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The Evolution of Seed Plants. The Evolution of Seed Plants. The Seed Plants The Gymnosperms: Naked Seeds The Angiosperms: Flowering Plants. The Seed Plants. The seed plants are the most recent group to appear in the evolution of the tracheophytes.
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The Evolution of Seed Plants • The Seed Plants • The Gymnosperms: Naked Seeds • The Angiosperms: Flowering Plants
The Seed Plants • The seed plants are the most recent group to appear in the evolution of the tracheophytes. • The seed plants include the gymnosperms (such as pines and cycads) and the angiosperms (flowering plants). • There are four living phyla of gymnosperms and one phyla of angiosperms.
The Seed Plants • In seed plants, the gametophyte generation is reduced, with the haploid gametophyte being attached to and nutritionally dependent on the diploid sporophyte.
The Seed Plants • The seed plants are heterosporous. • Separate megasporangia and microsporangia are formed on structures that are grouped on short axes. • The megaspores are not shed, but develop into female gametophytes within the megasporangia. • Only one of the meiotic products in the megasporangium survives. • This surviving haploid nucleus produces a multicellular female gametophyte that is retained within the megasporangium, where it matures and is eventually fertilized.
The Seed Plants • Male gametophytes are called pollen grains and are formed by the division of microspores produced meiotically within the microsporangia. • Pollen grains produce a slender pollen tube that elongates and digests its way through the sporophyte tissue toward the female gametophyte. • When the pollen tube reaches the female gametophyte, two sperm are released and fertilization occurs. • The resulting diploid zygote divides until an embryonic stage is reached, then growth is temporarily halted. The end product of this stage is a seed.
The Seed Plants • A seed may contain tissues from three generations. • The seed coat and megasporangium develop from tissues of the diploid sporophyte parent. • Within the megasporangium is the haploid female gametophyte tissue of the next generation. • The center of the seed package contains the third generation, in the form of the embryo of the new diploid sporophyte. • The possession of seeds is a major reason for the enormous evolutionary success of seed plants.
The Gymnosperms: Naked Seeds • The gymnosperms do not produce flowers. Gymnosperm means “naked-seeded,” meaning their ovules and seeds are not protected by flower or fruit tissue. • There are four clades of living gymnosperms today. • Phylum Cycadophyta, the cycads • Phylum Ginkgophyta has a single species, Ginkgo biloba. • Phylum Gnetophyta • Phylum Pinophyta, the conifers
The Gymnosperms: Naked Seeds • All gymnosperms exhibit secondary growth—their stems and roots grow larger in diameter. • All gymnosperms but the Gnetophyta have only tracheids as water-conducting and support cells in their xylem. • Wood is secondary xylem produced by gymnosperms.
The Gymnosperms: Naked Seeds • Fir, cedar, spruce, and pine all belong to the phylum Pinophyta, the conifers or cone-bearers. • A cone is a modified stem bearing a tight cluster of scales specialized for reproduction. • A strobilus is a conelike cluster of scales that are modified leaves inserted on an axis. • Megaspores are produced in seed cones, and microspores are produced in the much smaller pollen strobili.
The Gymnosperms: Naked Seeds • The ovule consists of the integument, the megasporangium inside it, and the tissue attaching it to the maternal sporophyte. • Pollen grains enter through a small opening in the integument at the tip of the ovule called the micropyle. • The ovules are borne on the upper surfaces of the cone scales. • About half of the conifer species have fruitlike tissues associated with their seeds that may be eaten by animals, which in turn disperse the seeds in their feces, but they are not true fruits.
The Angiosperms: Flowering Plants • The phylum Angiospermae consists of over 257,000 species of flowering plants. • Angiosperm means “enclosed seed.” • The angiosperms represent the extreme of an evolutionary trend in the tracheophytes—the sporophyte generation becomes larger and more independent of the gametophyte, while the gametophyte becomes smaller and more dependent on the sporophyte.
The Angiosperms: Flowering Plants • A number of synapomorphies, or shared derived traits, characterize the angiosperms: • They have double fertilization. • They produce triploid endosperm. • Their ovules and seeds are enclosed in a carpel. • They have flowers. • They produce fruit. • Their xylem contains vessel elements and fibers. • Their phloem contains companion cells.
The Angiosperms: Flowering Plants • In double fertilization, two male gametes participate in fertilization events within the megagametophyte. • One sperm combines with the egg to produce a diploid zygote. • The other sperm combines with two other haploid nuclei of the female gametophyte to form a triploid nucleus. • This gives rise to the endosperm, triploid tissue that nourishes the embryonic sporophyte during its early development.
The Angiosperms: Flowering Plants • The carpel is a modified leaf that encloses the ovules and seeds of the angiosperms. • Vessel elements are specialized water-transporting cells present within the xylem of angiosperms. • Fiber is another distinct cell type found in the xylem of angiosperms. It plays an important role in supporting the plant body. • Companion cells are a unique type of cell found in angiosperm phloem.
The Angiosperms: Flowering Plants • All the parts of a flower are modified leaves. • Stamens are composed of a filament that bears an anther containing the pollen-producing microsporangia. • The pistil is composed of one carpel or two or more fused carpels. It has a swollen base called the ovary that contains the megasporangia. • The style is the apical stalk of the pistil, and the terminal surface that receives pollen grains is called the stigma.
The Angiosperms: Flowering Plants • Flowers often have several other specialized leaves: • The inner ones are called petals (collectively, the corolla). • The outer ones are called sepals (collectively, the calyx). • These leaves often play roles in attracting animal pollinators to the flower. • The calyx commonly protects the immature flower in bud. • The sepals, petals, stamens, and carpels are usually in circular arrangements called whorls and attached to a stalk called the receptacle.
The Angiosperms: Flowering Plants • Perfect flowers have both microsporangia and megasporangia. • Imperfect flowers have either functional megasporangia or microsporangia, but not both. • Monoecious species produce both types of imperfect flowers on the same plant. • In dioecious species, a plant produces either megasporangiate or microsporangiate flowers but not both.
The Angiosperms: Flowering Plants • The flowers of the most evolutionarily ancient angiosperms have a large and variable number of tepals, carpels, and stamens. • Evolution within the angiosperms has resulted in many modifications of this early condition: reduction in number of floral organs, differentiation of petals and sepals, changes in symmetry, and fusion of parts.
The Angiosperms: Flowering Plants • Carpels and stamens are thought to have evolved from leaflike structures. • The carpels later fused and became more buried in the receptacle tissue. • Natural selection has favored pistils with long styles and stamens with long filaments, probably to increase the likelihood of successful fertilization.
The Angiosperms: Flowering Plants • Most angiosperms are animal-pollinated. Animals visit flowers to obtain nectar or pollen, and in the process often carry pollen from one plant to another. • The animals have affected the evolution of plants, and plants have affected the evolution of animals. • This coevolution has resulted in some highly specific interactions, with certain plant species being pollinated by only one or very few animal species. • Most plant–pollinator interactions are not highly specific.
The Angiosperms: Flowering Plants • Angiosperms are heterosporous. • The ovules are contained within carpels, rather than being exposed on the scales as in gymnosperms. • The ovule develops into a seed containing the products of double fertilization, a diploid zygote and a triploid endosperm. • The embryo consists of an embryonic axis and one or two cotyledons, or seed leaves. The cotyledons may digest the endosperm, and later expand and become photosynthetic.
The Angiosperms: Flowering Plants • The ovary of a flowering plant develops into a fruit after fertilization. • A fruit may consist only of the mature ovary and its seeds, or it may include other parts of the flower or structures associated with it. • Simple fruits develop from a single carpel or several united carpels. • Aggregate fruits develop from several separate carpels of a single flower. • Multiple fruits are formed from a cluster of flowers. • Accessory fruits are derived from parts in addition to the carpel and seeds.
The Angiosperms: Flowering Plants • Angiosperm clades: • The monocots have a single embryonic cotyledon and include the grasses, cattails, lilies, orchids, and palms. • The eudicots have two cotyledons, and include the majority of familiar seed plants, including most herbs, vine trees, and shrubs. • Clades other than the eudicots and monocots include the the water lilies, star anise, and the magnoliid complex.
Figure 30.13 Evolutionary Relationships among the Angiosperms
Figure 30.14 Monocots and Eudicots Are Not the Only Surviving Angiosperms (Part 1)
Figure 30.14 Monocots and Eudicots Are Not the Only Surviving Angiosperms (Part 2)
The Angiosperms: Flowering Plants • The question of how the angiosperms first arose is still unanswered. Several questions complicate efforts to answer this question. • What morphological characters should be selected as important? • What algorithms should be applied to computerized analysis of data? • Are all molecular differences and similarities significant? • Which fossils should be chosen for comparisons? • What is the likelihood that evidence of double fertilization can be found in ancient fossils?
