Summary

Concept 21.1: Primary Endosymbiosis Produced the First Photosynthetic Eukaryotes

• Primary endosymbiosis gave rise to chloroplasts and the subsequent diversification of the Plantae. The descendants of the first photo-synthetic eukaryote include glaucophytes, red algae, several groups of green algae, and land plants, all of which contain chlorophyll a. Review Figure 21.1
• Streptophytes include the land plants and two groups of green algae. Green plants, which include the streptophytes and the remaining green algae, are characterized by the presence of chlorophyll b (in addition to chlorophyll a). Review Figure 21.1
• Land plants, also known as embryophytes, arose from an aquatic green algal ancestor related to today’s stoneworts. Land plants develop from embryos that are protected by parental tissue. Review Figure 21.1 and Table 21.1

Concept 21.2: Key Adaptations Permitted Plants to Colonize Land

• The acquisition of a cuticle, stomata, gametangia, a protected embryo, protective pigments, thick spore walls with a protective polymer, and a mutualistic association with a fungus were all adaptations of land plants to terrestrial life.
• All land plant life cycles feature alternation of generations, in which a multicellular diploid sporophyte alternates with a multicellular haploid gametophyte. Review Figure 21.4
• The nonvascular land plants comprise the liverworts, hornworts, and mosses. These groups lack specialized vascular tissues for the conduction of water or nutrients through the plant body.
• The life cycles of nonvascular land plants depend on liquid water. The sporophyte is usually smaller than the gametophyte and depends on it for water and nutrition.
• In many land plants, spores form in structures called sporangia and gametes form in structures called gametangia. Female and male gametangia are, respectively, an archegonium and an antheridium. Review Figure 21.6 and ANIMATED TUTORIAL 21.1

Concept 21.3: Vascular Tissues Led to Rapid Diversification of Land Plants

• The vascular plants have a vascular system consisting of xylem and phloem that conducts water, minerals, and products of photosynthesis through the plant body. The vascular system includes cells called tracheids.
• The rhyniophytes, the earliest known vascular plants, are known to us only in fossil form. They lacked true roots and leaves but apparently possessed rhizomes and rhizoids.
• Among living vascular plant groups, the lycophytes (club mosses and relatives) have only small, simple leaflike structures (microphylls). True leaves (megaphylls) are found in monilophytes (which include horsetails and ferns). The monilophytes and the seed plants are collectively called euphyllophytes. Review ACTIVITY 21.1
• Roots may have evolved either from rhizomes or from stems. Microphylls probably evolved from sterile sporangia, and megaphylls may have resulted from the flattening and reduction of a portion of a stem system with overtopping growth. Review Figure 21.10
• The earliest-diverging groups of vascular plants are homosporous, but heterospory—the production of distinct megaspores and microspores—has evolved several times. Megaspores develop into female megagametophytes; microspores develop into male microgametophytes. Review Figure 21.11 and ACTIVITIES 21.2 and 21.3

Concept 21.4: Seeds Protect Plant Embryos

• All seed plants are heterosporous, and their gametophytes are much smaller than (and dependent on) their sporophytes. Review Figure 21.12
• Seed plants do not require liquid water for fertilization. Pollen grains, the microgametophytes of seed plants, are carried to a megagametophyte by wind or by animals. Following pollination, a pollen tube emerges from the pollen grain and elongates to deliver gametes to the megagametophyte. Review Figure 21.14
• An ovule consists of the seed plant megagametophyte and the integument of sporophytic tissue that protects it. The ovule develops into a seed. Review Figure 21.15
• Seeds are well protected, and they are often capable of long periods of dormancy, germinating when conditions are favorable.
• The two major groups of living seed plants are the gymnosperms and angiosperms. Review Figure 21.1B
• The gymnosperms produce ovules and seeds that are not protected by ovary or fruit tissues. The major gymnosperm groups are the cycads, ginkgos, gnetophytes, and conifers. Review Figure 21.16
• The megaspores of conifers are produced in woody cones called megastrobili; the microspores are produced in herbaceous cones called microstrobili. Pollen reaches the megagametophyte by way of the micropyle, an opening in the integument of the ovule. Review Figures 21.17 and 21.18, ACTIVITY 21.4, and ANIMATED TUTORIAL 21.2

Concept 21.5: Flowers and Fruits Increase the Reproductive Success of Angiosperms

• Flowers and fruits are unique to the angiosperms, distinguishing them from the gymnosperms.
• The xylem of angiosperms is more complex than that of the gymnosperms. It contains two specialized cell types: vessel elements, which function in water transport, and fibers, which play an important role in structural support.
• The floral organs, from the base to the apex of the flower, are the sepals, petals, stamens, and one or more carpels. Stamens bear microsporangia in anthers. The carpel includes an ovary containing ovules and a receptive surface called the stigma. Review Figure 21.14B and ACTIVITY 21.5
• The structure of flowers has evolved over time. A flower with both megasporangia and microsporangia is referred to as perfect; a flower with only one or the other is imperfect. Some plants with perfect flowers have adaptations to prevent self-fertilization. Review Figures 21.22 and 21.23
• A monoecious species has megasporangiate and microsporangiate flowers on the same plant. A dioecious species is one in which megasporangiate and microsporangiate flowers occur on different plants.
• Flowers may be pollinated by wind or by animals. Many angiosperms have coevolved with their animal pollinators.
• Nearly all angiosperms exhibit double fertilization, resulting in the production of a diploid zygote and an endosperm (which is triploid in most species). Review Figure 21.25 and ANIMATED TUTORIAL 21.3
• The oldest evolutionary split among the angiosperms is between the clade represented by the single species in the genus Amborella and all the remaining flowering plants. Review Figure 21.27
• The magnoliids are the sister group to the monocots and eudicots. The most species-rich angiosperm clades are the monocots and the eudicots.