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Attractant, Acting As a Homing Device for the Swimming Sperm. Sperm r 62 CHAPTER 3 EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS UN[T 11 EVOLUTION AND DIVERSITY OF PLANTS 63 gemmae propagules 2 rows of 1 row of sperm cells dorsal leaves ventral leaves (sterile “jacket” layer) neck sperm cells “ ‘fl gemmae cup I / pore dorsal (upper) ventral (lower) A B view view thalloid liverwort leafy liverwort FIGURE 3.10 A. Antheridia. B. Archegonia. Both are apomorphies of land plants. 4 (,, ••1• attractant, acting as a homing device for the swimming sperm. of the liverworts, mosses, and hornworts is relatively small, Sperm cells enter the neck of the archegonium and fertilize ephemeral, and attached to and nutritionally dependent upon the egg cell to form a diploid (2n) zygote. In addition to the gametophyte (see later discussion). effecting fertilization, the archegonium serves as a site for The relationships of the liverworts, mosses, and hornworts embryo/sporophyte development and the establishment of a to one another and to the vascular plants remain unclear. nutritional dependence of the sporophyte upon gametophytic Many different relationships among the three lineages have archegonium tissue. been proposed, one recent of which is seen in Figure 3.6. (n) The land plants share other possible apomorphies: the presence of various ultrastructural modifications of the sperm LIVERWORTS cells, fiavonoid chemical compounds, and a proliferation of Liverworts, also traditionally called the Hepaticae, are one of archegoniophore (n) archegoniophore (n) (longitudinal-section) heat shock proteins. These are not discussed here. the monophyletic groups that are descendents of some of the (longitudinal-section) first land plants. Today, liverworts are relatively minor com ponents of the land plant flora, growing mostly in moist, fertilization DIVERSITY OF NONVASCULAR LAND PLANTS shaded areas (although some are adapted to periodically dry, hot habitats). Among the apomorphies of liverworts are During the early evolution of land plants, three major, (1) distinctive oil bodies and (2) specialized structures called capsule monophyletic lineages diverged before the vascular plants elaters, elongate, nonsporogenous cells with spiral wall thick (discussed in Chapter 4). These lineages may collectively be enings, found inside the sporangium. Elaters are hygroscopic, called the nonvascular land plants or “bryophytes” and meaning that they change shape and move in response to include the liverworts, mosses, and hornworts. “Bryophytes” changes in moisture content. Elaters function in spore are a paraphyletic group, defined by the absence of derived dispersal; as the sporangium dries out, the elaters twist out of features; the name, placed in quotation marks, is no longer the capsule, carrying spores with them (Figures 3.11, 3.12K). formally recognized. There are two basic morphological types of liverwort Liverworts, mosses, and homworts differ from the vascular gametophytes: thalloid leafy and (Figures 3.11—3.13). Ri II I) sporophyte plants in lacking true vascular tissue and in having the game— Thalloid elater liverworts consist of a thallus, a flattened mass of (2n) tophyte as the dominant, photosynthetic, persistent, and free- tissue; this is likely the ancestral form, based on cladistic living phase of the life cycle. It is likely that the ancestral studies. As in hornworts and mosses, the gametophyte bears antheridiophore (n) gametophyte of the land plants was thalloid in nature, similar rhizoids, uniseriate, filamentous processes that function in (longitudinal section) antheridium spore (0) to that of the hornworts and many liverworts. The sporophyte anchorage and absorption. Pores in the upper surface of the (n) germinating spore F IGURE 3.11 Liverwort morphology and life cycle. r 62 CHAPTER 3 EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS UN[T 11 EVOLUTION AND DIVERSITY OF PLANTS 63 gemmae propagules 2 rows of 1 row of sperm cells dorsal leaves ventral leaves (sterile “jacket” layer) neck sperm cells “ ‘fl gemmae cup I / pore dorsal (upper) ventral (lower) A B view view thalloid liverwort leafy liverwort FIGURE 3.10 A. Antheridia. B. Archegonia. Both are apomorphies of land plants. 4 (,, ••1• attractant, acting as a homing device for the swimming sperm. of the liverworts, mosses, and hornworts is relatively small, Sperm cells enter the neck of the archegonium and fertilize ephemeral, and attached to and nutritionally dependent upon the egg cell to form a diploid (2n) zygote. In addition to the gametophyte (see later discussion). effecting fertilization, the archegonium serves as a site for The relationships of the liverworts, mosses, and hornworts embryo/sporophyte development and the establishment of a to one another and to the vascular plants remain unclear. nutritional dependence of the sporophyte upon gametophytic Many different relationships among the three lineages have archegonium tissue. been proposed, one recent of which is seen in Figure 3.6. (n) The land plants share other possible apomorphies: the presence of various ultrastructural modifications of the sperm LIVERWORTS cells, fiavonoid chemical compounds, and a proliferation of Liverworts, also traditionally called the Hepaticae, are one of archegoniophore (n) archegoniophore (n) (longitudinal-section) heat shock proteins. These are not discussed here. the monophyletic groups that are descendents of some of the (longitudinal-section) first land plants. Today, liverworts are relatively minor com ponents of the land plant flora, growing mostly in moist, fertilization DIVERSITY OF NONVASCULAR LAND PLANTS shaded areas (although some are adapted to periodically dry, hot habitats). Among the apomorphies of liverworts are During the early evolution of land plants, three major, (1) distinctive oil bodies and (2) specialized structures called capsule monophyletic lineages diverged before the vascular plants elaters, elongate, nonsporogenous cells with spiral wall thick (discussed in Chapter 4). These lineages may collectively be enings, found inside the sporangium. Elaters are hygroscopic, called the nonvascular land plants or “bryophytes” and meaning that they change shape and move in response to include the liverworts, mosses, and hornworts. “Bryophytes” changes in moisture content. Elaters function in spore are a paraphyletic group, defined by the absence of derived dispersal; as the sporangium dries out, the elaters twist out of features; the name, placed in quotation marks, is no longer the capsule, carrying spores with them (Figures 3.11, 3.12K). formally recognized. There are two basic morphological types of liverwort Liverworts, mosses, and homworts differ from the vascular gametophytes: thalloid leafy and (Figures 3.11—3.13). Ri II I) sporophyte plants in lacking true vascular tissue and in having the game— Thalloid elater liverworts consist of a thallus, a flattened mass of (2n) tophyte as the dominant, photosynthetic, persistent, and free- tissue; this is likely the ancestral form, based on cladistic living phase of the life cycle. It is likely that the ancestral studies. As in hornworts and mosses, the gametophyte bears antheridiophore (n) gametophyte of the land plants was thalloid in nature, similar rhizoids, uniseriate, filamentous processes that function in (longitudinal section) antheridium spore (0) to that of the hornworts and many liverworts. The sporophyte anchorage and absorption. Pores in the upper surface of the (n) germinating spore F IGURE 3.11 Liverwort morphology and life cycle. 64 CHAPTER 3 EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS UNIT II EVOLUTION AND DIVERSITY OF PLANTS 65 FIGURE 3.13 Hepaticae — Leafy liverworts. A. Bazania trilobata, a leafy liverwort. B. Porella, a leafy liverwort, showing third row of reduced leaves at arrows (lower side facing). thallus of some species function in gas exchange (Figure entirely of a sporangium or capsule (Figure 3.11, 3. 12F,J). 3.12B,L). These pores are not true stomata (discussed later), At a certain stage, the internal cells of the capsule divide mei as they have no regulating guard cells. Some liverworts, like otically, forming haploid (n) spores (see Figure 3.7). In liver the hornworts (discussed later), have a symbiotic relationship worts the spores are released by a splitting of the capsule into with cyanobacteria. On the upper surface of the gametophytes four valves. The spores may land on a substrate, germinate of some thalloid liverworts, such as Marchantia, are special (under the right conditions), and grow into a new gameto ized structures called gemma cups, which contain propagules phyte, completing the life cycle. called gemmae (Figure 3.11, 3.12B). These structures func tion in vegetative (asexual) reproduction; when a droplet of MOSS water falls into the gemma cup, the gemmae themselves may The mosses, or Musci, are by far the most speciose and be dispersed some distance away, growing into a haploid diverse of the three major groups of nonvascular land plants genetic clone of the parent. and inhabit a number of ecological niches. Mosses may share Leafy liverworts have gametophytes consisting of a stem one apomorphy with the hornworts (discussed later) and vas axis bearing three rows of thin leaves. In most leafy liver cular plants: possession of stomates (Figure 3.6). Stomates worts, the stem is prostrate and the leaves are modified such (also termed stomata) are specialized epidermal cells gener that the upper two rows of leaves are larger and the lower ally found on leaves, but sometimes on stems. Stomata con most row (on the stem underside) are reduced (Figures
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