What was the main basis for grouping all plants together in the Two-Kingdom classification?

Correct answer: Presence of a cell wall

Direct answer: B, presence of a cell wall. In the old Two-Kingdom system, organisms were broadly placed in either Plantae or Animalia using a few easily observed features. A major feature associated with plants was the presence of a cell wall. This simple basis caused many unrelated organisms with cell walls to be grouped together in Plantae, even though they differed in cell organization, nutrition and evolutionary history. A, mobility, is not the best answer because many plant-like organisms may be motile at some stage, and lack of movement is not a complete classification criterion. C, mode of nutrition, does not explain the historical grouping as accurately because organisms placed in the plant kingdom did not all have exactly the same nutritional method. D, size is highly variable and has little taxonomic value. The question asks about the historical basis, not the modern criteria used in classification. Memory cue: old plant grouping was strongly linked with a cell wall, while modern systems use several structural, nutritional and evolutionary characters.