Aquatic Plants of Pennsylvania: A Complete Reference Guide

Chapter 5. Thalloid and/or Tiny Floating Plants

Chapter 5. Thalloid and/or Tiny Floating Plants Plants in this group have a reduced structure lacking well-defined stems and leaves, or have tiny floating stems with minute leaves. They are small plants that float at or just below the water surface, or in a few cases grow attached to submerged rocks or other solid surfaces. Included in this group are flowering plants (the duckweeds and riverweed), and non-flowering plants including aquatic liverworts and a fern. AQUATIC LIVERWORTS The aquatic liverworts are small floating plants that may also exist stranded on exposed mud. They are described as thalloid because they lack true roots, stems, or leaves; they also lack specialized vascular tissues. Aquatic liverworts are generally flat and often strap-shaped with dichotomous branching. Rhizoids may be present. Liverworts do not produce flowers or fruits. They reproduce by spores, but the sporangia are not often seen. Fragmentation is a more common means of reproduction. Aquatic liverworts are usually found near lake margins, and may be floating among emergent plants. We have included descriptions of two floating aquatic liverworts , purple-fringed riccia (Ricciocarpus natans) and slender riccia (Riccia fluitans). Both are widespread in Pennsylvania (Manville 2001); however, others may also be encountered. Many additional species of liverwort grow on wet rocks or tree bases along the edges of lakes and streams. Additional references should be sought for more specific information on this group of plants. The plant body of purple-fringed riccia is 2-lobed, often forming a floating fan-shaped thallus about 1 cm wide. Conspicuous dark rhizoids extend downward from the lower surface. Slender riccia has a strap-like thallus that branches repeatedly in a dichotomous pattern forming a radiating mass 4–8 cm in diameter that floats just below the water surface. Rhizoids are not present. Ricciocarpus natans ×2 Riccia fluitans ×1.5 Block-Rhoads_Chap05_Final.indd 163 3/24/11 4:46 PM 164 C H A R O P H Y T E A L G A E CHAROPHYTE ALGAE MUSK-GRASS (CHARA SPP.) AND STONEWORT (NITELLA SPP.) The Charophyte algae (musk-grasses and stoneworts) could be confused with higher plants, due to their upright, branching structure. Close observation will reveal that the “stems” are of a homogeneous consistency and lack vascular tissues. “Stems” are anchored to the substrate and support “branches” arising in whorls along the main axis. The primary stems give rise to additional whorls and a dense tufted appearance in some species. The plants range from light yellow-green to very dark green that can appear almost black. The Charophyte algae have been grouped with the green algae in the past but are currently considered a sister group to the land plants (McCourt et al. 2004) (Figure 1.1). These plants do not produce flowers or fruits; rather, they reproduce by eggs and sperm borne in special structures located at nodes. The egg- and sperm-producing structures of both the stoneworts and the muskgrasses are pear-shaped, often with a spiral pattern on the surface. Distinguishing characteristics of the musk-grasses include their stiffness and rough texture, which is due to encrustations of lime, and their garlic-like odor. In addition, the surface of musk-grass “stems” has a longitudinally striated appearance when viewed under 10X magnification (Dillard 1999). Muskgrasses are commonly found in calcium-rich (soft) waters. Stoneworts, on the other hand, do not have a distinctive odor, they are smooth to the touch, and their cells lack striations. Stoneworts are more often found in soft water (low pH) lakes. Musk-grasses and stoneworts often form a continuous ground cover at depths of up to 3 m in lakes with clear water. Stands of these plants, which are 0.15–1 m in height, provide cover for small fishes; in addition, they are eaten by waterfowl. Chara sp. ×1 Chara sp.—reproductive structure ×10 Block-Rhoads_Chap05_Final.indd 164 3/24/11 4:46 PM C H A R O P H Y T E A L G A E 165 Nitella sp. ×1 Nitella sp.—reproductive structure ×5 Block-Rhoads_Chap05_Final.indd 165 3/24/11 4:46 PM 166 D U C K W E E D S DUCKWEED, WATERMEAL, WATER-FLAXSEED, AND BOGMAT LEMNA L., WOLFFIA, WOLFFIELLA, SPIRODELA, AND LANDOLTIA Arum Family—Araceae The duckweeds and their relatives include the smallest flowering plants in the world. They are tiny free-floating plants with a highly simplified growth form...