1. Chapter 29: Plants

2. Characteristics of Plants
Plants are multicellular, photosynthetic organisms adapted to a land existence with features such as a waxy cuticle.
Plants resemble algae in using chlorophylls a and b and carotenoid pigments.
But unlike algae, all plants protect the developing embryo from drying out.

3. Seedless vascular plants Vascular plants with seeds
The many divisions of plants can be grouped into three main groups:
Nonvascular plants
Seedless vascular plants
Vascular plants with seeds
Plants are adapted to living on land where light is more available and carbon dioxide diffuses freely.

4. Common plants today Top photograph: Moss, a bryophyte
Second photograph: Fern, a seedless vascular plant
Third photograph: Conifer, a gymnosperm
Bottom photograph: Flowering plant, and angiosperm

5. Life Cycle of Plants
Plants have a two-generation life cycle called alternation of generations.
The sporophyte (2n) produces haploid spores and the spores develop into a gametophyte that produces the gametes.
When the sperm fertilizes the egg, the zygote develops into a sporophyte.
Some plants have a dominant gametophyte (haploid generation) and others have a dominant sporophyte (diploid generation).

6. Nonvascular plant life cycle
The nonvascular plant life cycle features a dominant gametophyte, flagellated sperm,and a dependent sporophyte. Homospores disperse the species.

7. Seedless vascular plant life cycle
The seedless vascular plant life cycle features a dominant gametophyte, flagellated sperm, and an independent sporophyte. Homospores disperse the species.

8. Seed plants are well adapted to reproduction n land.
They produce heterospores: microspores and megaspores.
A microspore develops into a pollen grain.
The megaspore develops into an egg-producing gametophyte within an ovule.
The ovule becomes a seed enclosing the embryonic sporophyte.

9. Seed plant life cycle
The seed plant life cycle features a dominant sporophyte, is heterosporous, has dependent microgametophytes and megagametophytes, has pollen grains, and seeds disperse the species.

10. Nonvascular Plants
Nonvascular plants include the liverworts, hornworts, and the mosses.
Nonvascular plants lack vascular tissues throughout their life cycle; they lack true roots, stems, and leaves.
The gametophyte is the dominant generation; the sporophyte is dependent on the gametophyte.
Sperm require water to swim to the egg.

11. Liverworts
The liverwort Marchantia has a flattened, lobed body known as thallus.
Rhizoids (rootlike hairs) project from the lower surface into the soil.
Marchantia reproduces asexually by forming gemmae, groups of cells in gemmae cups on the upper surface of thallus.
In sexual reproduction, umbrella-like gametophores produce gametes.
Gemmae cups can detach from the main plant and start new plants.

12. Liverwort, Marchantia
Marchantia can reproduce asexually by means of gemmae – minute bodies that give rise to new plants. As shown here, gemmae are located in cuplike structures called gemmae cups.

13. Mosses
In the moss life cycle, antheridia produce swimming sperm that use external water to reach the eggs in the archegonia.
Following fertilization, the dependent moss sporophyte consists of a foot, stalk, and a capsule or sporangium within which windblown spores are produced by meiosis.
Each spore germinates to produce a gametophyte.

14. The gametophyte of mosses has two stages.
First, there is the alga-like protonema, a branching filament of cells.
Next, upright leafy shoots are seen at intervals along the protonema.
Rhizoids anchor the shoots, which bear the antheridia and archegonia.

15. Moss life cycle
The gametophyte is dominant in bryophytes, such as mosses (lower photo).
The leafy shoots bear separate antheridia and archegonia.
Flagellated sperm are produced in antheridia, and these swim in external water to an archegonium that contains a single egg.
When the egg is fertilized, the zygote and developing sporophyte are retained within the archegonium. In some species of mosses, a hoodlike covering (calyptra; top photo) derived from the archegonium is carried upward by the growing sporophyte.
The mature sporophyte growing atop a gametophyte shoot consists of a foot that grows down into the gametophyte tissue, a stalk, and an upper capsule, or sporangium, where meiosis occurs and spores are produced.
When the covering and capsule lid (operculum) fall off, the spores are mature and are ready to escape. The release of spores is controlled by one or two rings of “teeth” that project inward from the margin of the capsule. The teeth close the opening when the weather is wet but curl up and free the spores when the weather is dry.
Spores are released at times when they are most likely to be dispersed by air currents.
When a spore lands on an appropriate site, it germinates into a protonema, the first stage of the gametophyte.

16. Adaptations and Uses of Nonvascular Plants
Mosses are usually found in moist habitats because the sperm are flagellated.
However, mosses can live in shady cracks of hot, exposed rocks.
Sphagnum is bog or peat moss that is used to hold water in garden soil.
Dried peat is sometimes used as fuel.
For certain microhabitats, such as stone walls, or shady crevices of hot exposed rocks, mosses have the selective advantage of being small and simple. Larger, more complex plants could not exploit the same microhabitats.

17. Seedless Vascular Plants
Vascular plants have vascular tissue: xylem (conducts water and minerals from the soil) and phloem (transports organic nutrients within the plant).
In the life cycle of vascular plants, the sporophyte is dominant.
Vacular plants have true roots, leaves, and stems.
Waxy cuticles prevent leaves from drying out.
An advantage of having a diploid sporophyte as the dominant plant is that diploidy allows for recessive, and possibly deleterious, genes to be masked by dominant alleles.

18. Ferns and Their Allies
Whisk ferns, club mosses, horsetails, and ferns are the seedless vascular plants that were prominent in swamp forests during the Carboniferous period.
Their incomplete decomposition formed much of the coal we burn today.
When the spores of these plants germinate, the larger gametophyte is independent of the sporophyte for nutrition.

19. The Carboniferous period
Growing in the swamp forests of the Carboniferous period were treelike club mosses (left), treelike horsetails (right), and lower, fernlike foliage (left). When the trees fell, they were covered with water and did not decompose well. Sediment built up and turned to rock, whose pressure caused the organic material to become coal, a fossil fuel which still helps run our industrialized society today.

20. Whisk Ferns
Whisk ferns (psilotophytes) are represented by Psilotum in which an erect stem that forks repeatedly is attached to a rhizome.
There are no leaves and sporangia are located at the ends of short branches that photosynthesize.
It closely resembles a primitive vascular plant (rhyniophyte) known only from the fossil record.
The independent gametophyte produces the gametes, and sperm are flagellated.
There are several species of whisk ferns.

21. Whisk fern, Psilotum
Whisk ferns have no roots or leaves – the branches carry on photosynthesis. The sporangia are yellow.

22. Club Mosses
In club mosses, a branching rhizome sends up aerial stems less than 30 cm tall.
The sporangia are formed on terminal clusters of leaves called stroboli that are club-shaped.
These plants are common in moist temperate woodlands, but the majority live in the tropics where many of them are epiphytes.
There are 1,000 species of club mosses.

23. Club moss, Lycopodium
Green photosynthetic stems are covered by scale-like leaves, and sporangia are found on leaves arranged as strobili.

24. Horsetails
Rhizomes of horsetails produce aerial stems about 1.3 meters tall.
Whorls of side branches give the appearance of a green horse’s tail.
Some have stroboli on regular stems; some have special stems for stroboli.
Silica in cell walls provide an abrasive grit that made horsetails useful as an abrasive cleanser.
There are 15 species of horsetails. They can be found in moist habitats worldwide.

25. Horsetail, Equisetum
Whorls of branches and tiny leaves occur at the joints of the stem. The sporangia are borne in strobili.

26. Ferns
Ferns have very large fronds (leaves) that grow from a rhizome; ferns have vascular tissue and have true roots, stems and leaves.
Sporangia are within sori on the underside of the leaflets of a frond; meiosis occurs within a sporangium, producing spores.
A windblown spore develops into a separate gametophyte, a heart-shaped prothallus, that bears both egg-producing archegonia and sperm-producing antheridia.
The fronds differ in appearance according to the particular fern.

27. When a flagellated sperm fertilizes an egg, the zygote develops into a young sporophyte.
Although ferns are likely to be found in moist habitats due to flagellated sperm, vegetative (asexual) reproduction is used to disperse ferns in dry habitats.
Ferns are used to decorate bouquets and as ornamental plants in homes and gardens.
Wood from tropical tree ferns is used as a building material, and fiddleheads are sometimes eaten as a delicacy.

28. Fern diversity
On the left is the maidenhair fern (Adiantum pedatum). In the center is the royal fern (Osmunda regalis). On the right is Hart’s tongue fern (Campyloneurum scolopendrium).

29. Fern life cycle The sporophyte is dominant in ferns.
In the fern shown here, the sori (sing., sorus), protected by an indusium, are on the underside of leaflets. Within a sporangium, meiosis occurs and spores are produced.
As a band of thickened cells on the rim of the sporangium (the annulus) dries out, it moves backward, pulling the sporangium open, and the spores are released.
A spore germinates into a prothallus, which bears the antheridia and archegonia on the underside. Typically, the archegonia are at the notch, and antheridia are toward the tip, between the rhizoids.
Fertilization takes place when moisture is present, because the flagellated sperm must swim in a film of water from the antheridia to the egg within the archegonium.
The resulting zygote begins its development inside an archegonium, but the embryo soon outgrows the available space. As a distinctive first leaf appears above the prothallus and as the roots develop below it, the sporophyte becomes visible. Often the sporophyte tissues and the gametophyte tissues are distinctly different shades of green.
The young sporophyte develops a root-bearing rhizome from which the fronds project.

30. Seed Plants Both gymnosperms and angiosperms disperse by seeds.
A seed has a seed coat and contains an embryonic sporophyte and stored food that supports growth when the seed germinates.
Gymnosperms have exposed or “naked” seeds.
In angiosperms (flowering plants), seeds are enclosed by a fruit.

31. Gymnosperms This group includes cycads, the ginkgo, and conifers.
Cycads are palm-like, tropical and subtropical plants that flourished during the era of dinosaurs.
The single species of ginkgo is planted in parks because it does well in polluted areas.
Conifers are the largest group of gymnosperms and include cone-bearing pine, spruce, fir, and redwood trees.
The female ginkgo tree tends to produce rather smelly seeds, thus it is customary to plant only male ginkgoes in parks and gardens. These trees can be propagated vegetatively.
Most conifers are evergreen trees that lose their leaves continuously rather than losing all their leaves during a short period of time like deciduous trees. In this way, they have leaves all year long.

32. Gymnosperm diversity
a. Cycads resemble palm trees, but are gymnosperms that produce cones. Shown here is the Kaffir bread cycad, Encephalartos altensteinii.
b. Ginkgoes exist only as a single species – the maidenhair tree, Ginkgo biloba.
c. Conifers are the most common gymnosperms. Shown here is the Eastern hemlock, Tsuga canadensis.

33. Life Cycle of a Conifer
The gymnosperm microspore develops into a pollen grain; this microgametophyte develops in a pollen cone.
The megagametophyte develops within an ovule located on the scale of a seed cone.
Following wind pollination and fertilization that do not require external water, the ovule becomes a winged seed that is dispersed by wind.
Conifers produce heterospores within cones.

34. Pine life cycle
The sporophyte is dominant, and its sporangia are borne in cones.
There are two types of cones: pollen ones and seed cones. Typically, the pollen cones are quite small and develop near the tips of lower branches.
Each scale of a pollen cone has two or more microsporangia on the underside.
Within these sporangia, each microsporocyte (microspore mother cell) undergoes meiosis and produces four microspores.
Each microspore develops into a microgametophyte, which is the pollen grain. The pollen grain has two wings and is carried by the wind to the seed during pollination. The seed cones are larger than the pollen cones and are located near the tips of higher branches.
3) Each scale of the seed coat has two ovules that lie on the upper surface. Each ovule is surrounded by a thick, layered integument, having an opening at one end. The megasporangium is within the ovule, where a megasporocyte (megaspore mother cell) undergoes meiosis,
4) Producing four megaspores.
5) Only one of these spores develops into a megagametophyte, with two to six archegonia, each containing a single large egg lying near the ovule opening.
6) Once a pollen grain is enclosed within the seed coat, it develops a pollen tube that digests its way slowly toward a megagametophyte. The pollen tube discharges two nonflagellated sperm. One of these fertilizes an egg in an archegonium and the other degenerates. Fertilization, which takes place one year after pollination, is entirely separate from pollination.
7) After fertilization, the ovule matures and becomes the seed, composed of the embryo, the reserve food, and the seed coat. Finally, in the fall of the second season, the sporophyte embryo develops into a new pine tree, and the cycle is complete.

35. Adaptation and Uses of Conifers
Conifers supply much of the wood used for construction of buildings and production of paper.
Many valuable chemicals are extracted from resin, a substance that protects conifers from fungi and insects.
The oldest trees in the world, at 4,500 years old, are bristlecone pines in Nevada.

36. Angiosperms
Angiosperms are flowering plants and include tropical and subtropical deciduous trees.
All hardwood trees are angiosperms.
Angiosperms are the source of clothing, food, medicines, and many other products used by humans.
Angiosperms are divided into monoots (such as the grass family) and dicots (such as the maple and rose families).
The many uses of angiosperms are discussed in the Ecology Focus on pages 602–603.

37. Life Cycle of Angiosperms
Like conifers, angiosperms produce heterospores, except angiosperms do so within their flowers.
Microspores develop into pollen grains within the pollen sacs of the anther.
The megaspore develops into an embryo sac within an ovule.
Pollen is windblown or carried by bees (or other animals) to the pistil.
The pollinator and the flower have coevolved, and therefore they are specific to one another. For example, bee-pollinated flowers are usually blue or yellow and have ultraviolet shadings that lead the pollinator to seek nectar at the base of the flower. The mouthparts of bees are fused into a long tube, through which the bee is able to obtain nectar from this location. As the bee collects nectar, pollen is deposited on its body and then inadvertently carried to the next flower.

38. Double fertilization occurs where one sperm joins with the egg to produce a zygote, and a second sperm joins with the polar nuclei to produce triploid (3n) endosperm, which becomes stored food.
The ovule develops into a seed consisting of a seed coat, stored food, and an embryo, but the ovary and adjacent parts of the flower develop into a fruit.
Fruits aid in seed dispersal.

39. Flowering plant life cycle
The parts of the flower involved in reproduction are the stamens and the pistil. Reproduction has been divided into development of the megagametophyte, development of the microgametophyte, double fertilization, and the seed. Development of the megagametophyte: The ovary at the base of the pistil contains one or more ovules.
Within an ovule, a megasporocyte (megaspore mother cell) undergoes meiosis to produce four haploid megaspores.
Three of these megaspores disintegrate, leaving one functional megaspore, which divides mitotically.
The result is the megagametophyte, or embryo sac, which typically consists of eight haploid nuclei embedded in a mass of cytoplasm. The cytoplasm differentiates into cells, one of which is an egg and another of which is a diploid cell formed by the union of two polar nuclei.
Development of the microgametophyte:
The anther at the top of the stamen has pollen sacs, which contain numerous microsporocytes (microspore mother cells).
Each microsporocyte undergoes meiosis to produce four haploid cells called microspores. When the microspores separate, each one becomes a microgametophyte, or pollen grain.
At this point, the young microgametophyte contains two nuclei: the generative cell and the tube cell. Pollination occurs when pollen is windblown or carried by insects, birds, or bats to the stigma of the same type of plant.
Only then does a pollen grain germinate and produce a long pollen tube. This pollen tube grows within the style until it reaches an ovule in the ovary. Before fertilization occurs, the generative sperm is the mature microgametophyte. Double fertilization:
Upon reaching the ovule, the pollen tube discharges the sperm. One of the two sperm migrates to and fertilizes the egg, forming a zygote; the other unites with the two polar nuclei, producing a 3n (triploid) endosperm nucleus. The endosperm nucleus divides to form endosperm, food for the developing plant. This so-called double fertilization is unique to angiosperms. The seed:
The ovule now develops into the seed, which contains an embryo and food enclosed by a protective seed coat. The wall of the ovary and sometimes adjacent parts develop into a fruit that surrounds the seeds. Therefore, angiosperms are said to have covered seeds.

40. The Flower The flower accounts for the success of angiosperms.
The flower both attracts animals that aid in pollination and produces seeds enclosed by fruits that aid dispersal.
Sepals form a whorl around the colored petals.

41. The reproductive parts of a flower are the pistil and the stamens.
A stamen consists of a filament and anther with two pollen sacs.
The pistil consists of a stigma, style, and ovary.

42. Generalized flower
A flower has four main kinds of parts: sepals, petals, stamens, and pistils. A stamen has anthers at the end of filaments. A pistil has a stigma, a style, and an ovary. An ovary contains ovules.

43. Chapter Summary
Plants resemble algae in using chlorophylls a and b and carotenoid pigments, but unlike algae, plants protect the embryo; this is an adaptation that facilitates land existence.
Presence of vascular tissues and variation in reproductive strategies are used to classify plants.

44. Nonvascular plants are low-growing and lack a means of water transport and internal support, whereas vascular plants have a system that transports water and also provides internal support.
In nonseed plants, spores disperse the species; in seed plants, seeds disperse the species.
In seed plants, a germinating pollen grain transports sperm to the egg.
Angiosperms and gymnosperms have unique adaptations.