Question

Which of the following inhabits the ventral surface of the Anthoceros thallus?
A) Chlamydomonas
B) Nostoc
C) Volvox
D) Chara

Answer 1

Certain organisms live in symbiotic relationships with each other which is a kind of relationship in which organisms benefit from one another by living together

Complete answer:
Anthoceros thallus is characterised by the presence of small to medium sized thallus. These plants grow in moist clay soils and belong to the family of anthocerotaceae. They live in a symbiotic relationship with certain organisms for fixed nitrogen. Let us analyze the option to find out the correct answer of nitrogen fixing organisms.

Option A: Chlamydomonas are single celled green algae that have cells that are able to perform photosynthesis. They reproduce by zoospores and obtain their nutrition by absorption through small pores present on their surface (sometimes through photosynthesis). Therefore, this is the incorrect option.
Option B: Nostoc is a cyanobacteria which lives under the ventral surface of Anthoceros thallus. They live in a symbiotic relationship with anthoceros in which they provide fixed nitrogen to these plants. Cells called heterocysts carry out nitrogen fixation in Nostoc. Therefore, this is the correct option.
Option C: Volvox is a green algae that belongs to the family of volvocaceae. They are found in freshwater and sometimes in ponds. Therefore, this is the incorrect option.
Option D: Chara is a green algae and belongs to the family of characeae. Other cyanobacteria live in association with them to provide these plants with fixed nitrogen. Therefore, this is the incorrect option.

Thus, the correct answer is option (B) Nostoc.

Additional information:
i) The heterocysts in nostoc are able to carry out photosynthesis.
ii) Nostoc is present in the form of blue-green patches on the ventral surface of Anthoceros thallus.
iii) Anthoceros thallus means ‘flower horn’.

Note: Anthoceros thallus is found associated with a cyanobacteria Nostoc through a symbiotic relationship. However, Chara is also associated with cyanobacteria in a symbiotic relationship but these cyanobacteria don't include Nostoc.