Vocabulary

mold yeast mycology hypha chitin mycelium septum coenocyte dimorphism sporangiophore sporangium sporangiospore conidium conidiophore fragmentation budding figure 26-1

The hyphae of some fungi have separating walls called septa (a). The hyphae of other fungi do not have septa and are called coenocytic hyphae (b).

figure 26-2

This light micrograph shows the dimorphic fungus Paracoccidioides brasiliensis as it changes from its unicellular yeast form at 37°C (98.6°F) to a mycelial form at 25°C (77°F).

figure 26-2

This light micrograph shows the dimorphic fungus Paracoccidioides brasiliensis as it changes from its unicellular yeast form at 37°C (98.6°F) to a mycelial form at 25°C (77°F).

Word Roots and Origins dimorphic from the Greek dis, meaning "twice," and morphe, meaning "shape"

figure 26-3

Fungi reproduce asexually in many ways. Yeasts, such as Saccharomyces cerevisiae, form buds that eventually pinch off to produce new cells.

figure 26-3

Fungi reproduce asexually in many ways. Yeasts, such as Saccharomyces cerevisiae, form buds that eventually pinch off to produce new cells.

Fungal species that have hyphae lacking septa are called coenocytes (SEE-noh-SIETS). The general structures of septate and nonseptate hyphae are shown in Figure 26-1. Hyphae increase in length by cellular growth and division at the tip. As the hyphae grow, the size of the mycelium increases. When hyphae encounter organic matter, such as a tree trunk or dead animal, they secrete digestive enzymes and then absorb the digested nutrients. Fungi range in size from the microscopic yeast to the largest single organism in the world. Fungi of the genus Armillaria, or honey mushrooms, can occupy an area of up to 800 hectares (3.1 mi2).

Several species of fungi can change form in response to changes in their environment. For example, Histoplasma capsulatum, which causes a severe disease in humans that can resemble tuberculosis, normally grows as a mycelium in soil. However, when it invades a human, the increased temperature and available nutrients cause the fungus to grow unicellularly as a yeast. This ability to exist in two different forms, demonstrated in Figure 26-2, is called dimorphism (die-MAWR-Flz-uhm).

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