Spirogyra
(Campbell 7th Ed. 550 Fig. 28.2d)
Spirogyra is a free-floating genus of filamentous algae belonging to the division Chlorophyta. The organism occurs in a variety of freshwater habitats often forming bright green masses on the surfaces of ponds. The most obvious feature of the organism is the spiral shaped chloroplasts (usually one or two per cell) located in a parietal position within the cell protoplast. There are numerous pyrenoids associated with each chloroplast. The majority of the cell protoplast is occupied by vacuole, with the haploid nucleus often being suspended centrally by narrow strands of cytoplasm. The cell wall is relatively rigid and composed of cellulose, a feature of most of the members of the Chlorophyta and all of the higher plant groups. Contractile vacuoles, found in such organisms as Chlamydomonas and Euglena, are lacking.
Growth of the multicellular organism is indeterminate. Any cell or group of cells is capable of mitotic cell division. The daughter cells enlarge, increasing the size of the filament. Asexual reproduction is restricted to fragmentation; motile zoospores are lacking. Sexual reproduction occurs by conjugation. In this process two filaments come to lie side by side and lateral outgrowths, termed papillae, are formed. The papillae fuse and, eventually, the common wall dissolves forming an open conjugation tube between two cells.
The entire protoplast of one of the isogametes migrates through the conjugation tube and fuses with the other isogamete, forming a diploid zygote exhibiting a spherical protoplast. The zygote undergoes meiosis forming four haploid nuclei, three of which disintegrate. The now uninucleate cell divides by mitosis to form a new filament.
Note: Spirogyra exhibits a haplontic life cycle.
Lichens
(Campbell 6th Ed. 627-628; 7th Ed. 621-622)
Lichens are a symbiotic association, an example of mutualism, between a heterotrophic fungus (often an Ascomycota) and a photosynthetic organism (often a green alga). Lichens are often categorized into morphological types termed crustose , foliose, and fruticose and are frequently found in harsh environments that would normally not support either of the symbiotic partners alone. Lichens are often the first colonists of rock outcrops and set the stage for the ecological succession of other organisms.
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