Question

Plants like clover and beans, that have nitrogen-fixing bacteria in their roots, belong to the family:
a. Fabaceae
b. Brassicaceae
c. Solanaceae
d. Asteraceae

Answer 1

Hint: The root nodules are found in the leguminous plants as we have studied from our lower classes. This question can be answered easily by knowing the other name of the family Leguminosae or the pea family as the pea plants are the classic example for both the family and the leguminous plants.

Complete answer:
For hundreds of years, the plant families were grouped based on morphology only but now we classify plants based on molecular analysis and evolutionary history.
The distinguishing features of the above-mentioned plant families are as follows.

Fabaceae:
• Fabaceae is one of the largest plant families.
• Roots: tap root with root nodules which contain nitrogen-fixing bacteria.
• Stem: erect or climber. Leaves petiolate, stipulate.
• Flower: Racemose inflorescence. Complete, bisexual, zygomorphic and hypogynous.
• Calyx: Five in number, fused, aestivation is imbricate.
• Corolla: Five in number, not fused, aestivation is vexillary.
• Androecium: Stamens ten in number
• Gynoecium: Single carpel, superior ovary.
• Fruit: Legume, non-endospermic.

Brassicaceae:
• Roots: tap root with storage modifications.
• Stem: erect, glabrous or hairy.
• Flower: Racemose inflorescence. Complete or incomplete, bisexual, actinomorphic and hypogynous.
• Calyx: Four in number, polysepalous, aestivation is imbricate.
• Corolla: Four in number, cruciform petals, aestivation is valvate.
• Androecium: Stamens usually six in number but can vary from two to sixteen.
• Gynoecium: Fused gynoecium, two carpels and rarely three, superior ovary.
• Fruit: Siliqua or lomentum

Solanaceae:
• Roots: tap root.
• Stem: erect or climber.
• Flower: Racemose or cymose inflorescence or solitary. Complete, bisexual, actinomorphic.
• Calyx: Five in number, fused, aestivation is valvate.
• Corolla: Five in number, fused, aestivation is valvate.
• Androecium: Stamens five in number
• Gynoecium: Fused gynoecium, two carpels, superior ovary.
• Fruit: Berry or a capsule and seeds with endosperm.

Asteraceae:
• Asteraceae is the largest family in the plant kingdom.
• Roots: tap root or fibrous roots.
• Stem: erect or prostrate. Leaves often contain secretory canals with resin or latex.
• Flower: Head or capitulum with bracts. Complete or incomplete, bisexual or unisexual, actinomorphic and zygomorphic. Ray florets – female, zygomorphic, epigynous.
• Calyx: Absent or modified as pappus.
• Corolla: Five in number, fused, aestivation is valvate.
• Androecium: Absent or 5 stamens.
• Gynoecium: Absent or if present they are with two carpels, inferior with one style and bifid stigma.
• Fruit: cypsela, seeds are non-endospermic.

Hence, the correct answer is option (A).

Additional information:
The root nodules have a symbiotic association with the nitrogen-fixing bacteria, Rhizobium. The bacteria get energy from the plants in the form of amino acids and they fix nitrogen into the soil by converting it into ammonia which can be easily taken up and utilized by the plants which cannot directly use the atmospheric nitrogen.

Note: It can be noted that Rhizobia normally exist in the soil and it doesn’t need a plant to survive although it is safer and easier to survive in the plant tissues. There are other species of bacteria that exist in the soil and fix nitrogen as well such as Azotobacter, Anabaena, Nostoc etc. but they don’t form any symbiotic associations with roots of plants.