Themes of Plant Biology
Fungi: Not Plants
All fungi have some features in common, but other special structural and reproductive features separate the four phyla (see Table
1 ).
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Structure
The fungi are eukaryotic and have membrane-bound cellular organelles and nuclei. They have no plastids of any kind (and no chlorophyll).
The hyphae of the fungi are of two general kinds: Some are septate, and are divided by septa (walls) that separate the cylindrical hypha into cells; in the nonseptate fungi, the hypha is one long tube. (The septa are perforated, however, permitting the cytoplasm to flow throughout the length of the filament.) Mitosis occurs in the nonseptate hyphae, but there is no accompanying cytokinesis (division of the cytoplasm) so the hyphae are multinucleate (with many nuclei). The special name for this condition—an organism or part of an organism with many nuclei not separated by walls or membranes—is coenocytic, and the organism is a coenocyte.
A few fungi—called by the general name yeasts—are single-celled, and nonfilamentous much of the time. The only flagellated cells in the kingdom are the flagellated gametes of the chytrids.
Metabolism
The fungi are all heterotrophic, but unlike animals and many other heterotrophs that ingest their nutrients as bits or bites of food, the fungi secrete digestive enzymes into their surroundings, in effect digesting their food outside of their bodies. They then can absorb the smaller particles and incorporate the nutrients into their own cells. Some are parasites obtaining nutrients from living organisms, but more are saprobes (saprotrophs) that digest and recycle materials from dead organisms.
In addition to potent digestive enzymes, some fungi manufacture powerful alkaloids that, when ingested by humans, assail the nervous system, causing hallucinations and even death. The “death angel,” Amanita, is one such well-known poisonous fungus; ergot ( Claviceps purpurea) is another.
Fungal hyphae, like the roots of vascular plants, grow primarily at the tip, elongating and branching repeatedly. The filaments are in direct contact with their environment, obviating in the fungal body the need for separate absorbing and conducting systems (and precluding the need for storage tissues). Materials readily pass through the plasma membrane and cell walls of the hyphae along their entire length, although the most active metabolism and material exchange is concentrated near the hyphal tips. Most of the cytoplasm is located at the tips also.
