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Chapter 14: Diversity: vascular plants.

No fossil evidence suggests that life on land existed prior to 450 million years ago. Various marine organisms must have repeatedly washed up on shore, but the rocky, barren environment was inhospitable to them. This period also coincided with the receding of the oceans and the exposure of algae to the land. The seawater offered protection from desiccation, and the sun's destructive ultraviolet radiation acted as a buffer so that temperature fluctuations were moderate and allowed organisms to absorb nutrients directly from the minerals dissolved in it. Marine organisms did and still do cope with the osmotic problems of a saline environment, but otherwise growth and development in the ocean was without many of the problems that were to be encountered on land.

Objectives

After completing this chapter, you should be able to:

* Discuss the cycle that plants have gone through in the past 500 million years

* Name the primitive vascular plants

* Describe how these plants developed their vascular system

* Name which one of the four primitive vascular plant families actually gave rise to the living seedless vascular plants

* Discuss the progression of the Pterophyta, ferns that contain almost 12,000 living species

* Draw and label the fern life cycle

* Identify the differences between a non-seed bearing vascular plant and seed bearing vascular plant

* Compare and contrast the gymnosperms and angiosperms

* Discuss the co-involvement of flowering plants and insects

* List and tell how different insects and animals are involved in the co-involvement

* Explain secondary plant compounds that have strong physiological effects on humans

Key Terms

ozone

multicellular green alga

bryophytes

vascular plants

antheridium

archegonium

conduction systems

sporophyte

gametophyte

paleozoic era

carboniferous period

seeds

seed ferns

gymnosperms

carboniferous period

fronds

fiddlehead

ovule

nucellus

integuments

seed coat

after-ripening dormancy

cycads

ginkgos

conifers

gentophytes

angiosperms

trimerophytes

progymnosperms

cone (stobilus)

microsporangia

microspore mother cells

microspores

prothallium cells

tube cell

seed-scales complexes

polyembryony

maidenhair tree

mormon's tea

ephedrine

bronchodilator

paleobotanists

coevolution

protected ovules

inferior ovary

pseudocopulation

alkaloids

isozymes

Movement to Land

Although some oxygen was apparently produced by electrical discharges that split water molecules, it was not until cyanobacteria began photosynthesizing, and thus obtained their reducing source ([H.sup.+]) from a photolysis of water that oxygen was put into the atmosphere in appreciable quantities. The oxygen molecule, [O.sub.2], automatically forms a small amount of ozone, [O.sub.3], which has the ability to absorb ultraviolet radiation. Although the proportion of ozone is relatively small compared with the concentration of oxygen, it is a significant factor in the Earth's atmosphere. The fossil record indicates but evidence suggests the significant factor in the Earth's atmosphere. The fossil record indicates that cyanobacteria began photosynthesis about 3 billion years ago, but evidence suggests that significant oxygen accumulation did not occur until about 1 billion years ago. It was not until approximately 450 million years ago that a sufficient concentration of ozone finally began to absorb enough harmful radiation that organisms could survive when exposed directly to the sun's rays.

Single-celled marine organisms existed as the sole life forms for a long time. Then true multicellularity evolved, first as a single-dimensional filament, then as a two-dimensional sheet of cells, and finally as three-dimensional organisms with considerable differences in the external and internal environment of the organism as a whole. Colonial organisms such as the green alga Volvox developed modified internal concentrations of nutrients and gases. As multicellular marine plants became larger and more complex, specialized cells formed to accommodate and alleviate problems of "communication" between cell tissues. Survival is more likely for organisms that evolve a system of coping with environmental changes.

No evidence exists that any of these prokaryotes or eukaryotes survived on land until a multicellular green alga established itself on land in the Silurian period, about 400 million years ago. Evidence suggests that it may have evolved symbiotically with a fungus, perhaps like the modern lichens. Probably thousands of algae were washed onto land before successful establishment was achieved. At best, the terrestrial habitat was precarious, but surviving offspring had improved structure/function relationships, and the landscape finally had its first occupants.

Adaptations of Land Plants

The fossil record suggests that early land plants were not significantly different from marine progenitors. Some of the earliest plants evolved as bryophytes, a group that could be considered an evolutionary dead end. Bryophytes failed to develop specialized transport systems for water and nutrients, which in turn limited their stature. They also failed to develop an effective cuticle, the root system did not become extensive enough to carry the plant through periods of stress, and gas exchange mechanism (stomata) failed to develop or regulate efficiently.

While the bryophytes followed one line of evolution, another group developed a level of specialization not before achieved on land. Elongated, thin-walled cells had already developed in some of the multicellular marine algae, and mutation of those cells gradually led to conducting cells that eventually evolved into xylem and phloem. At first the cells had obstructions in the end walls, and long-distance transport was rather slow. With a relatively inefficient cuticle and system of gas exchange, plants could not grow very tall, and the surface area of aboveground parts had to remain small. Such plants were branched and had almost no leaves. As xylem and phloem became more efficient, water and nutrients moved faster, leaves expanded, and the area available for capturing light energy increased; thus, more photosynthate could accumulate, creating greater biomass. Those plants that developed one efficient transport system for water and another for the transport of organic solutes are called vascular plants.

The vascular plants and bryophytes diverged long ago, shortly after the progenitor alga made the transition from ocean to land. Some bryophytes are fossilized and calculated to be more than 350 million years old; the ancient specimens are similar to the alternation of generations, it is believed that the green algal ancestor must also have had the characteristic.

The rate of evolution was rapid after the movement to land. Extreme environments lead to a more expeditious selection of variants than does a stable environment; therefore, fluctuations in temperature, changes in light conditions, nutritional scarcity, changing relative humidity, and unpredictable water availability promoted swift plant evolution on land. One of the critical features of survival on land is the protection of sex cells; organisms successful in making the transition must have gametes with an outer layer of protective cells. The development of the antheridium (male multicellular sex organ) and the archegonium (female multicellular sex organ) were important advancements in the success of land plants.

Early in the movement to land, organisms developed multicellular sporangia well protected by walls that prevented desiccation. Such structures could be dispersed really by wind. At the same time, plants were beginning to develop a cuticle to keep the plant itself from drying. Efficient conduction systems evolved from primitive transport cells hampered by friction to more streamlined cells with little or no frictional loss. Gradually the alternation of generations evolved an increasingly important role for the sporophyte; the gametophyte was much reduced in size and became more dependent on the sporophyte. In the flowering plants, the gametophyte represents only a tiny fraction of the life cycle.

During the latter stages of the Paleozoic era, the climate was stable and mild, similar to the climate of modern subtropical and tropical regions. Although mountains were forming in what is now the eastern United States, Texas, and Colorado, larger areas of the United States were flat, and the far western portion of the continent was still covered by the sea. In most places, the saline waters were not deep, some being vast coal swamps. Very stable, warm, humid regions gave rise to lush plants with a great deal of biomass. This era, known as the Carboniferous period, produced most of the vegetation that decomposed and formed vast deposits of coal, oil, and natural gas.

Evolution and Distribution of Vascular Plants

The vascular plants include those with and without seeds. A great deal has been said in older botanical literature about the seed ferns, a group of plants that is now extinct. The fossil record clearly shows that they were not ferns at all, but primitive gymnosperms and therefore seed bearing. Vascular plants do not produce seeds but reproduce by spores. There are three extinct (Rhyniophyta, the oldest; Timerophyta, fossil found 350 million years ago, and Zosterophyllophyta, early Devonian 375 millions years ago) and four living divisions (Lycophyta, a group of herbaceous line; Sphenophyta, horsetail's jointed stems; Psilophyta, epiphytes; and Pterophyta, the ferns). Pterophyta is currently the largest and the most evident group. There are 12,000 living species, and they are cosmopolitan (throughout the world). The age of the ferns in the fossil record dates from the Carboniferous period, a time in which they were the dominant vegetation. Water was plentiful, and the climate was warm, humid, and unchanging. In many respects, the world was like a giant greenhouse with ideal growing conditions for many plants. Some ferns grew to 8 m tall and had broad trunk bases with many aerial roots as props.

Most extant ferns are tropical, but some are found in temperate regions, including the mountainous regions of the United States; a few are adapted to aridity and inhabit deserts. Living species vary in size from the tiny aquatic Azolla microphylla with fronds or fern leaves less than 2 cm long, to the giant tree ferns, some of which may reach almost 25 m in height and 30 cm or more in diameter. All this stem tissue is primary in origin; and only one species of fern has a vascular cambium. Fern fronds come in myriad shapes and sizes, and many are highly dissected. Unlike the seed plants, ferns produce leaves from a coiled position. The frond expands by unrolling from base to tip with new growth produced at the apex; such new leaves are often covered with surface hairs. The coiled frond is called a fiddlehead because of its resemblance to the neck and head of a violin, as shown in Figure 14-1.

[FIGURE 14-1 OMITTED]

Reproduction of Ferns (Nonseed Plants)

In almost every case, ferns are homosporous, that is, produce only a single kind of spore. These spores are born on the underside of fertile fronds, as opposed to the non-spore-bearing sterile fronds. Spore mother cells in the fronds undergo meiosis. The spores are produced in sporangia, and many sporangia are grouped together in a region to make up a sorus. The sori are arranged on the underside of the leaf in varying patterns, as shown in Figure 14-2, depending on the species, and are often mistaken for insect eggs.

[FIGURE 14-2 OMITTED]

Each sorus may be covered with a membrane-like protective layer called the indusium. When the spores mature, the indusium shrivels or breaks, and the spores are released. They are catapulted through the air for maximum dispersal.

If the spore lands on a suitable substrate, the spore starts to germinate; the haploid spore will give rise to a chain of cells called a protonema. Mitosis takes place, the result is a free-living bisexual gametophyte. The absorption of water and nutrients is achieved by rhizoids, which grow like root hairs on the underside of the haploid plant body.

The aquatic-dwelling ancestry of the ferns is revealed by a reproductive system in which motile sperm are produced within the antheridia, swim to the neck of the archegonium, and fertilize the egg to produce a new diploid zygote (see Figure 14-3).
MAINTAINING DIVERSITY

In 1990, the U.S. Congress authorized establishment of a National
Genetic Resources Program (NGRP). It is the NGRP's responsibility
to acquire, characterize, preserve, document, and distribute to
scientists germplasm of all life forms important for food and
agricultural production.

The Germplasm Resources Information
Network (GRIN) Web server provides germplasm information about
plants, animals, microbes, and invertebrates. This program is
within the U.S. Department of Agriculture's Agricultural Research
Service.

In terms of plants, the National Plant Germplasm System
(NPGS) is a cooperative effort by public (state and federal) and
private organizations to preserve the genetic diversity of plants.
The world's food supply is based on intensive agriculture, which
relies on genetic uniformity. But this uniformity increases crop
vulnerability to pests and stresses. Scientists must have access to
genetic diversity to help develop new varieties that can resist
pests, diseases, and environmental stresses. The NPGS aids the
scientists and the need for genetic diversity by acquiring,
preserving, evaluating, documenting, and distributing crop
germplasm important for food and agricultural production.

Since many important crop species originate outside the United States,
the first steps toward diversity are acquisition and introduction.
New germplasm (accessions) enter NPGS through collection, donation
by foreign cooperators, or international germplasm collections.

GRIN provides NGRP personnel and germplasm users with continuous
access to databases for the maintenance of passport,
characterization, evaluation, inventory, and distribution data
important for the effective management and utilization of national
germplasm collections.

For more information, start at the Web site
for Germplasm Resource Information System: www.ars-grin.gov.


[FIGURE 14-3 OMITTED]

The Seed Plants

Spores produced by ferns and lower plants are adapted to many environmental stresses and therefore are able to survive under extreme conditions; however, such spores fail to exhibit the survivability of seeds. Seed plants represent a significant jump in adaptation to more extreme climates. The oldest seeds have been found in rocks from the Devonian period, some 350 million years old.

Modern seeds develop from a mature ovule, the female gametophyte being embedded within a fleshy nucellus. One or more layers of the integuments, which develop into the seed coat, enclose the nucellus. The thickness and chemical composition of the integuments ultimately determine the nature of the seed coat and thus its protective quality. Some seed coats become exceedingly thick and highly impermeable to water and gases. The survival capability of such seeds is very high.

In most modern seeds, the embryo matures before dispersal, whereas ancient seeds apparently completed embryo development after dispersal. Survival ability would appear to be best with embryo development prior to dispersal, and such embryos might have a selective advantage. Even today some seeds have after-ripening dormancy because the embryo is not fully developed. Newly harvested sugar beet seeds exhibit this characteristic.

Storage of reserved food within the seed is critical for survival. Within the endosperm, or cotyledons, various storage molecules (starch, protein, and lipids) become available for energy during the germination process. As germination proceeds, respiration rates increase dramatically, and storage molecules are hydrolyzed to produce the glucose substrate for respiration. This food reserve is usually more than adequate and sometimes stored reserves are available weeks or months after the germination process begins. Such storage capacity is vital to survival during long periods of dormancy, perhaps hundreds of years for some species.

This remarkable adaptation--the seed--increased the chances of survival on land. It allows for continuation of the life cycle under conditions unfavorable for growth. Frozen soil, drought, extreme heat, and other environmental factors usually inhospitable to plant growth are endured in the seed stage; this resistant, reproductive structure simply goes dormant until unfavorable conditions have passed.

There are five extant groups of seed plants: the cycads, the ginkgos, the conifers, the gentophytes, and the angiosperms. The first four of these comprise the gymnosperms, or plants with naked seeds, the angiosperms, or flowering plants, have enclosed seeds.

Gymnosperms

The gymnosperms apparently evolved during the Paleozoic era from an intermediate group of plants that were more highly vascularized than the lower plants. These now-extinct groups were the trimerophytes. They had no leaves; the main axis formed lateral branches that divided several times. Some of the lateral branches terminated in sporangia, which produced a single type of spore. These progymnosperms eventually gave rise to plants with leaves and seeds.

The movement of land and increasing aridity presented the problem of moving male gametes to the female gamete for fertilization. Whereas ferns and lower plants facilitate the reproductive process by having motile sperm swim through water to reach the egg, most large land plants failed to develop a mechanism for ensuring that water-based reproduction will continue. Only in the cycads and in Ginkgo do the relic motile sperm swim to the region of fertilization. Instead, airborne pollen evolved, and pollination had to be accomplished by wind or by animals. In most gymnosperms, the immature male gametophytes, the pollen grain, is borne by the wind to the vicinity of the female gametophyte within an ovule. Following pollination, the pollen grain germinates and produces a pollen tube, which delivers the sperm directly to the archegonium.

Conifers

Although the total number of conifer species is not large, they are the largest group of living gymnosperms, comprising about 50 general and 550 species. These plants have adapted remarkably to aridity by means of sunken stomata and thick cuticles, and the needlelike and scale leaves have a much reduced surface area, as shown in Figures 14-4a and 14-4b. There is reason to believe that the evolution of conifers coincides with tremendous selection pressure during the worldwide aridity of the Permian period. During the early Tertiary period, some genera were more prominent than they are in the modern landscape. On all the northern continents, vast regions were and still are covered with conifers. The pines are unquestionably the most economically important conifers. There are about 90 species widely distributed throughout North America, Europe, and Asia.

[FIGURE 14-4a OMITTED]

[FIGURE 14-4b OMITTED]

Reproduction

The reproduction structure in conifers is a cone (stobilus), and male and female gametes are produced on separate cones. Both types of cone are found on the same plant, the male cones occurring on the lower branches and the more conspicuous female cones being borne above. It is not uncommon to find 1-, 2-, and even 3-year-old female cones on the same tree in pines, although most conifers produce mature cones in one season. Compared with that of angiosperms, the reproductive cycle in conifers is very long. The period between pollination and fertilization may be many months, whereas in angiosperms the period is a matter of hours or days. Male cones are quite small, usually no more than 1 or 2 cm in length. Each of the spirally arranged microsporophylls (scales) contains two microsporangia, and each microsporangia contains many microspore mother cells, which undergo meiosis to produce four haploid microspores. Each of these develops into a winged pollen grain with two prothallium cells (having no apparent function), a generative cell, and a tube cell. The mature pollen grains are shed by the male cones in great abundance in the spring, and sometimes coniferous forest are shrouded in yellow clouds of pollen, even covering the entire surface of lakes. The bladderlike air sacs increase the buoyancy of the pollen, and it can travel hundreds of miles before falling inside the scales of an ovulate cone.

The female cone is technically a modification branch with spirally arranged scales known as seed-scales complexes. Each scale bears two ovules on the upper surface. Each ovule consists of a nucellus surrounded by the integument, which eventually gives rise to the seed coat. A tiny opening at the base of the integument, the micropyle, is the site of penetration of the pollen tube. Within the megasporangium located between the micropyle and nucellus, a megaspore mother cell undergoes meiosis, producing four megaspores. The three nearest the micropyle disintegrate, leaving a single megaspore to produce the gametophyte containing the egg. Pollination occurs in the spring. The ovulate cones open the scales while in an upright position so that the pollen grain can fall downward into the crevice. A drop of sticky fluid at the micropyle captures the pollen grain and pulls it closer to the ovule. During this process, the scales again close, providing additional protection from the environment. When the pollen grain comes in contact with the nucellus, it forms a pollen tube that grows slowly toward the megagametophyte. At this time, the megaspore mother cell has not undergone meiosis. This process is exceedingly slow, sometimes requiring as long as 6 months for initiation and an additional 6 months for complete development. Fertilization is not accomplished until approximately 15 months after pollination (see Figure 14-5).

[FIGURE 14-5 OMITTED]

In gymnosperms, two sperm cells are eventually produced, but one disintegrates and the other fertilizes the egg. Even though more than one egg may be fertilized and more than one embryo may start to develop, a process known as polyembryony, all but one generally aborts, and a single embryo develops to maturity within each seed. The mature cone completes development during the fall of the second year following pollination. The mature conifer embryo consists of a root/shoot axis with several, usually eight, cotyledons or true seeds. The environmental conditions essential for germination are similar to those for the angiosperms.

Ecology and economic importance

Conifers' adaptation to aridity, particularly true of the pines, has allowed colonization in many parts of the world. Vast areas are covered by coniferous forest, particularly the temperate regions of the northern hemispheres. Secondary growth is rapid and extensive; thus, conifer wood is usually considered to be softwood, and angiosperm wood is considered to be hardwood, but the distinction is far from absolute. Many conifers produce relatively hard wood. The largest and oldest organisms in the world are conifers.

Ginkgo

One of the broad leaf deciduous street trees of many cities throughout the world is the maidenhair tree (Ginkgo biloba), as shown in Figure 14-6. Few nonbotanists recognize this tree as a gymnosperm, yet its morphological features, including naked seed, place it within this group. The unusual leaves are fan shaped and sometimes deeply lobed. Gingko is the only living species of a group of organisms that extended into the late Paleozoic era and were widespread in the Mesozoic area. Gingko is probably extinct in the wild, and all specimens now growing have been cultivated. The species has spread rapidly during recent years because it is an excellent street tree for metropolitan areas, and it is particularly resistant to air pollution. Tokyo's famous cherry trees, many of which have succumbed to industrial pollution, have been replaced with Gingko.

[FIGURE 14-6 OMITTED]

Gingkos are dioecious trees; that is, male and female sex structures are borne on different trees. The ovules are borne in pairs on the end of short stalks, and seeds are produced in the fall. Fertilization is very much delayed and may not occur until after the ovules have been shed from the tree. The seeds produce butyric acid, which has an offensive odor, and consequently male trees are usually propagated vegetatively, and therefore are easy to perpetuate only the male plants (see Figure 14-7).

[FIGURE 14-7 OMITTED]

Cycads

These are palmlike gymnosperms, very different from either the conifers or Gingko and native to subtropical and tropical regions. In the Mesozoic era, during the reign of the dinosaurs, cycads were dominant vegetation. There are approximately 10 genera with 100 species living at the present time. The only species native to the United States is Zamia pumila, which grows in southern Florida. Cycads, which are shown in Figure 14-8, produce a trunk with some secondary tissue, and the leaves are usually clustered at the apex to produce a palmlike effect. Pollen and seed cones are borne on separate plants, and the cone can be very large in some species.

[FIGURE 14-8 OMITTED]

Gnetophytes

This unusual group of plants consists of three genera with some 70 species. Like the angiosperms, this single group of gymnosperms conducts water via vessels. Ephedra, which inhabits arid regions, is a shrubby gymnosperm with extensive branching and scalelike leaves. It is the only genus native to the United States. One species Ephedra antisyphilitica, the so-called Mormon's tea, was brewed by the early settlers as a substitute for true tea, and the liquid was thought to have medical properties. Ephedra also produces ephedrine, an important bronchodilator. Gnetum is a group of trees and climbing vines found throughout tropical rain forest. They have thick, leathery leaves and are easily mistaken for typical dicots. Welwitschia is one of nature's strangest products. It grows only in the desert regions of Africa, and most of the plant is covered with sand. The aerial parts consist of woody, straplike leaves. On old plants, the leaves are torn and tattered, the splits giving the impression of more than two leaves.

Angiosperms

Unquestionably today's dominant worldwide vegetation consists of the flowering plants. In addition to their agricultural importance, their applications are abundant in commerce and industry. Most of the plant products for consumer use come from the angiosperms. This giant group of plants, which include some 200,000 species of dicots (grouped into about 250 families) and some 70,000 species of monocots (grouped into about 60 families), is characterized by a diversity not seen in any of the other plant groups.

From the sedimentary strata of the fossil record, one can determine layer upon layer of ferns and primitive gymnosperms, the lush vegetation of the Carboniferous forest about 200 million years ago. Then suddenly, angiosperms appear in sediments from about 127 million years ago. Paleobotanists have generally concluded that angiosperms evolved considerably earlier than the fossil record indicates, perhaps as long as 175 to 200 millions years ago. The argument suggests that conditions that lead to selection pressures (changing and diverse climates) existed in the tropical highlands, not in the valleys and lowlands where the sedimentation would be expected to occur. It would be possible for a group of organisms to have evolved in such high elevations and have no fossil record for many millions of years.

Known distribution patterns and the fossil record suggest that angiosperms did evolve from now-extinct broad-leafed gymnosperms in the hills and uplands of Gondwana, which began breaking apart just about the same time the angiosperms appeared. The separation of South America and Africa was not complete in the tropical regions until about 90 million years ago. Presumably the early angiosperms radiated into Laurasia in the region of what is now North Africa, the Iberian Peninsula, and the Middle East. Even though the Indian subcontinent was moving rapidly northward at this time and eventually collided with Asia, there is little evidence of major angiosperms crossing this link; the Indian subcontinent had undergone such major climate changes during its move northward that most species became extinct.

The fossil record also suggests that by 75 million years ago many flowering plant families were well established, and some of these are common in today's angiosperm flora including; birch (Betula), alder (Alnus), oak (Quercus), elm (Ulmus), sycamore (Platnus), basswood (Tilia), chestnut (Castanea), maple (Acer), beech (Fagus), sweet gun (Liquidamber), hickory (Carya), and Magnolia.

A number of modern angiosperms have returned to an aquatic habitat where the environmental stresses are not serious. Whether the plant is submerged or floating, water stress in an aquatic environment is seldom a factor in growth and reproduction. Light and gas exchange can be compromised, but species that have managed to survive did so through specific anatomical and morphological adaptations. In Lemna minor (duckweed), for example, the tiny leaves float on the surface of the water, and roots lie just below the surface.

Angiosperm evolution, as in other organisms, is characterized by the development of new species as a world climate change. Unquestionably one of the major selective forces is the lack of a constant water supply, and many species have managed to survive because variants within the species were capable of coping with aridity. Because lack of water is such a compelling selective factor, proportionately fewer organisms manage to compete well in arid ecosystems. Species that do survive are often very different morphologically from their ancestors.

Evolution of the Flower

The shoot apex of flowering plants is a remarkable structure: It sometimes produces leaves and sometimes produces flowers, fruits, and seeds depending on a number of complex internal and external factors. A vegetative apex that produces leaves is said to be indeterminate in that new leaves arise whenever climate conditions are suitable for growth and development. Even in periods of dormancy, the vegetative apex remains intact, and primordial leaves wait for the onset of favorable conditions. This same shoot apex, cued by certain environmental and/or hormonal factors, suddenly stops producing a flower, fruit, and seeds; thus, it becomes a determinate organ. The conversion from indeterminate to determinate status is characteristic of angiosperms. Perhaps no single event of plant evolution has had such a significant impact as has the evolution of the flower. It is important to remember that floral parts have evolved from leaves, and indeed flowers are really nothing more than modified leaves. Most of what we know about evolution of flowers is based on morphological comparison of modern forms because flowers are generally too delicate to be preserved in the fossil record. A primitive flower was found in sedimentary deposits in Sweden. Apparently the progenitor of the rose, it is thought to be approximately 80 million years old. Such discoveries are rare, and the fossil record is not likely to reveal the minute, step-by-step history of floral evolution.

Recall that a flower is a determinate shoot bearing various leaflike appendages. The carpel is derived from a folded blade without a welldefined stigmatic surface to which pollen grains adhere. Instead, hairs on the margin of the blade acted as pollen traps. Gradually, with the evolution of carpels, the stigma became more specialized and relocated near the top of the structure. In early angiosperms, the ovules were probably arranged in rows near the edges of the inner surface. As evolution proceeded, the number of ovules decreased. Likewise, the primitive flower contained a number of separate carpels within the ovary, which have been fused and/or reduced in number with increasing specialization.

Stamens too, have evolved from leaves, although most modern stamens bear them little resemblance. One of the primitive flowering plants, Magnolia, does produce flat, broad stamens, however. It is thought that ancestral stamens were nothing more than leaf blades with sporangia near the center of the blade. As specialization took place, the blade narrowed to produce what is now the filament, and the sporangium was left at the tip of the modified leaf. Increased specialization has led to fused stamens. In some cases, they are fused to the corolla. Some very advanced species have sterile stamens that have become modified to produce nectar, although typical nectaries are not derived from stamens.

Most sepals are green, photosynthetic, and leaflike. They have apparently been derived directly from leaves with little modification. Petals are occasionally derived from sepals, but generally they are specialized from stamens and have become broadened, pigmented, and otherwise modified for pollinator attraction. Petals, like stamens, generally have a single vascular strand, whereas sepals, like leaves, have three or more vascular connections. In the more advanced families, the petals are fused into a tube, and the stamens are often fused to that tube. Likewise, the sepals may fuse into a tube in the more recently evolved groups.

The typical primitive flower was made up of many carpels, stamens, petals, and sepals (all distinctly separated), and all structures were spirally arranged on the tip of the stem. Figure 14-9 shows a modern flower that still retains those characteristics: the Magnolia. By comparing Magnolia with an advanced species such as an orchid, one can make some generalization about the trends in floral evolution:

1. Flowers have evolved from many indefinite parts to a few definite parts.

2. The number of kinds of parts has been reduced from four in the primitive flower to three, two, or one in modern flowers. (Actually, there were only carpels and stamens in the primitive flower, then complete flowers with petals and sepals, then flowers reduced to the three, two, or one part.)

3. The primitive position of the ovary was superior, the advanced position inferior.

4. The radial symmetry of primitive flowers has given way to bilateral symmetry in the more advanced flowers.

[FIGURE 14-9 OMITTED]

The most specialized dicot family is the sunflower family (Asteraceae), and the most specialized monocot family is the orchid family (Orchidaceae), as shown in Figure 14-10. Both also have the largest number of species.

[FIGURE 14-10 OMITTED]

Evolution of Fruit

A fruit is a mature ovary. In some cases it retains floral parts. Depending on the arrangement of the carpels, fruits can be simple, multiple, or aggregated. Modifications of floral parts have, in some cases, led to fruits that include the ovary but are largely composed of the organs. In the apple, for example, the receptacle enlarges, growing around the carpels to become the fleshy portion of the fruit. The ovary itself becomes the apple core (and is thus of secondary importance as a food source).

Many fruits have developed hard walls that protect the seeds inside. Such a protective mechanism helps to ensure survival of the species, even though it can make seed germination and establishment more difficult.

Coevolution

The evolutionary processes are normally described in terms of individuals and populations of a given species changing in adaptive (or nonadaptive) ways. The variability in sexually reproducing individuals allows natural selection to function in several ways. The selective forces, however, are usually described in general terms of the organism's "environment," which all too often brings to mind only abiotic and climate features. The complete environment acting on organisms also includes interactions with other living organisms. The term coevolution refers to two or more groups of organisms evolving in parallel and interdependently. Neither group "causes" the other to change; both groups develop new types independently. However, whether a new type succeeds is often a direct function of the interrelationships between the groups. Coevolution then refers to successful adaptations only.

Insect-flower coevolution

One of the most striking examples of coevolution is that of flowers and insects. Although today there are some 245,00 species of flowering plants and over 750,00 species of insects, 200 million years ago angiosperms were just beginning to evolve, and only a few different insect species since that time attests to the high development of this relationship. It is one of the most successful examples of coevolution known to scientists.

This interdependence revolves around the selective advantage it provides both groups. Flowers are more efficiently cross-pollinated by insect visitors, and the insects are provided a reliable food source. Since gymnosperms are wind-pollinated today, as they were when angiosperms evolved, undoubtedly insect pollination is one of the advantages that allowed flowering plants to become more successful than gymnosperms.

The earliest visitations were probably by beetles feeding randomly on soft plant tissues, pollination droplets of the gymnosperm ovules, and sap or resin exuded from leaves and stems (see Figure 14-11). During their random foraging, the beetles accidentally carried pollen from plant to plant. If primitive flowers formed by broad-leaved gymnosperms accidentally produced a more nutritious tissue or fluid that increased beetle visitations, pollen sticking to their bodies would be carried from flower to flower. This would have increased the effectiveness of cross-pollination over wind-pollination, and increased cross-pollination frequencies resulted in more ovules developing into seed and greater total genetic variability in successive generations of offspring.

[FIGURE 14-11 OMITTED]

As mentioned, most flowers are self-incompatible-their own garments cannot fertilize them. Thus, new genetic material from a different plant of the same species can constantly be introduced. Cross-pollination ensures maximum genetic variability possible through sexual reproduction.

As the number of surviving offspring and variability increase, flowers having even greater insect attraction developed, producing even for regular and frequent visitation and cross-pollination. For the insects, more plants with more flowers meant greater amounts of nutritional material and a reliable food source. These flowers were numerous enough to become primary or even exclusive food sources for the insects. This interdependence has occurred in a spiraling way: More and different types of insects, resulting in increase crosspollination efficiency, which resulted in increased flower diversity from which insects could feed, and so on. Unlike the chicken and the egg quandary (which came first?), flowers and insects diversified, multiplied, and succeeded in parallel--they coevolved.

It is probable that several features unique to angiosperms evolved as a direct response to insect pollination. Beetles are generally not very dainty or specific feeders; thus, an ovule is as desirable a food as other floral tissue. Early flower types that lacked sufficient surrounding protective tissues for their ovules did not survive. Today, all flowering plants have protected ovules. Subsequent development of entire ovaries surrounded by protective tissue has resulted in inferior ovary position.

As insect groups other than beetles evolved, even greater flower variability and insect-flower specificity evolved. From approximately 50 million years ago on, the fossil records indicate continual diversification among insect groups such as the bees, butterflies, and moths. Correspondingly, flower size, shape, color, and organizational complexity also have undergone remarkable change. The efficiency of bisexual flowers, in which an insect is able to pick up and deliver pollen in the same visit, exemplifies continuing flower evolution. Grouping of flowers into inflorescence has also expedited pollination by insect visitors. The development of petals from stamens resulted in an additional floral structure having a phenomenal number of different shapes and specific modifications.

The larger number of flower and insect types that exist today is a result of this process of coevolution. Visitation specificity ranges from general, as in the primordial relationships, to specific one-to-one relationships. It is important to point out that, although the general trend in pollinator-flower coevolution is toward greater specificity, the resulting advantages are there only as long as both partners are successful in all other ways as well. If either member of a highly interdependent association fails, the other will fail (unless it is able to survive by previously unused mechanisms).

Dispersal into new areas is one of the critical requirements for plant success that is very closely tied to pollinator range expansion. In a sense, pollinators are the only legs plants have. Even if seed is dispersed into new areas, unless those plants are able to sexually reproduce, they are ultimately doomed to failure in that habitat. Interestingly, most successful colonizers are weedy plants found in new, disturbed, and changing habitats; these plants have unspecialized pollination systems.

Pollinator Specificity

The importance of pollinators for sexual reproduction in most flowering plants cannot be overemphasized. Because plants lack mobility, cross-pollination and the genetic variability it allows would be very limited without the activities of pollinators. The role of the "birds and the bees" is vital, since these two groups participate in some of the most reliable and species-specific pollinator-flower relationships. A brief and much generalized description of the most common pollinator organisms and the kind of flowers they visit will provide a better perspective of the proceeding discussions.

Beetles

Even though beetles were probably the earliest pollinators and their role in the successful evolution of angiosperms is important, today they are relatively overlooked as pollinators (see Figure 14-11). Generally, beetles are large, awkward, and poorly adapted as a flower pollinator. Their mouthparts are adapted for chewing; thus, they feed on edible tissues and pollen, seldom nectar. Most beetles that do visit flowers do not depend exclusively on flowers for their nourishment; rather, they primarily feed on other plant parts or dead animals, and as pollinators they are undependable. Certainly, not all beetle visitations are random; there are many examples of regular, predictable, and even highly specific visitations. There are even some beetles with mouthparts modified for nectar feeding and flowers that have developed attractants for beetle visitation.

Beetles pollinate even highly modified flowers, such as some orchids, but the typical flowers visited by beetles are far less specialized. Usually large and solitary or small and grouped into a large inflorescence, these flowers are often dull white to greenish with strong fruity or decaying proteinlike odors. Beetles' sense of smell is much more acute than is their vision, and they are attracted by odors that simulate their common food sources-fruit, carrion, and dung.

[FIGURE 14-12 OMITTED]

The flowers are usually open, flat or bowl shaped, and shallow, with plentiful pollen and accessible sex organs. In addition, the ovary is normally well protected, safe from the beetles' indiscriminate feeding habits and chewing mouthparts. Some well-known solitary flowers pollinated by beetles are species of the Magnolia genus, poppies, cactus, and lily. Inflorescence in the carrot family (umbels), dogwoods, and tropical members of the Fagaceae (beech family) are also beetle pollinated.

Bees

Of all the flower-visiting pollination organisms, bees are definitely the most well adapted, specific, and numerous. From small nonsocial bees to the larger social honeybee and bumblebees, these insects are experts in flower recognition, feeding, and pollination. Bees have hairy bodies, which are ideal for pollen transport, and bumblebees are known to carry as many as 15,000 grains per individual. Bees are able to readily learn shapes, patterns, and colors, and they have mouthparts modified for nectar feeding and pollen collecting. In addition, social bees have a large food demand, providing for themselves and their brood. They also have a communication system to inform others in their colony of the exact location of a bountiful food source, as shown in Figure 14-12.

An interesting phenomenon of bee vision is their range of color perception. Bees have a visible spectrum that is shifted into the ultraviolet wavelengths but out of the red range. Therefore, they are able to detect patterns produced on petals by ultraviolet-reflecting pigments but are essentially red colorblind. One of the groups of secondary plant compounds, the flavonoids, include some pigments often found in petals that reflect ultraviolet patterns. Two related species, both with the same color (to us) yellow flower, have distinct colors to a bee visitor--hence, differential visitation.

Bee and flower coevolution is specific and highly complex; as a result, some of the most unusual and highly modified flowers are bee pollinated. Typical bee-pollinated flowers are irregular, sturdy, fairly deep, and often have a "landing platform." Usually bright yellow or blue (but not red), bee-pollinated flowers commonly have color nectar guidelines running from the outer edges of the top surface of lower petals down the tube of the flower. Many also have semiclosed flower throats, which help prevent nectar thieving by smaller insect visitors too weak to force their way past the closure. Many flowers have hairy areas near the stigma that groom pollen off the bee's body to ensure pollination.

The flower tubes are often structurally designed so that only the appropriate bee species can get to the nectar--the reward for visiting that flower. In addition, the pathway to the nectar supply also requires that the visiting bee come in contact with the stigma, guaranteeing pollination. Pollen from the flower being visited is also deposited on the bee's legs, back, or underside, often by complicated modifications that break open anther sacs or force the departing bee to come into contact with the mature anther.

One of the most specialized and highly evolved bee-flower interdependencies involves a wild orchid in the genus Ophrys. The flowers of a given species open at a time in the spring when males of the coevolved bee species emerge. The flowers are the same size and shape as the female of the bee species (which does not emerge until later). The male bees are instinctively attracted to the flower and attempt to copulate with it. Although notably unsuccessful in this effort, the thrashing around on the flower does result in pollen deposition on the male bee's body. Repeated attempts to copulate with these flowers guarantee the next generation of bees. Pseudocopulation, as the bee-flower activity is termed, occurs between several specific members of the Ophrys genus species of bee, wasp, and even some flies.

Common to many bee flowers is a postpollination change in the appearance of the flower, probably induced chemically by the development of pollen tubes. Flowers are known to have their visual attractant markers change after pollination. The potential bee visitors then do not recognize the flower and thus pass it by. Were these bees to visit such flowers anyway, they would find no nectar and the desirability of future visits to flowers of that species would be diminished. Changes such as the fading of the nectar guidelines, wilting of the landing platform petal, general flower closure and withering, and color dullness are adaptations of some bee flowers that secure even greater visitation and pollination efficiency and success.

Butterflies and moths

Even though butterflies and moths are closely related and have similar morphology, as pollinators they are quite different. Butterflies, shown in Figure 14-13a, are active in the daytime and have good vision but a weak sense of smell. Moths, shown in Figure 14-13b, are nocturnal and have a well-developed sense of smell. Butterflies light on the flower, whereas moths hover. Both suck the thin nectar through their long, thin, hollow tongues, and neither has to provide food for developing young.

Usually yellow, blue, or in some cases red, butterfly flowers have a long, thin floral tube with a sturdy outer flower structure for adequate landing. Butterfly flowers include many members of the sunflower family (Asteraceae), Lantana, and various trumpet-shaped blossoms.

[FIGURE 14-13a OMITTED]

[FIGURE 14-13b OMITTED]

Moths visit white, pale yellow, or pink flowers that are open at night and produce a strong, heavily sweet perfume. These flowers also have deep tubes but with open or bent-back margins that allow hovering moths to reach the nectar with their long tongues. Generally, moth flowers produce more nectar than butterfly flowers because hovering expends more energy.

The hawk moth is a competitor of hummingbirds for their food sources. Essentially as large and remarkably similar in appearance to a hummingbird, this moth has a long proboscis for sucking up the thin sugary nectar of typical bird flowers. More commonly however, moth flowers are larger than bird flowers and open only at night. Yucca, evening primrose (Oenothera), and a number of night-blooming cactus species are moth-pollinated flowers.

The Yucca is pollinated only by the four species of a single moth genus (Tegeticula), which in turn depends on the Yucca flower for its entire reproductive cycle. The female moth collects pollen from one flower and transports it to another. It pierces the ovary wall and lays eggs inside the ovary. Only about 20% of the developing Yucca seeds are destroyed by the feeding larvae, which eat their way out of the ovary when they are mature so that they may pupate on the ground below the plant. The female yucca moth ensures a food source for her larvae while pollinating the flower.

Birds

Different kinds of birds act as pollinators by feeding on nectar or on insects within the flowers. Sparrows are known to visit spring crocus and have been observed pollinating pear trees. Honeycreepers pollinate the Hawaiian lobelias, and the sunbirds of Africa and Asia are known pollinators. The most well-known bird pollinators, however, are the hummingbirds of North America. These tiny animals expend incredible amounts of energy in flight, especially in hovering while feeding. They have keen eyesight, being most responsive to reds and some yellows, but do not have a well-developed sense of smell. Their long thin beaks enable them to reach the abundant, thin, sweet nectar. Their feathers carry large amounts of pollen, picked up primarily on the front and top of their heads when they come into contact with stamens extruded beyond the floral tube (see Figure 14-14).

Typical flowers pollinated by hummingbirds are red, with a good supply of thin, sweet nectar found at the bottom of a slender floral tube. The lips of the flowers are usually curved back out of the way, and they are normally a solid color, lacking nectar guides. These flowers usually have little or no scent, which, in combination with their color and long slender tubes, usually make them unnoticeable to most insect visitors. Sugar ants are a notable exception, but they provide very little competition to hummingbirds. Striking examples of bird flowers include Erythrina, Aquilegia (columbines), orchids, Salvia, Mimulus, and Lobelia species that have red flowers.

[FIGURE 14-14 OMITTED]

Flies

Flies display greater variation in their methods and tendencies of pollination than any other insect groups. Primitive flies parallel beetles in their lack of sophistication; highly specialized flies are comparable to bumblebees and hawk moths in complexity. Some South African flies have a 5 mm long proboscis and the ability to hover with nectar feeding. In spite of the range of variability, the best known pollinators are the carrion and dung flies. Because they lay their eggs in diseased or decaying animal flesh or on fresh dung, they are attracted by the putrid odors of decomposing protein. Flowers that are pollinated by these flies usually attract with strong odors but offer no reward to the visitor. The "carrion" plant Stapelia is the best known of the fly-pollinated flowering plants, carrying the attraction mimicry even a step further. Stapelia flowers vary in size depending on the species, but they are flat and open with the dull yellow color of decaying meat, complete with reddish streaks. Most notable, however, is the odor, which does not invite a second sniff.

Ants

Ants are exceptionally fond of sweets, from table sugar to flower nectar. They are also so small that they can raid a flower without touching anthers or stigma. In addition, their bodies are hard and not well adapted to pollen transport, and they are aggressive defenders of a newfound food source. As pollinators, therefore, ants are essentially noneffective, although there are a few isolated examples of larger ants providing nonspecialized or accidental pollination for some flowers.

Their aggressive defense of a food source often chases away other insects, including those which would actually effect pollination were they allowed to visit the blossom. Some flowering plants have actually evolved "ant guard" adaptations that deter ant visitations. One of the most successful is the ring of sticky glandular hairs on the stem immediately below the flower of Viscoria vulgaris. Stiff hairs projecting downward on the stem or outward in the throat of a corolla tube are other common ant guards that have evolved in some species.

Mosquitoes

Mosquitoes also are too small and ill designed for effective pollination; however, some flowers are, in fact, mosquitoes pollinated. Certain small and inconspicuous orchids are visited by both male and female mosquitoes, which feed on nectar rather than blood. The mouthparts of several mosquitoes' species are actually modified, which feed on nectar rather than bloodsucking. The food demands of mosquitoes are very small, and even though they carry pollen with them as they visit from flower to flower; they seldom need to visit from flower to flower beyond those on the same branch of one plant. As cross-pollinating agents, then, they are less effective than other insects that also visit many of the same flowers.

Bats

Like birds, bats, as shown in Figure 14-15, have several effective pollinating features. They are large, have a rough (furry) surface for holding large amounts of pollen, and can move rapidly across large distances. Most bats are insect eaters, but a number of vegetarian bat species exist. Fruit-eating bats are found worldwide, and it is hypothesized that nectar and pollen feeding developed from such lines; as it did in bird species.

[FIGURE 14-15 OMITTED]

Pollinator bats are nocturnal, have an acute sense of smell over great distances, and unlike most bats, have acute vision (although color-blind). Their sonar system is less developed; therefore, flying in densely vegetated areas is difficult. Many of these bats can also hover like hummingbirds and have long, slender noses and long tongues.

As these bats fly from flower to flower, they lap up the nectar, often eat parts of the flower, and transport large amounts of pollen in their head fur. Although some species ingest pollen only accidentally while feeding on nectar, others use long tongues to lick pollen off their heads, a major portion of their diets.

It is now known that some bats actually depend exclusively on nectar and pollen for their food, and the flowers that these bats visit contain high amounts of protein in their pollen.

Although flower-visiting bats enjoy mostly tropical distribution, some migrate in the summer as far north as the southern United States and northern Mexico, feeding on Agave (century plant) and cactus flowers. Bat pollination is also known in Australia (on an introduced Agave) and in Asia as far north as the Philippines. In Africa, bat pollination does not extend north of the Sahara, and in the Pacific Islands their distribution extends south to Fiji. Certain plant distribution can be explained by knowing about bat-pollinated flower types. For example, there is a banana plant (Musa fehi) that is adapted to bat pollination. However, it is thought to have been introduced to Hawaii, not native, because bats are not indigenous to Hawaii.

Typical bat-pollinated flowers open only at night and only for one night. They are often drab greenish to pink-purple and sometimes white or creamy. They emit a strong fruity or fermenting odor at night and have a very large quantity of both nectar, and pollen held in larger or numerous anthers. These are generally large, sturdy, and solitary or in the inflorescence positioned outside the foliage of the plant for increased pollinator accessibility.

Other Angiosperm-Animal Coevolution

In the evolution, diversification, and adaptation of the flowering plants, other specific coevolution with various animal groups has taken place. Fruit and seed dispersal mechanisms and morphological adaptations to prevent herbivory are obvious and easily demonstrated examples. Less evident are chemical groups that are now thought to exist as a result of coevolutionary interactions. These were once regarded by plant biochemists as dead-end or nonessential secondary compounds. But many of these chemical groups are now thought to be involved in specific processes of plant protection, pollinator attraction, and feeding stimulation.

We have already mentioned certain flavonoid compounds--ultraviolet wavelength pigments in some flower petals that are visible to bee pollinators. Closely related chemically are the anthocyanins, which are visible wavelength flower pigments. The Ophrys flowers provoke pseudocopulation because of visible patterns produced by anthocyanin pigment.

Plant Palatability

Plant palatability is significantly influenced by secondary compounds. Both insect and vertebrate animal groups including humans are attracted or repulsed by a range of secondary compounds that affect the sense of taste. Members of the mustard family, for example, contain compounds that deter many insects and people from feeding on them, whereas other insects feed only on these plants. The acid taste and pungent odor of horseradish, cabbage, cauliflower, radish, watercress, mustard, Brussels sprouts, and rutabaga is caused by the same class of chemical compounds. Scientists have proven that this group of plants when eaten by humans can cut down the cause of certain cancers. These undoubtedly coevolved with specific insect groups, protecting the plants from excessive herbivory by most insects while providing a constant food source for insects adapted to tolerate or even be actually attracted to these compounds.

Alkaloids

A large group of loosely related, nitrogen-containing compounds are alkaloids, which are toxic. Toxicity, combined with an unappealing bitter taste, protects plants containing such compounds from being eaten. In the milkweed family (Asclepiadaceae), for example, alkaloid compounds and cardiac glycosides combine to act as severe toxins to most vertebrate animals. Foxglove (Digitalis purpurea in the Scrophulariaceae) also produces cardioactive glycosides that are used as a treatment for heart disease but in higher doses can endure a heart attack in vertebrate consumers. Certain insects, however, are unaffected by these compounds and preferentially feed as larvae or as adults on the tissue of such plants. In fact, these insects even "advertise" that their bodies contain these compounds by being brightly colored and patterned. They are recognizable to potential predators, which are predominantly vertebrates, and after one experience of acute digestive upset, vomiting, and diarrhea, the predator avoids further ingestion of such insects.

Monarch butterflies feed on milkweeds, and their distinctive visibility to birds acts as a protective device rather than as an attractant. So effective is this coloration and pattern that the viceroy butterfly has an example of mimicry, a process that has occurred repeatedly in both plant and animal groups. For many plant species that have evolved in a parallel manner, they are protected by their close resemblance.

Many of the secondary compounds that have evolved in plants as protective adaptations or as attractants produce strong effects in humans. The opium poppy, the hemp plant (Cannabis, or marijuana), and peyote cactus are among these plants. All contain secondary compounds, mostly alkaloids that occur in nature because of specific coevolutionary phenomena.

Biochemical Evolution

As plants evolve into different forms and develop new strategies for adaptation, they may also be selected against if they do not also develop new modes of metabolism; in some cases, the biochemistry actually changes. There is strong evidence that both plants and animals adapt to environmental stresses by synthesizing isozymes, or alternate forms of an enzyme, which may allow the plant to carry on photosynthesis, respiration, protein synthesis, or other roles under changed environmental conditions. Given plants appear to synthesize isozymes for water stress, salt stress, heat stress, and cold stress. Those individuals of a population that do so may survive in a changing environment, whereas the plants capable of synthesizing only the normal enzyme may fall.

Biochemical evolution also includes the alternative strategies for carbon fixation (photosynthesis). Most plants fix carbon via the C3, or Calvin, cycle. Other plants that have evolved under conditions of high temperature, high light intensity, and water stress have developed an alternative method, the C4 pathway. The C4 phenomenon is widespread among monocot and dicot families, which have evolved in the hot and drier parts of the world. Such plants have managed to survive continental movements and changes in world climates largely because they have been able to change certain aspects of their metabolism.

Ecology and Importance of Angiosperms

A unifying theme of this text is the application of plant science in a human-dominated world. Everywhere one looks, flowering plants influence our lives. Except for the coniferous forest, which provides timber for housing and other construction, angiosperms are the dominant plants for food, forage for animals, commercial fibers, industrial products, and medicines. Even most of our landscaping materials are flowering plants. There is no accurate method to assess the absolute value of angiosperms in our lives. A price tag can be placed on food or other items of trade, but higher plants essentially and complexly influence our very being. Societies that depend on firewood for survival are exquisitely aware of the importance of angiosperms.

Finally, the angiosperms have so totally invaded the terrestrial ecosystems that species are found almost everywhere. No other group of plants has been so successful in filling niches under the most extreme environmental conditions. Such plants compete most effectively for available light energy and contribute incalculably as producer organisms.

Summary

1. Approximately 450 million years ago a primitive multicellular marine green algae washed up on shore and was able to survive the ultraviolet radiation of the sun because of an atmospheric ozone layer that had been accumulating since the first cyanobacteria began photosynthesizing some 3 billion years ago.

2. In addition to the bryophytes, primitive vascular plants began evolving. Their vascular system plus protected sex cells, spores, and cuticle development allowed them to succeed and diversify rapidly on land. Three extinct primitive vascular groups gave rise to the four major divisions of living seedless vascular plants.

3. Ferns have fiddlehead coiled leaf fronds, and the reproductive structures, sori, occur on the underside of the fronds. The sporangia within each sorus produce spores that germinate and develop into a prothallus, which grows into a heart-shaped gametophyte. Ferns are most important economically as ornamentals.

4. Seed plants are even better adapted to extreme climates than are spore-bearing plants. The two seed-producing groups are the gymnosperms and angiosperms.

5. Gymnosperms include the cycads, gnetophytes (including Ephedra), Gingko, and conifers. By far the largest and most evident group is the conifers, which reproduce by having separate male and female cones. From wind pollination to fertilization takes about 15 months in pine, and with an additional year before mature seeds are ready to be released by the female cone.

6. The angiosperms are today's dominant vegetation, comprising some 235,000 species. They are thought to have evolved from a broad-leaved gymnosperm between 125 and 200 million years ago. Flowering plants occupy a wide range of ecological habitats and vary in size from a few centimeters to 100 m tall.

7. Flower parts evolved from leaves, and primitive flowers commonly have many of each floral part. More highly derived flowers have reduced numbers of floral parts, an inferior ovary, and bilateral symmetry. The Magnoliaceae are considered to be one of the most primitive angiosperm families, the Orchidaceae and the Asteraceae are among the most highly evolved families. Fruits have also evolved into more complex types.

8. Flowering plants and insects have coevolved, providing both groups with several advantages. Originally only beetles acted as pollinators; now wasps, bees, butterflies, moths, ants, mosquitoes, birds, and bats all act as pollinators for an incredible array of flowering plants.

9. Beetles are clumsy, general visitors; bees and wasps are highly specialized visitors attracted by complex shapes, color (except red) markings, and even ultraviolet patterns. Butterflies are active in the daytime, visiting yellow, blue, and red flowers with thin nectar and sturdy corollas. Moths are nocturnal and prefer stronger-smelling light-colored flowers. Hummingbirds are the most common bird pollinators, visiting red, tubular flowers with a lot of thin nectar. Flies, ants, and mosquitoes effectively pollinate a wide range of flower types, including some that attract flies by their rotting meat smell. Bats visit large, nocturnal flowers with a strong perfume smell, thin nectar, and copious pollen.

10. Secondary plant compounds are thought to protect plants from herbivores and insects. Some of these compounds have strong physiological effects on humans. No other plant groups have more utility to human society than the angiosperms.

Something to Think About

1. What made the environment of the Earth allow plants to start photosynthesize?

2. Plants with vascular systems were able to survive and succeed on land because of what?

3. How many Pterophyta exist on Earth?

4. Diagram how Pterophyta reproduce.

5. Compare and contrast the spore-producing plants with the seed-producing plants.

6. Which is the largest group of plants?

7. List the difference in angiosperms and gymnosperms.

8. How long does it take for pollination and fertilization to complete?

9. Trace the evolution of angiosperms.

10. What did the flower evolve from?

11. Name the plant family that is considered to be one of the most primitive.

12. Describe flower symmetry.

13. Which two plant families are considered to be the most evolved?

14. List different pollinators.

15. Which plant group is considered to be the most utilitarian to human society?

Suggested Readings

Belly, P. R., and R. Hemsley. 2004. Green plants: Their origin and diversity. Cambridge, MA: Cambridge University Press.

Dowson, J., and R. Lucas. 2005. The natural habitats, challenges, and adaptations. Portland, OR: Timber Press.

Gaston, K. J., and J. I. Spicer. 2004. Biodiversity: An introduction. Oxford: Blackwell Publishing Professional.

Huston, M. 2002. Biological diversity. Cambridge, MA: Cambridge University Press.

Moran, R. C. 2004. A natural history of ferns. Portland, OR: Timber Press.

Raven, P., et al. 1999. Biology of Plants (6th ed.). New York: W. H. Freeman.

Internet

Internet sites represent a vast resource of information. The URLs for Web sites can change. Using one of the search engines on the Internet, such as Google, Yahoo!, Ask.com, or MSN Live Search, find more information by searching for these words or phrases: after riping, dioecious, generative cell, integument, microsporangia, microspore mother cell, nucellus, ovule, polyembryony, seed coat, self-incompatible, and tube cell.
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Title Annotation:PART 4: Evolution and Diversity
Publication:Fundamentals of Plant Science
Date:Jan 1, 2009
Words:10607
Previous Article:Chapter 13: Genetic engineering and biotechnology.
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