Chapter 30 Plant Diversity II: The Evolution of Seed Plants

Lecture Outline

Overview: Feeding the World

·        The seed arose about 360 million years ago.

°        Seed plants, including gymnosperms and angiosperms, have come to dominate modern landscapes and make up the great majority of plant biodiversity.

·        Agriculture, the cultivation and harvest of plants (especially angiosperms), began 13,000 years ago.

°        Humans began the cultivation of plants independently in various regions, including the Near East, East Asia, Africa, and the Americas.

°        This was the single most important cultural change in the history of humanity, and it made possible the transition from hunter-gatherer societies to permanent settlements.

 

Concept 30.1 The reduced gametophytes of seed plants are protected in ovules and pollen grains

·        A number of terrestrial adaptations contributed to the success of seed plants.

°        These adaptations include the seed, the reduction of the gametophyte generation, heterospory, ovules, and pollen.

·        Bryophyte life cycles are dominated by the gametophyte generation, while seedless vascular plants have sporophyte-dominated life cycles.

·        The trend to gametophyte reduction continued in the lineage of vascular plants that led to seed plants.

°        Seedless vascular plants have tiny gametophytes that are visible to the naked eye.

°        The gametophytes of seed plants are microscopically small and develop from spores retained within the moist sporangia of the parental sporophyte.

·        In seed plants, the delicate female gametophyte and the young sporophyte embryo are protected from many environmental stresses, including drought and UV radiation.

°        The gametophytes of seed plants obtain nutrients from their parents, while the free-living gametophytes of seedless vascular plants must fend for themselves.

 Heterospory is the rule among seed plants.

·        Nearly all seedless plants are homosporous, producing a single kind of spore that forms a hermaphroditic gametophyte.

°        Seed plants likely had homosporous ancestors.

·        All seed plants are heterosporous, producing two different types of sporangia that produce two types of spores.

°        Megasporangia produce megaspores, which give rise to female (egg-containing) gametophytes.

°        Microsporangia produce microspores, which give rise to male (sperm-containing) gametophytes.

 Seed plants produce ovules.

·        In contrast to the few species of heterosporous seedless vascular plants, seed plants are unique in retaining their megaspores within the parent sporophyte.

·        Layers of sporophyte tissue, integuments, envelop and protect the megasporangium.

°        Gymnosperm megaspores are surrounded by one integument.

°        Angiosperm megaspores are surrounded by two integuments.

·        An ovule consists of the megasporangium, megaspores, and integuments.

·        A female gametophyte develops from a megaspore and produces one or more egg cells.

 Pollen eliminated the liquid-water requirement for fertilization.

·        The microspores develop into pollen grains that are released from the microsporangium.

°        Pollen grains are covered with a tough coat containing sporopollenin.

°        They are carried by wind or animals.

°        The transfer of pollen to the vicinity of the ovule is called pollination.

·        The pollen grain germinates and grows as a pollen tube into the ovule, where it delivers one or two sperm into the female gametophyte.

·        Bryophytes and seedless vascular plants have flagellated sperm cells that swim a few centimeters through a film of water to reach the egg cells within the archegonium.

·        In seed plants, the female gametophyte is retained within the sporophyte ovule.

·        Male gametophytes travel long distances as pollen grains.

°        The sperm of seed plants lack flagella and do not require a film of water, as they rely on the pollen tube to reach the egg cell of the female gametophyte within the ovule.

·        The sperm of some gymnosperm species retain the ancestral flagellated condition, providing evidence of this evolutionary transition.

·        The evolution of pollen contributed to the success and diversity of seed plants.

 Seeds became an important means of dispersing offspring.

·        What is a seed?

°        When a sperm fertilizes an egg of a seed plant, the zygote forms and develops into a sporophyte embryo.

°        The ovule develops into a seed, consisting of the embryo and its food supply within a protective coat derived from the integuments.

·        The evolution of the seed enabled plants to resist harsh environments and disperse offspring more widely.

·        For bryophytes and seedless vascular plants, single-celled spores are the only protective stage in the life cycle.

°        Moss spores can survive even if the local environment is too cold, too hot, or too dry for the moss plants themselves to survive.

°        Because of their tiny size, the spores themselves can be dispersed in a dormant state to a new area.

°        Spores were the main way that plants spread over Earth for the first 100 million years of life on land.

·        The seed represents a different solution to resisting harsh environments and dispersing offspring.

°        In contrast to a single-celled spore, a multicellular seed is a much more complex, resistant structure.

°        After being released from the parent plant, a seed may remain dormant for days or years.

°        Under favorable conditions, it germinates and the sporophyte embryo emerges as a seedling.

 

Concept 30.2 Gymnosperms bear “naked” seeds, typically on cones

·        The ovules and seeds of gymnosperms (“naked seeds”) develop on the surfaces of modified leaves that usually form cones (strobili).

°        In contrast, ovules and seeds of angiosperms develop in enclosed chambers called ovaries.

·        The most familiar gymnosperms are the conifers, cone-bearing trees such as pine, fir, and redwood.

 The four phyla of extant gymnosperms are Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta.

·        There are four plant phyla grouped as gymnosperms.

·        Phylum Ginkgophyta consists of only a single extant species, Ginkgo biloba.

°        This popular ornamental species has fanlike leaves that turn gold before they fall off in the autumn.

°        Landscapers usually plant only male trees because the coats of seeds produced by female plants produce a repulsive odor as they decay.

·        Cycads (phylum Cycadophyta) have large cones and palmlike leaves.

°        130 species of cycads survive today.

°        Cycads flourished in the Mesozoic era, which was known as the “Age of Cycads.”

·        Phylum Gnetophyta consists of three very different genera.

°        Weltwitschia plants, from deserts in southwestern Africa, have straplike leaves that are among the largest known leaves.

°        Gentum species are tropical trees or vines.

°        Ephedra (Mormon tea) is a shrub of the American deserts.

·        The conifers belong to the largest gymnosperm phylum, the phylum Coniferophyta.

°        The term conifer comes from the reproductive structure, the cone, which is a cluster of scalelike sporophylls.

°        Although there are only about 600 species of conifers, a few species dominate vast forested regions in the Northern Hemisphere where the growing season is short.

·        Conifers include pines, firs, spruces, larches, yews, junipers, cedars, cypresses, and redwoods.

·        Most conifers are evergreen, retaining their leaves and photosynthesizing throughout the year.

°        Some conifers, like the dawn redwood and tamarack, are deciduous, dropping their leaves in autumn.

·        The needle-shaped leaves of some conifers, such as pines and firs, are adapted for dry conditions.

°        A thick cuticle covering the leaf and the placement of stomata in pits further reduce water loss.

·        Much of our lumber and paper comes from the wood (actually xylem tissue) of conifers.

°        This tissue gives the tree structural support.

·        Coniferous trees are amongst the largest and oldest organisms of Earth.

°        Redwoods from northern California can grow to heights of over 100 m.

°        One bristlecone pine, also from California, is more than 4,600 years old, and may be the world’s oldest living organism.

 The Mesozoic era was the age of gymnosperms.

·        The gymnosperms probably descended from progymnosperms, a group of Devonian plants that were heterosporous but lacked seeds.

·        The first seed plants to appear in the fossil record were gymnosperms dating from around 360 million years ago.

°        Angiosperms arose more than 200 million years later.

°        The two surviving clades of seed plants are gymnosperms and angiosperms.

·        Early gymnosperms lived in Carboniferous ecosystems dominated by seedless vascular plants.

·        The flora and fauna of Earth changed dramatically during the formation of the supercontinent Pangaea in the Permian.

°        Climatic conditions became warmer and drier, favoring the spread of gymnosperms.

°        Many groups of organisms disappeared while others emerged.

·        Amphibians decreased in diversity and were replaced by reptiles, which were better adapted to dry conditions.

·        The lycophytes, horsetails, and ferns that had dominated in Carboniferous swamps were largely replaced by gymnosperms.

·        The change in organisms was so dramatic that geologists use the end of the Permian, 251 million years ago, as the boundary between the Paleozoic (“old life”) and Mesozoic (“new life”) eras.

°        The terrestrial animals of the Mesozoic, including dinosaurs, were supported by a vegetation consisting mostly of conifers and cycads, both gymnosperms.

·        The dinosaurs did not survive the mass extinction at the end of the Mesozoic, but many gymnosperms persisted and are still an important part of Earth’s flora.

 The life cycle of a pine demonstrates the key reproductive adaptations of seed plants.

·        The life cycle of a pine illustrates the three key adaptations to terrestrial life in seed plants:

1.     Increasing dominance of the sporophyte.

2.     The advent of the seed as a resistant, dispersal stage in the life cycle.

3.     The evolution of pollen as an airborne agent bringing gametes together.

·        The pine tree is the sporophyte.

°        It produces its sporangia on scalelike sporophylls that are packed densely on cones.

·        Conifers, like all seed plants, are heterosporous.

·        Male and female gametophytes develop from different types of spores produced by separate cones: small pollen cones and large ovulate cones.

°        Most pine species produce both types of cones.

·        A pollen cone contains hundreds of microsporangia held on small sporophylls.

°        Each cone produces microspore mother cells that undergo meiosis to produce haploid microspores.

°        Each microspore develops into a pollen grain containing a male gametophyte.

·        A larger ovulate cone consists of many scales, each with two ovules.

°        Each ovule includes a megasporangium.

·        Ovulate cones produce megaspore mother cells that undergo meiosis to produce four haploid cells, one of which will develop into a megaspore.

°        Surviving megaspores develop into female gametophytes, which are retained within the sporangia.

°        Two or three archegonia, each with an egg, develop within the gametophyte.

·        During pollination, windblown pollen falls on the ovulate cone and grows into the ovule through the micropyle.

°        Fertilization of egg and sperm follows.

·        The pine embryo, the new sporophyte, has a rudimentary root and several embryonic leaves.

°        The female gametophyte surrounds and nourishes the embryo.

°        The ovule develops into a pine seed, which consists of an embryo (new sporophyte), its food supply (derived from gametophyte tissue), and a seed coat derived from the integuments of the parent tree (parent sporophyte).

·        It takes three years from the appearance of young cones on a pine tree to the formation of mature seeds.

°        The scales of ovulate cone separate and the seeds are typically dispersed by the wind.

·        A seed that lands in a habitable place germinates, and its embryo emerges as a pine seedling.

 

Concept 30.3 The reproductive adaptations of angiosperms include flowers and fruits

·        Angiosperms, commonly known as flowering plants, are vascular seed plants that produce flowers and fruits.

°        They are the most diverse and geographically widespread of all plants, including more than 90% of plant species.

·        There are about 250,000 known species of angiosperms.

°        All angiosperms are placed in a single phylum, the phylum Anthophyta.

 The flower is the defining reproductive adaptation of angiosperms.

·        The flower is an angiosperm structure specialized for sexual reproduction.

°        In many species of angiosperms, insects and other animals transfer pollen from one flower to female sex organs of another.

°        Some species that occur in dense populations, like grasses, are wind pollinated.

·        A flower is a specialized shoot with up to four circles of modified leaves: sepals, petals, stamens, and carpals.

·        The sepals at the base of the flower are modified leaves that are usually green and enclose the flower before it opens.

·        The petals lie inside the ring of sepals.

°        These are often brightly colored in plant species that are pollinated by animals.

°        They typically lack bright coloration in wind-pollinated plant species.

°        Sepals and petals are sterile floral parts, not directly involved in reproduction.

·        Stamens, the male reproductive organs, are sporophylls that produce microspores that will give rise to pollen grains containing male gametophytes.

°        A stamen consists of a stalk (the filament) and a terminal sac (the anther) where pollen is produced.

·        Carpals are female sporophylls that produce megaspores and their products, female gametophytes.

°        At the tip of the carpal is a sticky stigma that receives pollen.

°        A style leads to the ovary at the base of the carpal.

°        Ovules are protected within the ovary.

 Fruits help disperse the seeds of angiosperms.

·        A fruit usually consists of a mature ovary.

°        As seeds develop from ovules after fertilization, the wall of the ovary thickens to form the fruit.

°        Fruits protect dormant seeds and aid in their dispersal.

·        The fruit develops after pollination triggers hormonal changes that cause ovarian growth.

°        The wall of the ovary becomes the pericarp, the thickened wall of the fruit.

°        The other parts of the flower wither away in many plants.

°        If a flower has not been pollinated, the fruit usually does not develop, and the entire flower withers and falls away.

·        Mature fruits can be fleshy or dry.

°        Oranges and grapes are fleshy fruits, in which one or more pericarp layers soften during ripening.

°        Dry fruits include beans and grains.

°        The dry, wind-dispersed fruits of grasses are major food staples for humans.

°        The cereal grains of wheat, rice, and maize are fruits with a dry pericarp that adheres to the seed coat of the seed.

·        Fruits are classified according to whether they develop from a single ovary, from multiple ovaries, or from more than one flower.

·        By selectively breeding plants, humans have capitalized on the production of edible fruits.

·        Fruits are adapted to disperse seeds.

°        Winged seeds may function as kites or propellers to assist wind dispersal.

°        Coconuts are specialized for water dispersal.

°        Some fruits are modified as burrs that cling to animal fur.

°        Many fruits are edible, nutritious, sweet tasting, and colorful.

§        These fruits rely on animals to eat the fruit and deposit the seeds, along with a supply of fertilizer, some distance from the parent plant.

 The life cycle of an angiosperm is a highly refined version of the alternation of generations common to all plants.

·        All angiosperms are heterosporous, producing microspores that form male gametophytes and megaspores that form female gametophytes.

°        The immature male gametophytes are contained within pollen grains, which develop within the anthers of stamens.

§        Each pollen grain has two haploid cells: a generative cell that divides to form two sperm and a tube cell that produces a pollen tube.

°        The ovule, which develops in the ovary, contains the female gametophyte, the embryo sac.

§        The embryo sac consists of only a few cells, one of which is the egg.

·        The life cycle of an angiosperm begins with the formation of a mature flower on a sporophyte plant and culminates in a germinating seed.

1.     Anthers contain microsporangia, containing microspore mother cells that produce microspores by meiosis.

2.     Microspores form pollen grains, which are immature male gametophytes.

3.     In the ovule, the megaspore mother cell produces four megaspores by meiosis.

°        One megaspore survives and forms a female gametophyte, or embryo sac.

4.     The pollen is released from the anther and carried to the sticky stigma of the carpel.

°        Most flowers have mechanisms to ensure cross-pollination.

5.     The pollen grain germinates and is now a mature male gametophyte.

°        The pollen tube grows down within the style.

°        After reaching the ovary, the pollen tube penetrates the micropyle, a pore in the integuments of the ovule.

°        Two sperm are discharged into the female gametophyte.

§        One fertilizes the egg to form a diploid zygote.

§        The other fuses with two polar nuclei in the large central cell of the embryo sac to form the triploid endosperm nucleus.

°        Double fertilization is unique to angiosperms.

6.     The zygote develops into an embryo that is packaged with food into the seed.

°        The embryo has a rudimentary root and one or two seed leaves, or cotyledons.

7.     When a seed germinates, the embryo develops into a mature sporophyte.

·        Monocots store most of the food for the developing embryo as endosperm, which develops as a triploid tissue in the center of the embryo sac.

°        Beans and many dicots transfer most of the nutrients from the endosperm to the developing cotyledons.

·        One hypothesis for the function of double fertilization is that it synchronizes the development of food storage in the seed with development of the embryo.

°        Double fertilization may prevent flowers from squandering nutrients on infertile ovules.

·        Another type of double fertilization, in which two embryos are formed, has evolved independently in gymnosperms of the phylum Gnetophyta.

·        The seed consists of the embryo, endosperm, remnants of the sporangium, and a seed coat derived from the integuments.

·        As the ovules develop into seeds, the ovary develops into a fruit.

·        After dispersal by wind or animals, a seed germinates if environmental conditions are favorable.

°        During germination, the seed coat ruptures and the embryo emerges as a seedling.

°        It initially uses the food stored in the endosperm and cotyledons to support development.

 The origin and evolution of angiosperms is complex.

·        Earth’s landscape changed dramatically with the origin and radiation of flowering plants.

·        The oldest angiosperm fossils are about 140 million years old.

·        By the end of the Cretaceous period, 65 million years ago, angiosperms had become the dominant plants on Earth.

·        In the late 1990s, scientists in China discovered fossils of 125-million-year-old angiosperms named Archaefructus liaoningensis and Archaefructus sinensis.

·        These fossils display both derived and primitive traits.

°        A. sinensis has anthers and seeds inside closed carpels but lacks petals and sepals.

°        This species may be a “proto-angiosperm,” suggesting that the ancestors of flowering plants were herbaceous rather than woody.

°        It was found along with fish fossils and may be aquatic.

§        Some paleobotanists suggest that angiosperms originated as aquatic plants.

§        Others dispute this, pointing out that aquatic angiosperms tend to evolve simpler flowers such as the “primitive” flowers of Archaefructus.

·        An “evo-devo” approach, synthesizing evolutionary and developmental biology, has lead to a hypothesis about the evolution of bisexual flowers.

°        The “mostly male” hypothesis proposes that the ancestor of angiosperms had separate male and female structures, and that, as a result of a mutation, ovules developed on some microsporophylls, which evolved into carpels.

°        Flower-development genes are usually related to gymnosperm pollen-producing genes.

°        Certain mutations cause angiosperms to grow ovules on sepals and petals, demonstrating that the position of ovules can change.

 Angiosperms are very diverse.

·        Angiosperms have diversified into more than 250,000 species that dominate most terrestrial ecosystems.

·        Until the late 1990s, flowering plants were divided into monocots and dicots on the basis of number of cotyledons or seed leaves.

·        Current research supports the view that monocots form a clade but reveals that dicots are not monophyletic.

·        The majority of plants traditionally called “dicots” form a clade now known as “eudicots.”

·        The remaining plants are divided into several small lineages.

·        Three of these lineages are called basal angiosperms, because they include the oldest known lineages of flowering plants.

°        Amborella is a basal angiosperm that lacks vessels that are found in more derived angiosperms.

·        Another lineage is the magnoliids.

°        Magnoliids include 8,000 species, including magnolias.

°        These angiosperms share primitive traits such as spiral arrangement of floral parts with the basal angiosperms.

·        One quarter of angiosperms are monocots.

°        Monocot traits include single cotyledons, parallel venation, scattered vascular bundles, fibrous root systems, pollen grains with a single opening, and floral parts in multiples of three.

·        More than two-thirds of angiosperms—170,000 species—are eudicots.

°        Eudicot traits include two cotyledons, netlike venation, vascular bundles arranged as a ring, a taproot, pollen grains with three openings, and floral parts in multiples of four or five.

 Animals and angiosperms share evolutionary links.

·        Ever since they colonized the land, animals have influenced the evolution of terrestrial plants and vice versa.

·        Plants and animals have been important selective agents on one another.

°        Natural selection favored plants that kept their spores and gametophytes above the ground, rather than dropping them within the reach of hungry ground animals.

°        This may, in turn, have been a selective factor in the evolution of flying insects.

·        Some herbivores were beneficial to plants by dispersing their pollen and seeds.

°        The animals received a benefit in turn, as they ate the nectar, seeds, and fruits of plants.

·        Natural selection reinforced these interactions when they improved the reproductive success of both partners.

·        Pollinator-plant relationships are partly responsible for increased diversity of angiosperms and animals.

°        In many cases, a plant species may be pollinated by a group of pollinators, such as many species of bees or hummingbirds, and have evolved flower color, fragrance, and structures to facilitate this.

°        Conversely, a single species, such as a honeybee species, may pollinate many plant species.

°        Some individual species of flower can only be pollinated by a single animal species.

§        In Madagascar, one species of orchid has an 11-inch long nectary and can only be pollinated by a moth species with an 11-inch proboscis.

§        Such linked adaptations, involving reciprocal genetic modifications in two species, are coevolution.

·        The expansion of grasslands over the past 65 million years has increased the diversity of grazing animals such as horses.

°        Grasses are C₄ photosynthesizers that spread as declining atmospheric CO₂ levels gave them a selective advantage.

·        The shift from forests to grasslands in Africa between 10 and 2 million year ago was crucial to hominid evolution.

 

Concept 30.4 Human welfare depends greatly on seed plants

·        The absolute dependence of humans on Earth’s flora is a specific and highly refined case of the more general connection between animals and plants.

°        Like other organisms, we depend on photosynthetic organisms for food production and oxygen release.

°        However, we use technology to manipulate or select plants that maximize the harvest of plant products for human use.

°        We rely on seed plants for food, fuel, wood, and medicine.

 Agriculture is based almost entirely on angiosperms.

·        Flowering plants provide nearly all our food.

·        Just six crops—wheat, rice, maize, potatoes, cassava, and sweet potatoes—yield 80% of all calories consumed by humans.

·        Modern crops are the products of a relatively recent burst of genetic change, resulting from artificial selection after the domestication of plants 13,000 years ago.

°        In maize, key changes such as increased cob size and removal of the hard coating of the kernels may have been initiated by as few as five gene mutations.

·        How did wild plants change so dramatically so quickly?

°        The answer is likely a combination of deliberate and unconscious selection for plants with desirable traits, such as large fruits and lack of toxins.

·        Angiosperms also provide important nonstable foods such as coffee, chocolate, and spices.

·        Gymnosperms and angiosperms are sources of wood, which is absent in all living seedless plants and consists of an accumulation of tough-walled xylem cells.

°        Wood is the primary source of fuel for much of the world.

°        It is used to make paper, and is the world’s most widely used construction material.

·        Humans depend on seed plants for medicines.

°        Most cultures have a tradition of herbal medicine.

°        Scientific research has identified the relevant secondary compounds in many of these plants, leading to the synthesis of many modern medicines.

 Plant diversity is a nonrenewable resource.

·        Although plants are a renewal resource, plant diversity is not.

·        The demand for space and natural resources resulting from the exploding human population is extinguishing plant species at an unprecedented rate.

·        This is especially acute in the tropics, where more than half the human population lives and where population growth rates are highest.

°        Due primarily to the slash-and-burn clearing of forests for agriculture, tropical forests may be completely eliminated within 25 years.

·        As the forests disappear, thousand of plant species and the animals that depend on these plants also go extinct.

°        The destruction of these areas is an irrevocable loss of these nonrenewable resources.

°        The rate of loss is faster than in any other period, even during the Permian and Cambrian extinctions.

·        While the loss of species is greatest in the tropics, the threat is global.

·        In addition to the ethical concerns that many people have concerning the extinction of living forms, there are also practical reasons to be concerned about the loss of plant diversity.

·        We depend on plants for food, building materials, and medicines.

°        We have explored the potential uses for only a tiny fraction of the 290,000 known plant species.

°        Almost all of our food is based on cultivation of only about two dozen species.

·        Researchers have investigated fewer than 5,000 plant species as potential sources of medicines.

°        Pharmaceutical companies were led to most of these species by local people who used the plants in preparing their traditional medicines.

°        The tropical rain forests and other plant communities may be a medicine chest of healing plants that could be extinct before we even know they exist.

·        We need to view rain forests and other ecosystems as living treasures that we can harvest only at sustainable rates.