Which group of plants typically develops characteristic nitrogen fixing root nodules in association with symbiotic bacteria such as Rhizobium?

Introduction / Context:
Root nodules are small swellings found on the roots of certain plants, and they play a major role in biological nitrogen fixation. This question checks whether you know which group of plants typically forms these nodules in association with nitrogen fixing bacteria. The concept is very important in school level biology, agriculture and environmental science because it explains how atmospheric nitrogen becomes available to crop plants without synthetic fertilisers.

Given Data / Assumptions:
- Root nodules are specialised structures found on the roots of some plants.- These nodules often contain symbiotic bacteria such as Rhizobium.- The question asks which group of plants typically has such nodules.- Options include parasitic, epiphytic, aquatic and leguminous plants, as well as other plant groups.

Concept / Approach:
The key concept is biological nitrogen fixation. Atmospheric nitrogen gas is abundant but cannot be directly used by most plants. Certain bacteria, for example Rhizobium species, can convert nitrogen gas into ammonia and related compounds that plants can absorb. In many leguminous plants such as pea, bean, gram and clover, these bacteria live inside root nodules. In return for carbohydrates from the plant, the bacteria supply usable nitrogen. This symbiosis is a hallmark feature of legumes, and it improves soil fertility. Parasitic, epiphytic and most aquatic plants do not characteristically form such nodules with nitrogen fixing bacteria, so they cannot be the main answer.

Step-by-Step Solution:
Step 1: Recall that root nodules containing Rhizobium bacteria are commonly shown in diagrams of pea and gram plants.Step 2: Recognise that pea, gram and bean belong to the legume family, also called Fabaceae or Leguminosae.Step 3: Understand that nitrogen fixation in these plants takes place inside the nodules, which house the bacteria in a protected environment.Step 4: Examine the options and identify that leguminous plants are specifically mentioned in option D.Step 5: Conclude that leguminous plants are the group that typically have nitrogen fixing root nodules.

Verification / Alternative check:
You can verify the answer by recalling common agricultural practices. Farmers often grow legumes in crop rotation or as green manure to enrich soil nitrogen content.Botany textbooks consistently show pictures of nodulated roots of legumes with Rhizobium bacteria inside.In contrast, standard descriptions of parasitic, epiphytic or aquatic plants do not present root nodules as a routine feature, confirming that legumes are unique in this context.

Why Other Options Are Wrong:
Option A is wrong because parasitic plants such as Cuscuta tap nutrients from host plants and generally do not form nitrogen fixing root nodules.Option B is wrong because epiphytic plants live on other plants mainly for support and moisture, and they are not known for having Rhizobium filled nodules.Option C is wrong because most aquatic plants absorb dissolved nutrients from water and do not characteristically develop nodules with nitrogen fixing bacteria.Option E is wrong because gymnosperms such as pine and deodar may form mycorrhizae but do not typically develop bacterial root nodules like legumes.

Common Pitfalls:
Students sometimes assume that any unusual root structure must be a nodule, confusing it with storage roots or parasitic haustoria.Another pitfall is to think that all plants which improve soil fertility have nodules, although this feature is mainly associated with legumes.Remember that the well known Rhizobium legume symbiosis is the classic example of nitrogen fixing root nodules, making legumes the correct answer group.

Final Answer:
The plants that typically develop nitrogen fixing root nodules are leguminous plants such as pea, bean and gram.