What Is a Plant?
A. Characteristerics of Plants (kingdom Plantae)
1. Plants are multicellular eukaryotes with well-developed tissues and organs.
2. Plants are autotrophic by photosynthesis.
3. Plants have adaptations that reduce the loss of water: the cuticle and stomata.
4. All plants protect the embryo from desiccation and some protect their entire gametophyte generations.
5. In some plants, pollen grains are transported by wind or animals to the egg, and the embryo becomes a seed.
6. Plants are believed to be closely related to green algae: both utilize chlorophylls a and b,the food reserve is starch, and the cell wall of both contains cellulose, and both form a cell plate during cell division.
7. However, in contrast to green algae, plants have sex organs with an outer layer of nonreproductive cells.
8. Plants have a two-generation life cycle called alternation of generations.
a. The sporophyte generation is the diploid generation; it produces haploid spores by meiotic cell division.
The gametophyte generation is the haploid generation; it produces haploid gametes by mitotic division.
9. Plants are divided into teo main groups: nonvascular and vascular plants.
1. Nonvascular Plants Are Diverse
1. Hornworts, liverworts, and mosses are the nonvascular plants.
2. Nonvascular plants lack specialized tissues for transporting water, minerals, and organic nutrients.
3. They lack true roots, stems, and leaves, although they may have rootlike, stemlike, or leaflike structures.
4. The gametophyte is the dominant (most conspicuous) generation in the bryophytes.
a. Flagellated sperm swim to the vicinity of the egg in a continuous film of water.
b. The sporophyte is attached to and derives its nourishment from the photosynthetic gametophyte.
5. Lacking vascular systems, nonvascular are quite small, with the largest being no more than 20 cm tall.
a. This, along with sexual reproduction involving flagellated sperm, is bryophytes are usually found in moist habitats.
b. However, mosses compete well in harsh environments because the gametophyte can reproduce asexually.
6. If required, mosses can dry up and later, when water is available, rehydrate and photosynthesize again.
7. The three divisions of bryophytes are individual lines of descent; mosses are closely related to vascular plants.
C. Liverworts Are Nonvascular
1. The division Hepatophyta contains 10,000 species of liverworts.
2. The name arose in the ninth century when the plant was seen as similar to lobes of the liver.
3. Marchantia has a flat, lobed thallus about a centimeter in length; the upper surface of thallus is smooth and lower surface bears numerous rhizoids projecting into soil; reproduces asexually and sexually.
4. Rhizoids are the rootlike hair that anchors a bryophyte and absorbs water and minerals from the soil.
5. Asexual reproduction is by means of gemmae in gemmae cups on upper surface of the thallus.
6. Sexual reproduction depends on antheridia and archegonia.
a. Antheridia are on disk-headed stalks and produce flagellated sperm.
b. Archegonia are on umbrella-headed stalks and produce eggs.
c. Zygote develops into a tiny sporophyte (a few millimeters long) composed of foot, short stalk, and capsule.
d. Spores produced within the capsule of the gametophyte are disseminated by wind.
D. Mosses Are Nonvascular
1. About 12,000 species of mosses are in the division Bryophyta.
2. Mosses are found in the Arctic through the tropics to parts of the Antarctic.
3. Although preferring damp, shaded localities, some survive in deserts while others are in bogs and streams.
4. Mosses store large quantities of water; when they dry out, they become dormant; when it rains, become green.
5. Copper mosses live only in the vicinity of copper and serve as an indicator of ore deposits.
6. Luminous moss lives in caves; its cells, shaped like lenses, focus the scant light on chloroplast grana.
7. Some "mosses" are not true mosses: Irish moss is an edible alga, reindeer moss is a lichen, club mosses are vascular plants, and Spanish moss, which hangs from trees in the southern U.S., is a flowering plant.
8. Most mosses can reproduce asexually by fragmentation.
9. Life cycle of mosses begins with an algalike protonema developing from germination of a haploid spore.
a. After three days of favorable growing conditions, upright shoots covered with leafy structures arise.
1) Rhizoids anchor the protonema, to which the shoots are attached.
2) The shoots bear antheridia and archegonia at their tips.
3) Antheridia produce flagellated sperm, which need external water to reach eggs in the archegonia.
4) Fertilization results in a diploid zygote that undergoes mitotic cell division to develop into a sporophyte.
b. The sporophyte consists of a foot (which grows down into the gametophyte tissue starting at the former archegonium), a stalk, and an upper capsule (sporangium) where spores are produced.
1) At first the sporophyte is green and photosynthetic.
2) At maturity it is brown and nonphotosynthetic.
2.Vascular Plants Include Seedless and Seed Plants
A. Vascular Tissue
1. Xylem is vascular tissue that conducts water and minerals up from the soil.
2. Phloem is vascular tissue that transports organic nutrients from one part of the plant to another.
B. Organs of Vascular Plants
1. The roots absorb water and minerals from the soil.
2. The stems conduct water and minerals to the leaves and conduct organic nutrients from the leaves to the roots.
3. The leaves are adapted to maximize photosynthetic activity; they are covered by a waxy cuticle.
C. Life Cycle of Vascular Plants
1. The diploid sporophyte generation is dominant in vascular plants; this is the generation that has vascular tissue.
D. Reproduction in Vascular Plants
1. The seedless vascular plants (ferns and their allies) disperse their species by producing windblown spores.
a. When the spores germinate, a large gametophyte develops independent of the sporophyte for nutrition.
b. Antheridia of gametophyte produce and release flagellated sperm, which swim in a film of water to archegonia of the gametophyte, where fertilization occurs.
2. Seed plants, which are fully adapted to land, produce heterospores.
a. Microspores develop within a microsporangium as a result of meiotic cell division.
1) Haploid microspore develop into grains, immature microgametophytes retained in microsporangium.
2) When released, the pollen grains develop into sperm-bearing microgametophytes.
3) In evolution of vascular plants, microgametophytes (pollen grains) replaced external swimming sperm.
b. Megaspores develop within a megasporangium as a result of meiotic cell division.
1) Haploid megaspores develop into egg-bearing megagametophytes, while still retained in an ovule.
2) The megagametophyte is dependent on the sporophyte, since it and the subsequent embryo are retained within the ovule that develops into the seed.
32.4. Ferns and Allies Are the Seedless Vascular Plants
A. Types of Seedless Vascular Plants
1. The seedless vascular plants include whisk ferns, club mosses, horsetails, and ferns.
B. Evolutionary History
1. Vascular plants arose during the Silurian period of the Paleozoic era, and dominated to the mid-Devonian.
2. These are photosynthetic stems (not true leaves or roots) with sporangia at tips; they are attached to a rhizome.
3. Cooksonia may have been the first vascular plant.
4. Rhynia, a rhynophyte, is better known; it has erect stems that fork repeatedly.
5. It is thought that ancestry of ferns and allies can be traced back to the rhyniophytes or closely related forms.
6. Although the first three groups are rather small and limited in diversity today, lycopods, horsetails, and ferns were also trees during the Carboniferous period.
C. Psilophytes Are Whisk Ferns
1. The whisk ferns are in the division Psilophyta and includes Psilotum, a plant that resembles Rhynia.
2. Whisk ferns occur in Arizona, Texas, Louisiana, and Florida as well as Hawaii and Puerto Rico.
3. Whisk ferns have no leaves or roots; they possess a branched rhizome that has rhizoids and a mycorrhizal fungus that helps gather nutrients.
4. Aerial stems with tiny scalefork repeatedly and carry on photosynthesis.
5. Sporangia are located at the ends of short branches.
6. The independent gametophyte is found underground penetrated by a mycorrhizal fungus; it produces flagellated sperm.
D. Club Mosses Have Club-Shaped Strobili
1. There are about 1,000 species of club mosses in the division Lycopodophyta.
2. They are common in temperate woodlands where they are called ground pine.
3. Typically a branching rhizome sends up aerial stems less than 30 cm tall.
4. Tightly packed, scalelike microphylls cover stems and branches; they contain one strand of vascular tissue.
5. Sporangia are borne on strobili, terminal clusters of leaves that bear sporangia.
6. The spores germinate into inconspicuous and independent gametophytes.
7. Majority of club mosses live in tropics and subtropics as epiphytes, plants that live on trees without harm.
8. Closely related are Selaginella (spike mosses); the resurrection plant curls up when dry and unfurls when moistened.
9. The phylogeny of club mosses can be traced to the Devonian period.
10. They are the only living plants that have microphylls; they are indirectly related to the ferns and their allies.
E. Horsetails Have Jointed Stems
1. The division Equisetophyta contains one extant genus, Equisetum, with 15 species of horsetails.
2. A rhizome produces aerial stems that stand about 1.3 meters tall.
3. Whorls of slender side branches encircle nodes of a stem, providing a resemblance to a horse's tail.
4. The leaves are reduced megaphylls with many very small strands of vascular tissue.
5. Many have a strobilus at the tip of stems, while others send up special buff-colored stems that bear the strobili.
6. Spores germinate into inconspicuous and independent gametophytes.
7. Stems are tough and rigid because of the presence of silica deposited in the cell walls; early Americans used them as "scouring rushes" and they are still used in a few abrasive powders.
F. Ferns Are Leafy
1. About 12,000 species of ferns belong to the division Pteridophyta.
2. Ferns are widespread, and especially abundant in warm, moist tropical regions.
3. Ferns range in size from low-growing mosslike forms to tall trees.
4. Fronds are variable in size and shape.
5. In nearly all species, the leaves first appear as a fiddlehead, which unrolls as it grows.
6. Fronds are megaphylls secondarily subdivided into leaflets; they may have evolved by uneven branching.
7. The life cycle of ferns can begin with the production of spores by meiotic cell division within sporangia, located in sori on the underside of the leaflets.
a. The spores are released and are dispersed largely by wind.
b. Each spore germinates into a prothallus, which grows and develops antheridia and archegonia underneath.
c. Fertilization occurs if water is present; flagellated sperm swim in a film from antheridia to archegonium.
d. Resulting zygote begins its development inside the archegonium, but the embryo soon outgrows the space.
e. Sporophyte becomes visible as a distinctive first leaf grows above and as roots develop below prothallus.
f. The young sporophyte develops a root-bearing rhizome from which the fronds project.
8. Uses of ferns include ornamental plants by florists, wood that is very decay resistant, and some medicines.
9. Adaptation of Ferns
1. The water-dependent gametophyte lacks vascular tissue and is separate from the sporophyte.
2. Flagellated sperm require an outside source of water in which to swim to the eggs.
3. Once established, the bracken fern can spread to drier areas by asexual reproduction.
Gymnosperms Have Naked Seeds
A. The Life Cycle of Seed Plants
1. There are separate microgametophytes (male) and megagametophytes (female).
2. Microspores develop immature microgametophytes, the pollen grains, still retained in a microsporangium.
3. After they are released, pollen grains develop into mature, sperm-bearing microgametophytes.
4. Pollination is the transfer of pollen to the vicinity of the megagametophyte.
5. Sperm is delivered to the egg through a pollen tube; therefore, no external water is required for fertilization.
6. The megaspore develops into an egg-bearing megagametophyte while still retained within an ovule.
7. An ovule is the sporophyte structure that holds the megasporangium and then the megagametophyte.
8. After fertilization, the ovule becomes an embryonic plant enclosed within the ovule, which becomes the seed.
9. Both the megagametophytes and microgametophytes are dependent upon the diploid sporophyte.
10. The sporophyte can evolve into diverse forms without any corresponding changes in the gametophyte.
11. Among seed plants, the seed disperses the sporophyte.
a. Seeds are mature ovules containing embryonic sporophyte and stored food enclosed in protective seed coat.
b. Seeds are resistant to adverse conditions including dryness and temperature extremes.
c. The food reserve supports the emerging seedling until it can exist on its own.
d. Survival value of seeds contributes greatly to success of seed plants, and their present dominance.
B. Gymnosperm Diversity
1. Gymnosperms include the conifers, cycads, ginkgo, and gnetophytes.
2. All gymnosperms produce naked seeds not enclosed in a fruit, but exposed on the surface of sporophylls.
3. Sporophylls are leaves that bear sporangia and are spirally arranged on a cone.
4. Gymnosperms did not begin to flourish until the Mesozoic era.
a. The supercontinent Pangaea had formed and mountain ranges arose producing deserts on the leeward side.
b. As a result, swamps became much drier and a mass extinction occurred; seedless vascular plants nearly vanished, providing opportunity for the first seed plants (including gymnosperms) to become dominant.
c. Tree seed plants developed well-developed roots and stems due to secondary growth of vascular tissue.
C. Conifers
1. About 550 species of conifers are in the division Pinophyta.
2. Conifers are cone-bearing trees and shrubs such as pines, hemlocks, and spruces.
3. Conifers usually have evergreen needlelike leaves and are well adapted to withstand extremes in climate.
4. Needles have a thick cuticle, sunken stomates, and a reduced surface area.
5. Conifers are found in nearly all habitats, from the equator to the subpolar regions, and comprise the taiga.
6. The oldest and largest trees in existence are conifers: the General Sherman tree in California's Sequoia National Park is 84 meters tall, 10 meters in diameter, and weighs 1,385 tons; redwoods grow over 90 meters high and exceed 2,000 years old; and bristlecone pines in the Nevada mountains are over 4,500 years old.
7. The life cycle of pines is typical of conifers.
a. The sporophyte is dominant and its sporangia are borne in cones.
b. There are two types of cones: pollen cones (small and near the tips of lower branches) and seed cones.
c. Each scale of a pollen cone has two or more microsporangia on the underside.
d. Within these sporangia, each microsporocyte undergoes meiosis and produces four microspores.
e. Each microspore develops into a microgametophyte, which is the pollen grain.
f. Each scale of a seed cone has two ovules surrounded by an integument and with an opening at one end.
g. A megasporangium is within an ovule; a megasporocyte undergoes meiosis producing four megaspores.
h. One spore develops into a megagametophyte with 2-6 archegonia, each containing a single large egg.
i. Once a pollen grain is enclosed within the seed cone, it develops a pollen tube that digests its way toward amegagametophyte and discharges two nonflagellated sperm.
j. Fertilization takes place one year after pollination.
k. The ovule matures and becomes the seed, composed of embryo, reserve food and seed coat.
l. The woody seed cone, opens to release winged seeds in the fall of the second season.
D. Cycads Fed Dinosaurs
1. About 100 species of cycads belong to the division Cycadophyta.
2. The trunk is stout and unbranched; the large leaves are compound giving a palmlike appearance.
3. Cycads flourished during the Mesozoic era and probably were food for herbivorous dinosaurs.
4. Today, cycads are found mainly in tropical and subtropical regions.
E. Ginkgo TrAdorn Parks
1. Only one species of ginkgo (maidenhair tree) survives in the division Ginkgophyta.
2. It grows up to 70 feet in height and is covered with forked-veined, fan-shaped leaves.
3. Microsporangia are in cones but the ovules are stalked; female trees produce seeds with a fleshy covering.
4. It was native to China, preserved in ornamental gardens; now male trees are propagated in city parks.
F. Gnetophytes
1. About 70 species of gnetophytes are in the division Gnetophyta.
2. Gnetophytes are divided into three genera and live mainly in the tropics.
3. Ephedra is found in U.S. desert regions, and is a many-branched shrub with small, scalelike leaves.
4. Welwitschia is found in desert Africa; most of it exists underground and its two enormous leaves may grow for hundreds of years.
G. Uses of Gymnosperms
1. Gymnosperms supply much of the wood used for building construction and paper production.
2. They also produce many valuable chemicals (e.g., those extracted from resin).
H. Adaptations of Gymnosperms
1. Typical gymnosperms withstand heat, dryness, and cold, as a result of having well-developed roots and stems, tough, small needles with a thick cuticle; and pollen production eliminating reliance on external water.
2. Enclosure of the dependent megagametophyte in an ovule protects it during its development.
3. Embryo is protected by seed and is provided with nutrients that support growth following germination.
Angiosperms Have Covered Seeds
A. Angiosperms Are the Flowering Plants
1. Over 235,000 species of angiosperms (flowering plants) belong to the division Magnoliophyta.
2. Angiosperms produce seeds that are enclosed in a fruit.
3. This group contains six times the number of species than all other plants combined.
4. Angiosperms live in all sorts of habitats, from freshwater to desert and from tropics to subpolar regions.
5. Flowering plants range in size from tiny duckweed (almost microscopic) to Eucalyptus exceeding
100 m tall.
6. It is impossible to exaggerate their importance to everyday human life.
B. Origin and Evolution of Angiosperms
1. Angiosperms evolved from ancient gymnosperms; by the Jurassic period in the middle of the Mesozoic era, many of the gymnosperms had features similar to those that we associate with angiosperms.
a. Some had vascular tissue resembling that of angiosperms.
b. Some had sporophylls that were beginning to look like flower parts, and were visited by beetles---many extant flowering plants depend on animals (e.g., insects) for pollination.
c. Most transitional gymnosperms became extinct, the gnetophyte Ephedra is considered the closest relative.
2. Angiosperms arose during Cretaceous period; in Cenozoic era, angiosperms diversified as climate grew cold.
3. Rather than being tall trees, the first angiosperms may have been fast-growing woody shrubs.
4. Today, angiosperms are either woody or herbaceous.
5. Angiosperms that are adapted to a wide range of climatic zones are perennials or annuals.
a. Perennials live two or more growing seasons; die back seasonally in herbaceous plants.
b. Deciduous trees have the characteristic of losing leaves in the autumn and are therefore perennials.
c. Annuals are plants that live for only one growing season.
C. How Flowering Plants Are Classified
1. Angiosperms are divided into two groups: dicotyledons and monocotyledons.
2. The dicotyledons are in the class Magnoliopsida and have these features: either woody or herbaceous, flower parts usually in fours and fives, leaves usually net-veined, vascular bundles arranged in a circle within the stem, and produce two cotyledons (seed leaves) at germination.
3. Dicots include buttercup, mustard, maple, cactus, pea, and rose families.
4. Monocotyledons are in the class Liliopsida and have these features: most are herbaceous, flower parts are in threes, leaves are usually parallel-veined, vascular bundles are scattered within the stem, and produce one cotyledon (seed leaf) at germination.
5. Monocots include lily, palm, orchid, iris, and grass families.
D. The Flower Contains Modified Leaves
1. Flowers consist of several kinds of highly modified leaves that are arranged in concentric rings and attached to a receptacle. (Fig. 32.13)
a. Receptacle is a modified stem tip to which flower parts are attached.
b. Sepals are outermost ring of modified leaves of flowers; usually green, they enclose flower before it opens.
c. Petals are a ring of modified leaves inside of sepals; often large and colorful, they help attract pollinators.
d. Stamens form a whorl within a ring of petals and around a pistil; slender filament has an anther at its tip.
1) Anther is a modified sporophyll containing microsporangia where microspores (pollen) are produced.
2) An anther likely evolved as a reduction of a sporangium's leaflike portion.
e. The pistil contains one or more fused carpels; it consists of a stigma, style, and ovary.
1) Carpels are modified sporophylls that contain ovules in which megasporangia are located.
2) A stigma is a landing platform for pollen and the site where the pollen tube enters the style.
3) The style is a slender column that extend from the upper part of a carpel and terminates in a stigma.
4) Ovary contains from one to many ovules which develop(s) into seeds; the ovary develops into a fruit.
5) A fruit is a mature ovary and provides a fleshy or dry covering for seeds; may aid in seed dispersal.
E. Life Cycle of a Flowering Plant
1. Microsporangia develop within the anther portion of a stamen; the megasporangia develop within ovules.
2. Pollination brings mature microgametophyte (pollen grain) to pistil and pollen tube brings sperm to the ovule.
3. Double fertilization: one sperm fertilizes egg; One sperm unites with polar nuclei to form triploid endosperm.
4. Endosperm is contained within a seed as a nutritive tissue that surrounds and serves as food for the embryo.
5. The ovule develops into the seed, and the ovary becomes the fruit.
6. Different kinds of fruit employ different kinds of dispersal mechanisms (i.e., wind, gravity, water, and animals) for dissemination of seed(s), which thereby promotes dispersal of the species to new locations.
7. Under favorable conditions, the seed germinates to produce a young sporophyte plant.
F. Flowers Aid Diversification
1. Flower diversification is related to the numerous means by which flowers are pollinated and fruits dispersed.
2. Although some flowers disperse pollen by wind, many attract specific pollinators (e.g., bees, wasps, flies, butterflies, moths, and even bats), which carry only a particular pollen from flower to flower.
3. Coevolution of pollinators and flowers increases probability a flower is pollinated with pollen from its species.
4. Flowers lend themselves to efficient cross pollination; they also aid in dispersal through production of fruits.
5. There are fruits that utilize wind, gravity, water, and animals for dispersal.
6. Because animals live in particular habitats and/or have particular migration patterns, they are apt to deliver the fruit-enclosed seeds to a suitable location for seed germination and development of the adult sporophyte.
G. Uses of Angiosperms
1. Humans use plant fibers to produce cloth, firewood, and building materials.
2. Plant oils, spices, and drugs are derived from different parts of angiosperm plants.
H. Adaptations of Angiosperms
1. Angiosperms have true roots, stems, and leaves.
2. The vascular tissue is well developed, and the leaves are generally broad.
3. The reproductive organs of angiosperms are in their flowers which often attract pollinators.
4. Flowers also produce seeds enclosed by fruit, which aids in dispersal.