Question

Describe the growth and development of an algae that develops into a multicellular leafy form.

Answer 1

Sargassum- It belongs to the class Phaeophyceae.

It has a sporophytic plant body bushy in habit; plant body is differentiated into main axis, lateral branches,. and leaf like structure (leaves).

Sargassum reproduces vegetatively and sexually.

1. Vegetative Reproduction- It takes place by fragmentation and also by the progressive growth of the apical region and the gradual decay of the older region.

2. Sexual Reproduction- It is of oogamous type. Both the sex organs, i.e. antheridia and oogonia are borne within the flask shaped conceptacles that are embedded in the receptacles. The thallus may be homothallic or heterothallic.

Homothallic species may have both antheridia and oogonia developed in the same conceptacle or in different conceptacles borne on the same plant.

In heterothallic species, male plants only bear male conceptacles containing antheridia and female plants only bear female conceptacles containing oogonia.

Fertilization:-

• The liberated antherozoids swim towards the extruded oogonia still attached to the wall of the female conceptacle.
• Antherozoids surround each oogonium and they attach to the oogonial wall through their anterior flagella and their posterior flagella remain moving in the water.
• Next one of the antherozoids penetrates the oogonial wall and ultimately fuses with the egg to form diploid zygote (2n).

Germination of the zygote:-​​​​​​

• The zygote immediately germinates without undergoing any period of rest.
• The germination of zygote starts within the enclosed oogonium which is still attached to the wall of the conceptacle.
• After sometime, the oogonial wall undergoes gelatinization and the zygote is liberated out in water.

The zygote divides and redivides, and finally develops into a new Sargassum plant (2n). There is no reduction of the zygote nucleus.

Alternation of generation:-

Sargassum does not show any alternation of generation. Sargassum plant, although cytologically a sporophyte, still it bears sex organs. Meiosis takes place only at the time of gamete formation- hence haplophase is restricted within the gametes only. The diploid zygote formed by the fusion of gametes directly develops into a new sporophytic plant without undergoing meiosis.

The life cycle of Sargassum is diagrammatically described below:-

Shown in the fig. below is a species of Sargassum.

B. A species of Sargassum swartzii showing enlarged male conceptacle.

C. A species of Sargassum swartzii showing enlarged female conceptacle containing many oogonia