Botany · Ch 2 — Plant Kingdom
General Characteristic features
General Characteristic features
Algae show one of the widest ranges of body form (thallus organisation) in the plant kingdom, running from single free-swimming cells to giant kelps many metres long. Recognised forms include unicellular motile (Chlamydomonas), unicellular non-motile (Chlorella), colonial motile (Volvox), colonial non-motile (Hydrodictyon), siphonous or coenocytic (Vaucheria), unbranched filamentous (Spirogyra), branched filamentous (Cladophora), discoid (Coleochaete), heterotrichous — combining a prostrate and an erect filament system (Fritschiella), foliaceous or sheet-like (Ulva), and giant kelps (Laminaria, Macrocystis).
Except for the blue-green algae (cyanobacteria, which are prokaryotic), all algae are eukaryotic, and none of them differentiate their body into distinct tissue systems. The cell wall is usually built from cellulose and hemicellulose, but with notable exceptions: diatoms deposit silica in their walls, and Chara encrusts its thallus with calcium carbonate. Some algae secrete algin, a polysulphate ester of polysaccharides that is the industrial source of alginate, agar-agar and carrageenan. Inside the cell, membrane-bound organelles (nucleus, chloroplast, mitochondria, ER, Golgi) are present as usual, along with proteinaceous storage bodies called pyrenoids embedded in the chloroplast that help synthesise and store starch. …
What this figure shows. Nine algal body forms in one figure, labelled (a)-(i): (a) Chlorella — unicellular, non-motile, with a nucleus and chloroplast; (b) Chlamydomonas — unicellular and motile, with flagella, a cup-shaped chloroplast and a pyrenoid; (c) Vaucheria — a siphonous (multinucleate, non-septate) thallus bearing an oogonium, antheridium and a large central vacuole; (d) Oedogonium — an unbranched filament with a hold-fast base and a distinctive apical cap cell; (e) Coleochaete — a flat discoid thallus bearing a bristle-like seta; (f) Fritschiella — a heterotrichous thallus with both a prostrate and an erect filament system; (g) Ulva — a broad, flat, sheet-like (foliaceous) thallus; (h) Fucus — a branched thallus with a hold fast, a stalk-like stipe and gas-filled …
What this figure shows. Six panels showing algal reproductive modes: (a) fragmentation in Spirogyra — a filament snaps into pieces that each regenerate a new thallus; (b) zoospore formation in Cladophora — flagellated motile asexual spores are released from a cell; (c) isogamy — fusion of two gametes that are morphologically and physiologically alike; (d) anisogamy — fusion of two motile gametes that differ in size; (e) oogamy — fusion of a large, non-motile egg with a small, motile sperm; (f) scalariform conjugation in Zygnema — two parallel filaments form lad …