By which of the following methods do red algae reproduce?

Correct — C. Algae reproduce vegetatively by fragmentation — the thallus breaks into pieces, and each piece grows into a complete new alga. Red algae reproduce this way, besides forming non-motile spores asexually and reproducing sexually.

The other three options are horticultural techniques that people perform on seed plants, not processes an alga carries out in the sea. The reproduction the stem asks for is option (c).

Why the others are wrong

• (a)
  Grafting — Grafting joins the scion of one plant to the rootstock of another, a nursery method for woody seed plants such as mango and apple. An alga has neither stem nor vascular tissue to graft.
• (b)
  Cutting — Cutting means rooting a detached piece of stem, as in rose or sugarcane. It resembles fragmentation, but it is a propagation method carried out by a gardener on a higher plant.
• (d)
  Micropropagation — Micropropagation is tissue culture — explants grown on a nutrient medium in the laboratory to raise many identical plantlets. It is a technique, not something red algae do in nature.

Algae reproduce in three ways: vegetative by fragmentation, asexual by spores, and sexual by the fusion of gametes.

Fragmentation is the simplest of the three. No special cells and no gametes are involved — a broken piece of the body simply regenerates the whole. Spirogyra is the textbook example, and the mechanism runs in red and brown algae as well.

In red algae the sexual reproduction is oogamous and the asexual spores are non-motile, which is a diagnostic feature of the class.

Key facts

• Algae reproduce vegetatively by fragmentation, each fragment of the thallus developing into a new thallus.
• Red algae are placed in the class Rhodophyceae and owe their colour to the pigment r-phycoerythrin.
• Sexual reproduction in red algae is oogamous and their asexual spores are non-motile.
• Grafting, cuttings and micropropagation are artificial propagation methods used on higher plants.

Common traps

• Choosing cutting because it also involves a detached piece. Cutting is done by a gardener, fragmentation happens on its own.
• Assuming red algae reproduce only sexually because that is the part textbooks describe in most detail.
• Treating micropropagation as a natural process because it produces whole plants.

No directly related past PYQ was found.