The Seaweed Hiding Inside Your China Grass Pudding

A money plant and a bitter gourd vine both climb, but that does not make them close relatives. Sorting plants by the right characters is the first challenge of the plant kingdom.

This part covers systems of classification, the three classes of algae, the importance of algae, and bryophytes.

Artificial systems classify plants by a few easily seen features such as habit, colour and leaf shape; natural systems use many features, including internal structure, embryology and chemistry, to reflect natural affinities; and phylogenetic systems group plants by evolutionary relationships and common ancestry.

- Artificial — few superficial characters; gave vegetative and sexual characters equal weight
- Natural — external features plus anatomy, embryology, ultrastructure and plant chemistry
- Phylogenetic — based on common ancestry, reflecting evolution

Worked example — a misleading grouping. Grouping by "climbing habit" would place a money plant with a bitter gourd vine. Their flowers, seeds and internal structure show they belong to entirely different families — even to different major groups of flowering plants.

Why vegetative characters mislead. Leaf size, shape and colour change with light, water and soil, so one species growing in sun and in shade can look like two.

An everyday example. A tulsi plant grown in deep shade has larger, softer leaves than one on a sunny terrace, though both are the same species.

The substance. Reproductive and internal characters are more stable than vegetative ones, so natural and phylogenetic systems rely on them more.

**Green algae (Chlorophyceae) have chlorophyll a and b and store starch, brown algae (Phaeophyceae) add fucoxanthin to chlorophyll a and c and store laminarin or mannitol, and red algae (Rhodophyceae) contain phycoerythrin with chlorophyll a and d and store floridean starch; flagella are equal in green algae, unequal in brown algae and absent in red algae.

Chlorophyceae (green algae):

- Pigments**: chlorophyll a and b
- Stored food: starch, often around pyrenoids, and oil
- Cell wall: inner cellulose, outer pectose
- Flagella: two to eight, equal, at the tip
- Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara

Phaeophyceae (brown algae):

- Pigments: chlorophyll a, c and fucoxanthin
- Stored food: laminarin or mannitol
- Cell wall: cellulose with a gelatinous coating of algin
- Flagella: two, unequal, attached at the side
- Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus

Rhodophyceae (red algae):

- Pigments: chlorophyll a, d and phycoerythrin
- Stored food: floridean starch
- Cell wall: cellulose, pectin and polysulphate esters
- Flagella: absent
- Examples: Polysiphonia, Porphyra, Gracilaria, Gelidium

An everyday example. The slimy green threads floating in a village pond are often Spirogyra, a green alga.

The substance. Red algae can live at great depths because phycoerythrin absorbs the blue-green light that penetrates deep water.

Algae carry out at least half of all the carbon dioxide fixation on Earth by photosynthesis, form the base of aquatic food chains, and provide foods and commercial products such as agar, algin and carrageen.

Ecological importance:

- Carbon dioxide fixation — at least half of the global total
- Oxygen — released into water, supporting aquatic animals
- Primary producers — the first link in most aquatic food chains

Economic importance:

- Food — species of Porphyra, Laminaria and Sargassum are eaten in several countries
- Algin — from brown algae; used as a thickener
- Carrageen — from red algae; used in food products
- Agar — from Gelidium and Gracilaria; used to grow microbes in laboratories and to set jellies and ice creams

Worked example — tracing a product. A jelly dessert set with agar comes from red algae (Gelidium or Gracilaria), while a food thickened with algin comes from brown algae.

An everyday example. "China grass", used to set puddings and falooda in Indian kitchens, is agar extracted from red seaweed.

The substance. Too many algae can harm water — nutrient pollution causes blooms that use up oxygen when they die and decay.

Bryophytes live on land but need water for their sperm to swim to the egg, so they are the amphibians of the plant kingdom; their main plant body is a haploid gametophyte, liverworts are usually flat and thalloid, and mosses grow from a filamentous protonema into leafy shoots.

General features:

- Damp, shady habitats; no true roots, stems or leaves — attached by rhizoids
- Dominant generation: the haploid gametophyte
- Sex organs: antheridia produce biflagellate antherozoids; flask-shaped archegonia hold one egg
- Water is needed for antherozoids to reach the egg
- Sporophyte grows attached to the gametophyte and depends on it for food; it makes spores by meiosis

**Liverworts (such as Marchantia):

- Thalloid, flat body pressed close to the ground
- Asexual reproduction by gemmae in gemma cups
- Sporophyte of foot, seta and capsule

Mosses (such as Funaria, Polytrichum, Sphagnum):

- Spore grows first into a protonema — creeping, green and filamentous
- Then a leafy stage with spirally arranged leaves on an upright axis
- Sporophyte more elaborate, with a well-developed capsule for spore dispersal

Uses.** Sphagnum forms peat, used as fuel and as packing material because it holds water well; mosses and lichens are among the first plants to colonise bare rock.

An everyday example. The soft green carpet that appears on walls and tree trunks in the monsoon is usually moss.

The substance. In bryophytes the gametophyte is the main, independent plant — the reverse of ferns and flowering plants.