Fungi Plantae Animalia

2.3 Kingdom Fungi — The Great Decomposers

If bacteria are invisible, fungi are modest. The mushroom pushing through damp grass, the grey fuzz on stale bread, the pink mould at the edge of a pickle jar — these are the obvious tips of a much larger filamentous world. Fungi are heterotrophic eukaryotes that drape themselves across substrates and absorb nutrients through the whole body surface.

Body Plan of a Fungus

Fungi span unicellular yeasts to sprawling multicellular networks. The multicellular forms are built from thread-like filaments called hyphae. A loose tangle of hyphae is the mycelium — the true body of a fungus.

• Septate hyphae carry cross-walls (septa) that partition the filament into cells.
• Coenocytic hyphae have no cross-walls — a continuous cytoplasm with many nuclei.

The cell wall is made of chitin (the same material as insect exoskeletons) plus other polysaccharides — a crucial clue that fungi are not plants.

Modes of Nutrition

• Saprophytic — absorb nutrients from dead organic matter (most common).
• Parasitic — live on or inside living hosts, often causing disease.
• Symbiotic — partner with other organisms; in lichens they team up with algae, and in mycorrhiza they wrap around plant roots and supply minerals in exchange for sugar.

Reproduction

Fungi reproduce with remarkable versatility:

• Vegetative — fragmentation, fission, budding (yeast).
• Asexual — by spores such as conidia, sporangiospores, or zoospores.
• Sexual — spores produced inside special structures: oospores, ascospores or basidiospores.

Sexual reproduction proceeds in three stages: plasmogamy (fusion of cytoplasm) → karyogamy (fusion of nuclei) → meiosis (in the zygote, producing haploid spores).

2.3.1 Phycomycetes

These are the "water moulds" and "bread moulds" — primitive fungi living on dung, decaying wood or as parasites of plants. Their mycelium is aseptate and coenocytic. Asexual reproduction occurs through zoospores (motile) or aplanospores (non-motile), produced inside a sporangium. Sexual reproduction yields a thick-walled zygospore.
Examples: Mucor, Rhizopus (the black bread mould), Albugo (parasite of mustard).

2.3.2 Ascomycetes — The "Sac Fungi"

Usually multicellular with branched, septate hyphae, though yeast (Saccharomyces) is a unicellular member. Asexual spores are conidia, borne on conidiophores. Sexually, the ascomycetes produce ascospores inside sac-like cells called asci, usually grouped into a fruiting body called an ascocarp.
Examples: Aspergillus, Penicillium (source of penicillin), Claviceps, Neurospora (a model organism in genetics), and the edible morels and truffles.

2.3.3 Basidiomycetes — The "Club Fungi"

The most familiar fungi — mushrooms, bracket fungi, puffballs. The mycelium is branched and septate. Asexual spores are rare; sexual reproduction produces basidiospores exogenously on club-shaped cells called basidia, themselves borne on large fruiting bodies known as basidiocarps.
Examples: Agaricus (edible mushroom), Ustilago (smut fungi of cereal crops), Puccinia (wheat rust).

2.3.4 Deuteromycetes — The "Imperfect Fungi"

A taxonomic waiting-room: these fungi are classified here because only their asexual stage is known. Once the sexual stage (ascus or basidium) is discovered, they are reshuffled into Ascomycetes or Basidiomycetes. Many produce antibiotics or help in decomposition.
Examples: Alternaria, Colletotrichum, Trichoderma.

Mushroom Anatomy — A Closer Look

Comparison of the Four Fungal Classes

2.4 Kingdom Plantae — Life Powered by Sunlight

Kingdom Plantae brings together all eukaryotic, chlorophyll-containing organisms — in other words, everything green that photosynthesises. The cell wall is made of cellulose; plastids (especially chloroplasts) are abundant; and most species are multicellular with tissue-level or organ-level organisation.

A few plants have adopted partial heterotrophy. Insectivorous species like the Venus flytrap and Bladderwort trap and digest small insects to supplement nitrogen in mineral-poor soils; parasitic plants such as Cuscuta (dodder) tap the vascular tissues of their hosts.

Plantae includes a broad sweep of lineages — algae, bryophytes, pteridophytes, gymnosperms and angiosperms — which the later chapters explore in detail.

Alternation of Generations

A distinctive feature of the plant life cycle is the alternation of generations — a haploid (n) gametophyte phase that produces gametes, and a diploid (2n) sporophyte phase that produces spores by meiosis. The two phases take turns across the life cycle.

2.5 Kingdom Animalia — The Eaters

Kingdom Animalia consists of multicellular, eukaryotic heterotrophs that ingest their food rather than absorb it — a mode called holozoic nutrition. Animal cells lack a cell wall — explaining the wide range of body shapes and rapid movement the kingdom is famous for.

Other hallmarks:

• Locomotion — most species move actively at some point in the life cycle.
• Sensory and neuromotor systems — nerves and muscles allow rapid responses to stimuli.
• Sexual reproduction predominates, often with complex courtship, parental care and development through distinct embryonic stages.
• Tissue, organ and organ-system organisation.

The details of the animal phyla — from sponges to chordates — are covered in a dedicated chapter later in the book.