6.6: Seed Plants - Angiosperms

Learning Objectives

By the end of this section, you will be able to do the following:

• Explain why angiosperms are the dominant form of plant life in most terrestrial ecosystems
• Describe the main parts of a flower and their functions
• Detail the life cycle of a typical gymnosperm and angiosperm
• Discuss the similarities and differences between the two main groups of flowering plants

 

Evolution of Angiosperms

The roughly 200 million years between the appearance of the gymnosperms and the flowering plants gives us some appreciation for the evolutionary experimentation that ultimately produced flowers and fruit. Angiosperms (“seed in a vessel”) produce a flower containing male and/or female reproductive structures. Fossil evidence (Figure 26.6) indicates that flowering plants first appeared about 125 million years ago in the Lower Cretaceous (late in the Mesozoic era), and were rapidly diversifying by about 100 million years ago in the Middle Cretaceous. Earlier traces of angiosperms are scarce. Fossilized pollen recovered from Jurassic geological material has been attributed to angiosperms. A few early Cretaceous rocks show clear imprints of leaves resembling angiosperm leaves. By the mid-Cretaceous, a staggering number of diverse flowering plants crowd the fossil record. The same geological period is also marked by the appearance of many modern groups of insects, suggesting that pollinating insects played a key role in the evolution of flowering plants.

New data in comparative genomics and paleobotany (the study of ancient plants) have shed some light on the evolution of angiosperms. Although the angiosperms appeared after the gymnosperms, they are probably not derived from gymnosperm ancestors. Instead, the angiosperms form a sister clade (a species and its descendents) that developed in parallel with the gymnosperms. The two innovative structures of flowers and fruit represent an improved reproductive strategy that served to protect the embryo, while increasing genetic variability and range. There is no current consensus on the origin of the angiosperms. Paleobotanists debate whether angiosperms evolved from small woody bushes, or were related to the ancestors of tropical grasses. Both views draw support from cladistics, and the so-called woody magnoliid hypothesis—which proposes that the early ancestors of angiosperms were shrubs like modern magnolia—also offers molecular biological evidence. 

The most primitive living angiosperm is considered to be Amborella trichopoda, a small plant native to the rainforest of New Caledonia, an island in the South Pacific. Analysis of the genome of A. trichopoda has shown that it is related to all existing flowering plants and belongs to the oldest confirmed branch of the angiosperm family tree. A few other angiosperm groups, called basal angiosperms, are viewed as having ancestral traits because they branched off early from the phylogenetic tree. Most modern angiosperms are classified as either monocots or eudicots, based on the structure of their leaves and embryos. Basal angiosperms, such as water lilies, are considered more ancestral in nature because they share morphological traits with both monocots and eudicots.

Figure 26.6 Ficus imprint. This leaf imprint shows a Ficus speciosissima, an angiosperm that flourished during the Cretaceous period. (credit: W. T. Lee, USGS)

 

From their humble and still obscure beginning during the early Jurassic period, the angiosperms—or flowering plants—have evolved to dominate most terrestrial ecosystems (Figure 26.13). With more than 300,000 species, the angiosperm phylum (Anthophyta) is second only to insects in terms of diversification. The success of angiosperms is due to two novel reproductive structures: flowers and fruits. The function of the flower is to ensure pollination, often by arthropods, as well as to protect a developing embryo. Most flowers have a mutualistic pollinator, with the distinctive features of flowers reflecting the nature of the pollination agent. The relationship between pollinator and flower characteristics is one of the great examples of coevolution. For example, some patterns are visible only in the ultraviolet range of light, which can be seen by arthropod pollinators. For some pollinators, flowers advertise themselves as a reliable source of nectar. Flower scent also helps to select its pollinators. Sweet scents tend to attract bees and butterflies and moths, but some flies and beetles might prefer scents that signal fermentation or putrefaction. Flowers also provide protection for the ovule and developing embryo inside a receptacle. Both fertilization and embryo development take place inside an anatomical structure that provides a stable system of sexual reproduction largely sheltered from environmental fluctuations.

Figure 26.13 Flowers. These flowers grow in a botanical garden border in Bellevue, WA. Flowering plants dominate terrestrial landscapes. The vivid colors of flowers and enticing fragrance of flowers are adaptations to pollination by animals like insects, birds, and bats. (credit: Myriam Feldman)

 

Following fertilization of the egg, the ovule grows into a seed. The surrounding tissues of the ovary thicken, developing into a fruit that will protect the seed and often ensure its dispersal over a wide geographic range. Not all fruits develop completely from an ovary; such “false fruits" or pseudocarps, develop from tissues adjacent to the ovary. Like flowers, fruit can vary tremendously in appearance, size, smell, and taste. Tomatoes, green peppers, corn, and avocados are all examples of fruits. Along with pollen and seeds, fruits also act as agents of dispersal. Different fruit structures or tissues on fruit—such as sweet flesh, wings, parachutes, or spines that grab—reflect the dispersal strategies that help spread seeds. Some may be carried away by the wind. Many attract animals that will eat the fruit and pass the seeds through their digestive systems, then deposit the seeds in another location. Cockleburs are covered with stiff, hooked spines that can hook into fur (or clothing) and hitch a ride on an animal for long distances. The cockleburs that clung to the velvet trousers of an enterprising Swiss hiker, George de Mestral, inspired his invention of the loop and hook fastener he named Velcro.

 

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Diversity of Angiosperms

Angiosperms are classified in a single phylum: the Anthophyta. Modern angiosperms appear to be a monophyletic group, which as you may recall means that they originated from a single ancestor. Within the angiosperms are three major groups: basal angiosperms, monocots, and dicots. Basal angiosperms are a group of plants that are believed to have branched off before the separation of the monocots and eudicots, because they exhibit traits from both groups. They are categorized separately in most classification schemes. The basal angiosperms include Amborella, water lilies, the Magnoliids (magnolia trees, laurels, and spice peppers), and a group called the Austrobaileyales, which includes the star anise. The monocots and dicots are differentiated on the basis of the structure of the cotyledons, pollen grains, and other structures. Monocots include grasses and lilies, and the dicots form a multi-branched group that includes (among many others) roses, cabbages, sunflowers, and mints.

 

Basal Angiosperms

The Magnoliidae are represented by the magnolias, laurels, and peppers. Magnolias are tall trees bearing dark, shiny leaves, and large, fragrant flowers with many parts, and are considered archaic (Figure 26.17). In the outer whorl of the magnolia flower the sepals and petals are undifferentiated and are collectively called tepals. The reproductive parts are arranged in a spiral around a cone-shaped receptacle, with the carpels located above the stamens (Figure 26.17). The aggregate fruit, with one seed formed from each carpel, is seen in Figure 26.18d. 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 (Figure 26.18a), and avocado tree.

Figure 26.17 Magnolia grandiflora. A cluster of carpels can be seen above the stamens, which have shed their pollen and begun to drop from the inflorescence. In the flower, the sepals and petals are undifferentiated and are collectively called tepals. (credit: Ianaré Sévi. http://bioimages.vanderbilt.edu/baskauf/10949)

Figure 26.18 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. The leaf venation resembles that of both the monocots (parallel) and the dicots (branched). (c) Water lilies, Nymphaea lotus. Although the leaves of the plant float on the surface of the water, their roots are in the underlying soil at the bottom of the lake. The aggregate fruit of a magnolia (d). The fruit is in its final stage, with its red seeds just starting to appear. (credit a: modification of work by Cory Zanker; credit b: modification of work by Franz Eugen Köhler; credit c: modification of work by Rl/Wikimedia Commons. d: modification of work by "Coastside2"/Wikimedia Commons).

 

Monocots

Plants in the monocot group are primarily identified by the presence of a single cotyledon in the seedling. Other anatomical features shared by monocots include veins that run parallel to and along 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 meristems form the trunk. The pollen from the first angiosperms was likely 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 scattered, not arranged in any particular pattern, but is organized in a ring in the roots. The root system consists of multiple fibrous roots, with no major tap root. Adventitious roots often emerge from the stem or leaves. The monocots include familiar plants such as the true lilies (Liliopsida), orchids, yucca, asparagus, grasses, and palms. Many important crops are monocots, such as rice and other cereals, corn, sugar cane, and tropical fruits like bananas and pineapples (Figure 26.19a,b,c).

Figure 26.19 Monocot and dicot crop plants. 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. (credit a: modification of work by David Nance, USDA ARS; credit b, c: modification of work by Rosendahl; credit d: modification of work by Bill Tarpenning, USDA; credit e: modification of work by Scott Bauer, USDA ARS; credit f: modification of work by Keith Weller, USDA)

 

Eudicots

Eudicots, or true dicots, are characterized by the presence of two cotyledons in the developing shoot. Veins form a network in leaves, and flower parts come in four, five, or many whorls. Vascular tissue forms a ring in the stem; in monocots, vascular tissue is scattered in the stem. Eudicots can be herbaceous (not woody), 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. The major differences between monocots and eudicots are summarized in Table 26.1. However, some species may exhibit characteristics usually associated with the other group, so identification of a plant as a monocot or a eudicot is not always straightforward.

Comparison of Structural Characteristics of Monocots and Eudicots

Characteristic	Monocot	Eudicot
Cotyledon	One	Two
Veins in Leaves	Parallel	Network (branched)
Stem Vascular Tissue	Scattered	Arranged in ring pattern
Roots	Network of fibrous roots	Tap root with many lateral roots
Pollen	Monosulcate	Trisulcate
Flower Parts	Three or multiple of three	Four, five, multiple of four or five and whorls

Table 26.1

 

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Angiosperm Life Cycle and Reproduction

The lifecycle of angiosperms follows the alternation of generations explained previously. The haploid gametophyte alternates with the diploid sporophyte during the sexual reproduction process of angiosperms. Flowers contain the plant’s reproductive structures.

 

Flowers

Flowers are modified leaves, or sporophylls, organized around a central receptacle. Although they vary greatly in appearance, virtually all flowers contain the same structures: sepals, petals, carpels, and stamens. The peduncle typically attaches the flower to the plant proper. 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 may display vivid colors to attract pollinators. Sepals and petals together form the perianth. 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 sexual organs, the female gynoecium and male androecium are located at the center of the flower. Typically, the sepals, petals, and stamens are attached to the receptacle at the base of the gynoecium, but the gynoecium may also be located deeper in the receptacle, with the other floral structures attached above it. 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 (male flowers), and carpellate flowers have only a gynoecium (female flowers). 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. In spite of the predominance of perfect flowers, only a few species of angiosperms self-pollinate. 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.

As illustrated in Figure 26.14, the innermost part of a perfect flower is the gynoecium, the location in the flower where the eggs will form. The female reproductive unit consists of one or more carpels, each of which has a stigma, style, and ovary. The stigma is the location where the pollen is deposited either by wind or a pollinating arthropod. The sticky surface of the stigma traps pollen grains, and the style is a connecting structure through which the pollen tube will grow to reach the ovary. The ovary houses one or more ovules, each of which will ultimately develop into a seed. Flower structure is very diverse, and carpels may be singular, multiple, or fused. (Multiple fused carpels comprise a pistil.) The androecium, or male reproductive region is composed of multiple stamens surrounding 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 haploid pollen grains, or male gametophytes.

Figure 26.14 Flower structure. This image depicts the structure of a perfect 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. (credit: modification of work by Mariana Ruiz Villareal)

 

The Life Cycle of an Angiosperm

The adult or sporophyte phase is the main phase of an angiosperm’s life cycle (Figure 26.15). Like gymnosperms, angiosperms are heterosporous. Therefore, they produce microspores, which will generate pollen grains as the male gametophytes, and megaspores, which will form an ovule that contains female gametophytes.

The male gametophyte develops and reaches maturity in an immature anther, which is at the end of the stamen—the long filament that supports the anther. Inside the anther’s microsporangia, which are usually bilobed, are pollen sacs in which the male sporocytes divide by meiosis to generate four haploid microspores, which, in turn, undergo mitosis and give rise to pollen grains. An inner layer of cells, known as the tapetum, provides nutrition to the developing microspores and contributes key components to the pollen wall. Mature pollen grains contain two cells: a generative cell and a pollen tube cell. The generative cell is contained within the larger pollen tube cell. Upon germination, the tube cell forms the pollen tube through which the generative cell migrates to enter the ovary. During its transit inside the pollen tube, the generative cell divides to form two male gametes (sperm cells). Upon maturity, the microsporangia burst, releasing the pollen grains from the anther. Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine. The exine contains sporopollenin, a complex waterproofing substance supplied by the tapetal cells. Sporopollenin allows the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without undergoing damage.

Visual Connection

Figure 26.15 Angiosperm life cycle. 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. (credit: modification of work by Mariana Ruiz Villareal)

 

The ovule, sheltered within the ovary of the carpel, contains the megasporangium protected by two layers of integuments and the ovary wall. The integument will develop into the seed coat after fertilization and protect the entire seed. The ovule wall will become part of the fruit. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization.

While the details may vary between species, the overall development of the female gametophyte has two distinct phases. First, in the process of megasporogenesis, a single cell in the diploid megasporangium undergoes meiosis to produce four megaspores, only one of which survives. During the second phase, megagametogenesis, the surviving haploid megaspore undergoes mitosis three times to produce eight nuclei distributed among the seven cells of the female gametophyte, also known as the megagametophyte or embryo sac (Figure 32.8). Two of the nuclei—the polar nuclei—move to the equator and fuse, forming a single, diploid central cell. This central cell later fuses with a sperm to form the triploid endosperm. Three cells are also located at each pole of the embryo sac. The three cells at the pole opposite the micropyle become antipodal cells and develop into the antipodal cells, which later degenerate. The three cells at the pole with the micropyle become the egg and two synergids. The nucleus closest to the micropyle becomes the female gamete, or egg cell. Once fertilization is complete, the resulting diploid zygote develops into the embryo, and the fertilized ovule forms the other tissues of the seed. The synergids help guide the pollen tube for successful fertilization, after which they disintegrate. When a pollen grain reaches the stigma, a pollen tube extends from the grain, grows down the style, and enters through the micropyle. 

Visual Connection

Figure 32.8 As shown in this diagram of the embryo sac in angiosperms, the ovule is covered by integuments and has an opening called a micropyle. Inside the embryo sac are three antipodal cells, two synergids, a central cell, and the egg cell.

 

After pollen is deposited on the stigma, it must germinate and grow through the style to reach the ovule. The microspores, or the pollen, contain two cells: the pollen tube cell and the generative cell. The pollen tube cell grows into a pollen tube through which the generative cell travels. The germination of the pollen tube requires water, oxygen, and certain chemical signals. As it travels through the style to reach the embryo sac, the pollen tube’s growth is supported by the tissues of the style. In the meantime, if the generative cell has not already split into two cells, it now divides to form two sperm cells. The pollen tube is guided by the chemicals secreted by the synergids present in the embryo sac, and it enters the ovule sac through the micropyle. A double fertilization event then occurs (Figure 32.18). One sperm and the egg combine, forming a diploid zygote—the future embryo. The other sperm fuses with the polar nuclei, forming a triploid cell that will develop into the endosperm—the tissue that serves as a food reserve for the developing embryo. After fertilization is complete, no other sperm can enter.

The fertilized ovule forms the seed, whereas the tissues of the ovary become the fruit, usually enveloping the seed. The seed consists of a toughened layer of integuments forming the coat, the endosperm with food reserves, and at the center, the well-protected embryo. 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. Ultimately, the embryo and cotyledons fill the seed, and the seed is ready for dispersal. Embryonic development is suspended after some time, and growth is resumed only when the seed germinates. The developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.

Figure 32.18 In angiosperms, one sperm fertilizes the egg to form the 2n zygote, and the other sperm fertilizes the central cell to form the 3n endosperm. This is called a double fertilization.

 

Fruit

As the seed develops, the walls of the ovary thicken and form the fruit. The seed forms in an ovary, which also enlarges as the seeds grow. Many foods commonly called vegetables are actually fruits. Eggplants, zucchini, string beans, tomatoes, and bell peppers are all technically fruits because they contain seeds and are derived from the thick ovary tissue. Acorns are true nuts, and winged maple “helicopter seeds” or whirligigs (whose botanical name is samara) are also fruits. 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 subtle distinction is that not all fruits are derived from just the ovary. For instance, strawberries are derived from the ovary as well as the receptacle, and apples are formed from the ovary and 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 orange, 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. Wind carries the light dry fruits of trees and dandelions. Water transports floating coconuts. Some fruits attract herbivores with their color or scent, or as food. Once eaten, tough, undigested seeds are dispersed through the herbivore’s feces (endozoochory). Other fruits have burrs and hooks to cling to fur and hitch rides on animals (epizoochory).

 

Development of Fruit and Fruit Types

After fertilization, the ovary of the flower usually develops into the fruit. Fruits are usually associated with having a sweet taste; however, not all fruits are sweet. Botanically, the term “fruit” is used for a ripened ovary. In most cases, flowers in which fertilization has taken place will develop into fruits, and flowers in which fertilization has not taken place will not. Some fruits develop from the ovary and are known as true fruits, whereas others develop from other parts of the female gametophyte and are known as accessory fruits. The fruit encloses the seeds and the developing embryo, thereby providing it with protection. Fruits are of many types, depending on their origin and texture. The sweet tissue of the blackberry, the red flesh of the tomato, the shell of the peanut, and the hull of corn (the tough, thin part that gets stuck in your teeth when you eat popcorn) are all fruits. As the fruit matures, the seeds also mature.

Fruits may be classified as simple, aggregate, multiple, or accessory, depending on their origin (Figure 32.22). If the fruit develops from a single carpel or fused carpels of a single ovary, it is known as a simple fruit, as seen in nuts and beans. An aggregate fruit is one that develops from more than one carpel, but all are in the same flower: the mature carpels fuse together to form the entire fruit, as seen in the raspberry. Multiple fruit develops from an inflorescence or a cluster of flowers. An example is the pineapple, where the flowers fuse together to form the fruit. Accessory fruits (sometimes called false fruits) are not derived from the ovary, but from another part of the flower, such as the receptacle (strawberry) or the hypanthium (apples and pears).

Figure 32.22 There are four main types of fruits. Simple fruits, such as these nuts, are derived from a single ovary. Aggregate fruits, like raspberries, form from many carpels that fuse together. Multiple fruits, such as pineapple, form from a cluster of flowers called an inflorescence. Accessory fruit, like the apple, are formed from a part of the plant other than the ovary. (credit "nuts": modification of work by Petr Kratochvil; credit "raspberries": modification of work by Cory Zanker; credit "pineapple": modification of work by Howie Le; credit "apple": modification of work by Paolo Neo)

Fruits generally have three parts: the exocarp (the outermost skin or covering), the mesocarp (middle part of the fruit), and the endocarp (the inner part of the fruit). Together, all three are known as the pericarp. The mesocarp is usually the fleshy, edible part of the fruit; however, in some fruits, such as the almond, the endocarp is the edible part. In many fruits, two or all three of the layers are fused, and are indistinguishable at maturity. Fruits can be dry or fleshy. Furthermore, fruits can be divided into dehiscent or indehiscent types. Dehiscent fruits, such as peas, readily release their seeds, while indehiscent fruits, like peaches, rely on decay to release their seeds.