Fertilization Of Flowers

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30.7 A Generalized Flower Not all flowers possess all the structures shown here, but they must possess a stamen (bearing microsporangia), a pistil (containing megasporangia), or both in order to play their role in reproduction. Flowers that have both, as this one does, are referred to as perfect.

Pictures Fertilization Flower

(a) Daucus carota

Compound umbel

(a) Daucus carota

Compound umbel mens, and carpels (which are referred to as the floral organs; see Figure 19.12) are usually positioned in circular arrangements or whorls and attached to a central stalk called the receptacle.

The generalized flower shown in Figure 30.7 has both megasporangia and microsporangia; such flowers are referred to as perfect. Many angiosperms produce two types of flowers, one with only megasporangia and the other with only microsporangia. Consequently, either the stamens or the carpels are nonfunctional or absent in a given flower, and the flower is referred to as imperfect.

Species such as corn or birch, in which both megasporan-giate and microsporangiate flowers occur on the same plant, are said to be monoecious (meaning "one-housed"—but, it must be added, one house with separate rooms). Complete separation is the rule in some other angiosperm species, such as willows and date palms; in these species, a given plant produces either flowers with stamens or flowers with pistils, but never both. Such species are said to be dioecious ("two-housed").

Flowers come in an astonishing variety of forms, as you will realize if you think of some of the flowers you recognize. The generalized flower shown in Figure 30.7 has distinct petals and sepals arranged in distinct whorls. In nature, however, petals and sepals sometimes are indistinguishable. Such appendages are called tepals. In other flowers, petals, sepals, or tepals are completely absent.

Flowers may be single, or they may be grouped together to form an inflorescence. Different families of flowering plants have their own, characteristic types of inflorescences, such as the compound umbels of the carrot family, the heads of the aster family, and the spikes of many grasses (Figure 30.8).

Flower structure has evolved over time

The flowers of the most basal lineages of angiosperms have a large and variable number of tepals (or sepals and petals),

Pictures Fertilization Flower

Ray flowers

(b) Echinacea purpurea

(c) Pennisetum setaceum

30.8 Inflorescences (a) The inflorescence of Queen Anne's lace is a compound umbel. Each umbel bears flowers on stalks that arise from a common center. (b) Coneflowers are members of the aster family; their inflorescence is a head. In a head, each of the long, petal-like structures is a ray flower; the central portion of the head consists of dozens to hundreds of disc flowers. (c) Grasses such as this fountain grass have inflorescences called spikes.

Ray flowers

(b) Echinacea purpurea

(c) Pennisetum setaceum

30.8 Inflorescences (a) The inflorescence of Queen Anne's lace is a compound umbel. Each umbel bears flowers on stalks that arise from a common center. (b) Coneflowers are members of the aster family; their inflorescence is a head. In a head, each of the long, petal-like structures is a ray flower; the central portion of the head consists of dozens to hundreds of disc flowers. (c) Grasses such as this fountain grass have inflorescences called spikes.

carpels, and stamens (Figure 30.9a). Evolutionary change within the angiosperms has included some striking modifications of this early condition: reductions in the number of each type of floral organ to a fixed number, differentiation of petals from sepals, and changes in symmetry from radial (as in a lily or magnolia) to bilateral (as in a sweet pea or orchid), often accompanied by an extensive fusion of parts (Figure 30.9b).

According to one theory, the first carpels to evolve were modified leaves, folded but incompletely closed, and thus differing from the scales of the gymnosperms. In the groups of angiosperms that evolved later, the carpels fused and became progressively more buried in receptacle tissue (Figure 30.10a). In the flowers of the most recent groups, the other flower parts are attached at the very top of the ovary, rather than at the bottom as in Figure 30.7. The stamens of the most ancient flowers may have appeared leaflike (Figure 30.10b), little resembling those of the generalized flower in Figure 30.7.

Why do so many flowers have pistils with long styles and anthers with long filaments? Natural selection has favored length in both of these structures, probably because length increases the likelihood of successful pollination. Long filaments may bring the anthers into contact with insect bodies, or they may place the anthers in a better position to catch the wind. Similar arguments apply to long styles.

30.9 Flower Form and Evolution

(a) A magnolia flower shows the major features of early flowers: It is radially symmetrical, and the individual tepals, carpels, and stamens are separate, numerous, and attached at their bases. (b) Orchids, like this ladyslipper, have a bilaterally symmetrical structure that evolved much later. One of the three petals evolved into the complex lower "lip." Inside, the stamen and pistil are fused. There are two anthers in this species, although most orchids have only a single anther.

Pictures Fertilization Flower

(a) Magnolia grandifolia

(b) Cypripedium reginae

(a) Magnolia grandifolia

(b) Cypripedium reginae

(a) Carpel evolution l| According to one theory, the carpel began as a modified leaf with sporangia.

2l In the course of evolution, leaf edges curled inward and finally fused.

l| According to one theory, the carpel began as a modified leaf with sporangia.

Stamens Evolution

Cross section

(b) Stamen evolution

3} At the end of the sequence, three carpels have fused to form a three-chambered ovary.

Fused carpel

Cross section

(b) Stamen evolution

3} At the end of the sequence, three carpels have fused to form a three-chambered ovary.

Fused carpel

Evolution Flower Carpels

i\ The leaflike portion of the structure was progressively reduced...

2Ï .until only the microsporangia remained.

Austrobaileya sp.

Modified leaf

Modified leaf

Sporangia

Austrobaileya sp.

Magnolia

Lily

Magnolia

Sporangia

Cross section

30.10 Carpels and Stamens Evolved from Leaflike Structures

(a) Possible stages in the evolution of a carpel from a more leaflike structure. (b) The stamens of three modern plants show the various stages in the evolution of that organ. It is not implied that these species evolved one from another; they simply illustrate the structures.

A long style may serve another purpose as well. If several pollen grains land on one stigma, a pollen tube will start growing from each grain down the style toward the ovary. If there are more pollen grains than ovules, there is a "race" to fertilize the ovules. The race down the style can be viewed as "mate selection" by the plant bearing the style.

Angiosperms have coevolved with animals

Pollen has played another crucial role in the evolution of the angiosperms. Whereas many gymnosperms are wind-pollinated, most angiosperms are animal-pollinated. Animals visit flowers to obtain nectar or pollen, and in the process often carry pollen from one flower to another, or from one plant to another. Thus, in its quest for food, the animal contributes to the genetic diversity of the plant population. Insects, especially bees, are among the most important pollinators; birds and some species of bats also play major roles as pollinators.

For more than 130 million years, angiosperms and their animal pollinators have coevolved in the terrestrial environment. The animals have affected the evolution of the plants, and the plants have affected the evolution of the animals. Flower structure has become incredibly diverse under these selection pressures.

Some of the products of coevolution are highly specific; for example, some yucca species are pollinated by only one species of moth. Pollination by just one or a few animal species provides a plant species with a reliable mechanism for transferring pollen from one of its members to another.

Most plant-pollinator interactions are much less specific; that is, many different animal species pollinate the same plant species, and the same animal species pollinate many different plant species. However, even these less specific interactions have developed some specialization. Bird-pollinated flowers are often red and odorless. Many insect-pollinated flowers have characteristic odors, and bee-pollinated flowers may have conspicuous markings, or nectar guides, that are evident only in the ultraviolet region of the spectrum, where bees have better vision than in the red region. Coevo-lution and other aspects of plant-animal interactions are covered in more detail in Chapter 55.

The angiosperm life cycle features double fertilization

The life cycle of the angiosperms is summarized in Figure 30.11. The angiosperm life cycle will be considered in detail in Chapter 39, but let's look at it briefly here and compare it with the conifer life cycle in Figure 30.6.

Like all seed plants, angiosperms are heterosporous. The ovules are contained within carpels, rather than being exposed on the surfaces of scales, as in most gymnosperms. The male gametophytes, as in the gymnosperms, are pollen grains.

The ovule develops into a seed containing the products of the double fertilization that characterizes angiosperms: a diploid zygote and a triploid endosperm. The endosperm

Microgametophytes develop from microspores in the anthers, the male flower part.

Sequence Fertilisation Flowers

Microgametophytes develop from microspores in the anthers, the male flower part.

Double Fertilization

Double Fertilization

Pollen grains (microgametophyte, n)

Pollen germinates on the stigma. A pollen tube grows through the pistil until it reaches the megagametophyte.

Mega-

gametophyte (n)

Pollen grains (microgametophyte, n)

Pollen germinates on the stigma. A pollen tube grows through the pistil until it reaches the megagametophyte.

Pollen Tube Pear

Tube cell nucleus

DIPLOID (2n) HAPLOID (n)

Mega-

gametophyte (n)

DIPLOID (2n) HAPLOID (n)

Pictures Fertilization Flower
Pollen grain

Tube cell nucleus

30.11 The Life Cycle of an Angiosperm The formation of a triploid endosperm distinguishes the angiosperms from the gymnosperms.

serves as storage tissue for starch or lipids, proteins, and other substances that will be needed by the developing embryo.

The zygote develops into an embryo, consisting of an embryonic axis and one or two cotyledons, or seed leaves. The cotyledons have different fates in different plants. In many, they serve as absorptive organs that take up and digest the endosperm. In others, they enlarge and become photosyn-thetic when the seed germinates. Often they play both roles.

Angiosperms produce fruits

The ovary of a flowering plant (together with the seeds it contains) develops into a fruit after fertilization. A fruit may consist only of the mature ovary and its seeds, or it may include other parts of the flower or structures associated with it. A simple fruit, such as a cherry (Figure 30.12a), is one that develops from a single carpel or several united carpels. A raspberry is an example of an aggregate fruit (Figure 30.12b)—one that develops from several separate carpels of a single flower.

Pineapples and figs are examples of multiple fruits (Figure 30.12c), formed from a cluster of flowers (an inflorescence). Fruits derived from parts in addition to the carpel and seeds are called accessory fruits (Figure 30.12d); examples are apples, pears, and strawberries. The development, ripening, and dispersal of fruits will be considered in Chapters 38 and 39.

There are several clades of angiosperms

The better-understood relationships among the angiosperm clades are shown in Figure 30.13. Two large clades include the great majority of angiosperm species: the monocots and the eudicots. The monocots are so called because they have a single embryonic cotyledon; the eudicots have two. We will describe other differences between these groups in Chapter 35.

Some familiar angiosperms belong to clades other than the monocots and eudicots (Figure 30.14). These clades include the water lilies, star anise and its relatives, and the magno-liid complex. The magnoliids are less numerous than the

Magnoliid Example

30.13 Evolutionary Relationships among the Angiosperms The monocots and the eudicots are the largest clades among the angiosperms.This diagram is a conservative interpretation of current data on relationships among the clades.

monocots and eudicots, but they include many familiar and often useful plants such as magnolias, avocados, cinnamon, and pepper.

The monocots (Figure 30.15) include grasses, cattails, lilies, orchids, and palms. The eudicots (Figure 30.16) include the vast majority of familiar seed plants, including most herbs, vines, trees, and shrubs. Among them are such diverse plants as oaks, willows, violets, snapdragons, and sunflowers.

Carpels; triploid endosperm; seeds in fruit

Gymnosperm-like ancestor

Vessel elements

Carpels; triploid endosperm; seeds in fruit

Gymnosperm-like ancestor

Carpel Endosperm Reduced Gametophytes

Vessel elements

Carpels fused by tissue connection

Pollen with three grooves

Carpels fused by tissue connection

Pollen with three grooves

Amborellas

(a) Amborella trichopoda

(b) Nymphaea odorata

(c) Illicium floridanum

(a) Amborella trichopoda

(b) Nymphaea odorata

(c) Illicium floridanum

Anatomie Aristolochia Longa

(d) Piper nigrum

(e) Aristolochia grandiflora

(d) Piper nigrum

(e) Aristolochia grandiflora

30.14 Monocots and Eudicots Are Not the Only Surviving Angiosperms

(a) Amborella, a shrub, is the closest living relative of the first angiosperms; its clade is sister to the remaining extant angiosperms. (b) The water lily clade is the next most basal clade after Amborellas. (c) Star anise and its relatives belong to another basal clade. (d-f) The largest clade other than the monocots and eudicots is the magno-liid complex, represented here by (d) a black pepper, (e) Dutchman's pipe, and (f) an avocado tree.The magnolia in Figure 30.9a is another magnoliid.

(a) Phoenix dactylifera

(a) Phoenix dactylifera

Phoenix Dactylifera Flower

30.15 Monocots (a) Palms are among the few monocot trees. Date palms are a major food source in some areas of the world. (b) Grasses such as this cultivated wheat and the fountain grass in Figure 30.8c are monocots. (c) Monocots also include popular garden flowers such as these lilies. Many orchids (Figure 30.9b) are highly sought-after monocot flowers.

30.15 Monocots (a) Palms are among the few monocot trees. Date palms are a major food source in some areas of the world. (b) Grasses such as this cultivated wheat and the fountain grass in Figure 30.8c are monocots. (c) Monocots also include popular garden flowers such as these lilies. Many orchids (Figure 30.9b) are highly sought-after monocot flowers.

Date Palm Fertilization

(c) Rosa rugosa

30.16 Eudicots (a) The cactus family is a large group of eudicots,with about 1,500 species in the Americas.This cactus bears scarlet flowers for a brief period of the year. (b) The flowering dogwood is a small eudicot tree. (c) Climbing Cape Cod roses are members of the eudicot family Rosaceae, as are the familiar roses from your local florist.

(c) Rosa rugosa

30.16 Eudicots (a) The cactus family is a large group of eudicots,with about 1,500 species in the Americas.This cactus bears scarlet flowers for a brief period of the year. (b) The flowering dogwood is a small eudicot tree. (c) Climbing Cape Cod roses are members of the eudicot family Rosaceae, as are the familiar roses from your local florist.

Determining the oldest angiosperm clade

Which angiosperms were the earliest flowering plants was long a matter of great controversy. Two leading candidates were the magnolia family (see Figure 30.9a) and another family, the Chloranthaceae, whose flowers are much simpler than those of the magnolias. At the close of the twentieth century, however, an impressive convergence of evidence led to the conclusion that the most basal living angiosperm belongs to neither of those families, but rather to a clade that today consists of a single species of the genus Amborella (see Figure 30.14a). This woody shrub, with cream-colored flowers, lives only on New Caledonia, an island in the South Pacific. Its five to eight carpels are in a single whorl, and it has 30 to 100 stamens. The xylem of Amborella lacks vessel elements, which appeared later in angiosperm evolution. The characteristics of Amborella give us a good sense of what the first an-giosperms might have been like. But are there extinct angiosperms that may represent still more ancient clades?

In 2002, Chinese and American botanists examined fossils of two species of a 125-million-year-old aquatic genus, Archaefructus (see Figure 22.16). Their studies established an extinct family, Archaefructaceae, that is posited to be the sister taxon of all other angiosperms. The flower of these plants had its ovules enclosed in carpels, as in all angiosperms. The flower had neither petals nor sepals, however, and its carpels and stamens were arranged spirally around elongated shoots. This arrangement of carpels and stamens is seen today in the magnolias.

The origin of the angiosperms remains a mystery

We have learned a lot about evolution within the angiosperm clade. But how did the angiosperms first arise? Are the an-giosperms sister to any single gymnosperm phylum? A few years ago, it seemed that we were on the verge of answering these questions. But the puzzle remains as vexing today as it ever was.

Why should this be? Different phylogenetic methods, applied by different investigators, have produced apparently contradictory results. It might seem a simple matter to rectify this situation, but several questions complicate such efforts: What morphological characters should be selected as important, or should they all be treated as equally important? What algorithms should be applied to computerized analysis of data? Are all molecular differences and similarities significant, or are some of them incidental? Which fossils should be chosen for comparisons? What is the likelihood that we can find evidence of double fertilization in ancient fossils? Furthermore, it is possible that the angiosperms have no close relatives at all among living seed plants.

We are left with our original question: Where did the first angiosperm come from? Current progress in methodology gives us reason to hope that our understanding of seed plant evolution will be much improved before the present decade ends. We will see in Chapters 32-34 whether our understanding of animal evolution is any more complete.

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