Which one of the following organisms depends primarily on a saprophytic mode of nutrition, obtaining food from dead and decaying organic matter?

Introduction / Context:
Organisms obtain food in different ways, and their nutrition can be autotrophic or heterotrophic. Saprophytic nutrition is a form of heterotrophic nutrition in which organisms feed on dead and decaying organic matter by secreting digestive enzymes and absorbing the released nutrients. This mode is common among fungi and some bacteria. Biology questions often ask you to identify which organism in a given list is saprophytic. This question focuses on that distinction among some common lower organisms.

Given Data / Assumptions:
• The mode of nutrition in focus is saprophytic, that is, feeding on dead and decaying organic matter.• The options include Agaricus, Ulothrix, Riccia, Cladophora, and Marchantia.• We assume typical ecological roles: mushrooms as fungi and the others as green plants or algae.• We differentiate autotrophic photosynthesis from saprophytic heterotrophy.

Concept / Approach:
Agaricus is a genus of mushroom forming fungi. These fungi grow on soil, manure heaps, and decaying organic matter. They secrete enzymes into the substrate and absorb dissolved nutrients, which is characteristic of saprophytic nutrition. Ulothrix and Cladophora are green algae that have chlorophyll and carry out photosynthesis, so they are autotrophic. Riccia and Marchantia are bryophytes, specifically liverworts, which also perform photosynthesis and are not saprophytes. Therefore, the only organism in the list that truly depends on saprophytic nutrition is Agaricus.

Step-by-Step Solution:
Step 1: Recall that saprophytes feed on dead and decaying organic matter using external digestion.Step 2: Identify Agaricus as a typical mushroom, which is a fungus commonly found on decaying plant material and manure.Step 3: Recognise that Ulothrix and Cladophora are green algae with chloroplasts and chlorophyll, so they are autotrophic.Step 4: Remember that Riccia and Marchantia are bryophytes, which are small green land plants performing photosynthesis.Step 5: Conclude that only Agaricus fits the definition of a primarily saprophytic organism.Step 6: Choose Agaricus as the correct option.

Verification / Alternative check:
Field observations of mushrooms such as Agaricus show them growing on logs, leaf litter, soil rich in organic matter, and compost piles. They appear rapidly after rain when moisture supports fungal growth on decomposing material. Algae such as Ulothrix and Cladophora are seen in aquatic environments attached to rocks or floating and are clearly green and photosynthetic. Bryophytes like Riccia and Marchantia form green mats on moist soil or rocks and are studied as simple land plants with chlorophyll. These ecological and morphological observations confirm that Agaricus is the saprophyte among the listed organisms.

Why Other Options Are Wrong:
Ulothrix is a filamentous green alga that uses sunlight, carbon dioxide, and water to produce its own food via photosynthesis, so it is autotrophic. Riccia is a liverwort with chlorophyll bearing tissues and does not depend on decaying matter for nutrition. Cladophora is a branched green alga found in freshwater or marine habitats, and like other green algae, it is photosynthetic. Marchantia, also a liverwort, makes its own food as an autotroph and is not saprophytic. These organisms therefore do not match the saprophytic mode of nutrition described in the stem.

Common Pitfalls:
A common confusion is to assume that any small, non flowering organism must be saprophytic or parasitic. However, many algae and bryophytes are fully capable of photosynthesis due to their chlorophyll content. Another mistake is to focus only on habitat and think that because certain algae grow on rocks or damp surfaces, they must be saprophytic, which is not correct. To avoid these errors, remember that most fungi, including mushrooms such as Agaricus, are classic saprophytes, while green algae and bryophytes are generally autotrophic.

Final Answer:
The organism that depends on a saprophytic mode of nutrition is Agaricus, a common mushroom that feeds on decaying organic matter.