Classifying Plants and the Algae: Quick Revision

A brief note on Classifying Plants and the Algae. Read it before a test. Try each question at the end before you open its answer.

Systems of classification

• Artificial: a few superficial characters, such as the number of stamens (Linnaeus). Vegetative and sexual characters weigh the same.
• Natural: many features, outside and inside: ultrastructure, anatomy, embryology, phytochemistry (Bentham and Hooker).
• Phylogenetic: evolutionary relationships; members of a taxon share a common ancestor.
• Numerical taxonomy: all characters, coded as numbers, of equal weight, compared by computer.
• Cytotaxonomy: chromosomes. Chemotaxonomy: chemical constituents.

The algae

• Simple, thalloid, autotrophic, with chlorophyll; mostly in water, also on moist soil, in lichens and on the sloth bear.
• Forms: single cells (Chlamydomonas), colonies (Volvox), filaments (Ulothrix, Spirogyra), huge kelps.
• Vegetative: fragmentation. Asexual: zoospores with flagella. Sexual: isogamy (equal gametes), anisogamy (unequal, both swim), oogamy (a still egg and a small swimming gamete).
• Uses: at least half of the world's carbon dioxide fixation; food (Porphyra, Laminaria, Sargassum); agar (Gelidium, Gracilaria); algin (brown); carrageen (red); Chlorella as protein.

The three classes

• Chlorophyceae (green): chlorophyll a and b; starch in pyrenoids; cellulose and pectose wall; 2 to 8 equal flagella at the tip.
• Phaeophyceae (brown): chlorophyll a and c with fucoxanthin; laminarin or mannitol; algin coat; 2 unequal flagella at the side; holdfast, stipe and frond.
• Rhodophyceae (red): chlorophyll a and d with r-phycoerythrin; floridean starch; no flagella; oogamous; can live deep.

A worked example

A seaweed from a deep rock pool is red, its cells have no flagella, and an iodine test on its stored food gives a colour unlike ordinary starch.

1. The colour. Red comes from r-phycoerythrin, so it is likely a red alga.
2. No flagella. Red algae have no flagellated cells.
3. The food. Floridean starch is stored, not ordinary starch.
4. Conclusion. It belongs to Rhodophyceae.

Practice with new numbers

A program to try

The program prints one line for each class, with its pigment, food and flagella.

Mistakes to avoid

• Calling Linnaeus's system natural. It used few characters, so it is artificial.
• Thinking isogamous gametes always swim. In Spirogyra they do not.
• Giving brown algae chlorophyll b. They have a and c.
• Saying red algae store starch. They store floridean starch.

Check yourself

1. Which branch of taxonomy uses all observable characters with equal weight?

2. Name the kind of sexual reproduction in which the gametes differ in size and both swim.

3. Give one pigment and one stored food of the brown algae.

4. Name two products that come from red algae.