Chytridiomycetes

If dead leaves or onion bulb scales are immersed in a little water' containing a bit of soil, an abundance of chytrids will likely appear in a short time. Their spores seem to be everywhere. They are the simplest of the fungi, reproducing primarily by zoospores. Some species exhibit alternation of generations. The simple, globe-shaped Chytridium sphaerocarpum (figure 18-1), which extends rhizoids into the substrate and produces unicellular sporangia and zoospores, and Allomyces arbuscula (figure 18-2), which shows alternation of generations, are two examples of chytrids. Chytrids consist of only a spherical cell and a few rhizoids that penetrate the host tissue.

'Chlorinated tap water excluded.

Figure 18-1 Chytridium sphaerocarpum growing on a dead leaf in water.

Figure 18-2 shows two types of sporangia on the Allomyces sporo-phyte plant: thick-walled sporangia and thin-walled sporangia. In the thick-walled sporangia, meiosis occurs and the resulting spores, called meiospores, are haploid. The meiospores grow into gametophytes. In the thin-walled sporangia, mitosis occurs, and the resulting spores, called mitospores, are diploid. Mitospores do not produce gametophytes but, rather, grow into sporophytes. If a culture of Allomyces were to be dried out, the thin-walled sporangia would not survive and the thick-walled sporangia would. Such treatment would therefore ensure alternation of generations. The rendering of the gametophyte in figure 18-2 shows two kinds of gametangia: the antheridia, situated below, and the oogonia, located above. Whereas the antheridia (male gametangia) are orange in color, the female gametangia are colorless. When the gametes unite, a biflagellated zygote forms, which, in turn, grows to a sporophyte thallus. Sperm cells are attracted to the egg by a hormonal substance having the romantic name sirenin.

Chytridiomycetes

Zoospores

Sporangium

Matured Sporangium

Dead Leaf in Water

Sexual Reproduction

Female Gametes

Oogonium

Sexual Reproduction

Female Gametes

Oogonium

Allomyces Arbuscula

Young Sporophyte

Figure 18-2 Allomyces arbuscula. Two kinds of sporangia are produced: thin-walled sporangia, which produce mitospores, and thick-walled sporangia, which produce meiospores. The mitospores produce sporophytes, and the meiospores produce gametophytes. The antheridia are orange in color. A fertilized egg grows into a sporophyte.

Meiospores (N)

Young Sporophyte

Figure 18-2 Allomyces arbuscula. Two kinds of sporangia are produced: thin-walled sporangia, which produce mitospores, and thick-walled sporangia, which produce meiospores. The mitospores produce sporophytes, and the meiospores produce gametophytes. The antheridia are orange in color. A fertilized egg grows into a sporophyte.

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  • forbes
    HOW IS TO GROW THE ORANGE?
    8 years ago
  • zain
    Where is the antheridia located?
    8 years ago

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