If you were to immerse dead leaves or flowers, old onion bulb scales, dead beetle wing covers, or other organic material in water that has been mixed with a little soil, within a day or two, thousands of microscopic chytrids (pronounced kitt-ridds) probably would appear on the surfaces of the immersed objects (Fig. 19.2). These simple, mostly one-celled organisms include many parasites of protists, aquatic fungi, aquatic flowering plants, and algae. Some parasitize pollen grains, and many other species are saprobic (feed on nonliving organic material). Some of the most common chytrids consist of a spherical cell with colorless, branching threads called rhizoids at one end. The rhizoids anchor the organism to its food source.
Other chytrids may develop short hyphae or even a complete mycelium whose hyphae contain many nuclei. Such multi-nucleate mycelia without crosswalls are said to be coenocytic (pronounced see-no-sitt-ik). The cell walls of a few chytrids have been reported to contain cellulose in addition to chitin.
Many chytrid species reproduce only asexually through the production of zoospores within a spherical cell. The zoospores, which each have a single flagellum, settle upon
Asexual Reproduction release and grow into new chytrids. Some species undergo sexual reproduction by means of the fusion of two motile gametes with haploid nuclei or by the union of two non-motile cells whose diploid zygote nuclei undergo meiosis. The zygote commonly is converted into a resting spore. The origin of chytrids is unknown, but the presence of a flagel-lum on the motile cells has led some authorities to suggest they may have originated from the protozoa.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.