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Charales (Stoneworts) There are several hundred species in this group. They are collectively known as stoneworts, because they commonly accumulate crusts of calcium carbonate 1CaCO32 over their surfaces. All are multicellular. Some species can be a meter or more in length. The stoneworts are freshwater algae. Certain species are specialized for growing in relatively deep waters, though most live in shallow water near lake beaches or pond edges. 637 Chapter 29 Green Plants Globularis species (tall) and C. fibrosa (short) Reproduction Primarily sexual reproduction occurs, with production of prominent, multicellular gametangia similar to those observed in early land plants. Eggs are retained on the parent plant, which supplies eggs with nutrients prior to fertilization. Life cycle Alternation of generations does not occur. Multicellular individuals are haploid; the only diploid stage in the life cycle is the zygote. Human and ecological impacts Some species form extensive beds in lake bottoms or ponds and provide food for ducks and geese as well as food and shelter for fish (Figure 29.29). Nonvascular Plants (“Bryophytes”) The most basal lineages of land plants are collectively known as nonvascular plants, or bryophytes. The three lineages with living representatives (liverworts, hornworts, and mosses) do not form a monophyletic group, but instead represent an evolutionary grade. All of the species present today have a low, sprawling growth habit. In fact, it is unusual to find bryophytes that are more than 5 5 cm FIGURE 29.29 Stoneworts Can Form Beds in Lake Bottoms to 10 centimeters (2 to 4 inches) tall. Individuals are anchored to soil, rocks, or tree bark by specialized structures called rhizoids. No bryophyptes have true vascular tissue where cells have reinforced walls. In the lineages present today, simple water-conducting cells and tissues are found only in some mosses. All of the bryophytes have flagellated sperm that swim to eggs through raindrops or small puddles on the plant surface. Spores are dispersed by wind. Hepaticophyta (Liverworts) Liverworts got their name because some species native to Europe have liver-shaped leaves. According to the medieval Doctrine of Signatures, God indicated how certain plants should be used by giving them a distinctive appearance. Thus, liverwort teas were hypothesized to be beneficial for liver ailments. (They are not.) About 6500 species are known. They are commonly found growing on damp forest floors or riverbanks, often in dense mats (Figure 29.30), or on the trunks or branches of tropical trees. Adaptations to land Liverworts are covered with cuticle. Some species have pores that allow gas exchange; in species that lack pores, the cuticle is very thin. Human and ecological impacts When liverworts grow on bare rock or tree bark, their dead and decaying body parts contribute to the initial stages of soil formation. Scapania undulata Reproduction Asexual reproduction occurs when fragments of a plant are broken off and begin growing independently. Some species also produce small structures called gemmae asexually, during the gametophyte phase. Mature gemmae are knocked off the parent plant by rain and grow into independent gametophytes. During sexual reproduction, sperm and eggs are produced in gametangia. Life cycle The gametophyte is the largest and longest-lived phase in the life cycle. Sporophytes are small, grow directly from the gametophyte, and depend on the gametophyte for nutrition. Spores are shed from the sporophyte and are carried away by wind or rain. 1 cm FIGURE 29.30 Liverworts Thrive in Moist Habitats