Slime molds and plant-like features (historical classification): Why were slime molds historically considered to resemble plants in some classifications, despite being heterotrophic protists?

Introduction / Context:
Slime molds are now grouped among amoeboid protists, but historically they were often compared with plants or fungi based on morphology and wall chemistry found in their reproductive structures. Understanding these legacy reasons helps students interpret older literature and exam items that reflect traditional views.

Given Data / Assumptions:

• Slime molds are not photosynthetic and lack chlorophyll in their trophic stages.
• Some stages (such as spores or stalks in cellular slime molds) may possess cellulose-rich walls.
• The question asks for the feature that led to plant-like comparisons.

Concept / Approach:
Plant cell walls are typically rich in cellulose. Although the feeding (amoeboid/plasmodial) stages of slime molds lack rigid cellulose walls, certain reproductive structures (e.g., spore walls, stalks) can contain substantial cellulose, making them appear “plant-like” to early taxonomists. This, along with their production of fruiting bodies, historically influenced classification debates.

Step-by-Step Solution:
Eliminate photosynthesis: slime molds do not harvest light energy like plants. Focus on wall chemistry: presence of cellulose in specific structures aligns with plant-like characteristics. Ignore superficial shape analogies (threads or leaf-like forms) as non-diagnostic. Select the cellulose-related option as the historically accurate rationale.

Verification / Alternative check:
Textbook treatments note cellulose in the stalk/spore walls of some slime molds and extensive fruiting resembling plant sporangia, which misled earlier classifiers.

Why Other Options Are Wrong:
They cannot photosynthesize; threadlike appearance is not a plant-specific trait; leaf-like structures are not formed; pigment composition does not include chlorophyll for photosynthesis.

Common Pitfalls:
Assuming vegetative plasmodia have cellulose walls; they typically do not and can be wall-less and motile.

Final Answer:
They have cell walls composed largely of cellulose (in certain stages/structures).