and (c) bananas are monocots, while (d) cabbage, (e) beans, and (f) peaches are dicots.

Eudicots

Eudicots, or true dicots, are characterized by the presence of two cotyledons in the developing shoot. Veins form a network in leaves, while flower parts come in four, five, or many whorls. Vascular tissue forms a ring in the stem whereas in monocots, vascular tissue is scattered in the stem. Eudicots can be herbaceous (like grasses), or produce woody tissues. Most eudicots produce pollen that is trisulcate or triporate, with three furrows or pores. The root system is usually anchored by one main root developed from the embryonic radicle. Eudicots comprise two-thirds of all flowering plants.