Two Kingdom classification system:

It is the oldest system and classifies all organisms into two kingdoms i.e. Plantae and             Animalia. According to it, all organisms that can prepare food from simple inorganic materials and thus can store energy, are autotrophs and are included in Kingdom Plantae. On the other hand, the organisms that cannot synthesize their food and depend on autotrophs or others are heterotrophs and are included in the kingdom Animalia. According to this system, bacteria, fungi, and Algae were included in Kingdom Plantae.

Drawbacks of Two-Kingdom Classification system:

Some taxonomists found the system unworkable because; many unicellular organisms like Euglena have both plant-like (presence of chlorophyll) and Animal-like (Heterotrophic mode of nutrition in darkness and lack of cell wall). Characters so there should be a separate Kingdom for such organisms. This system also ignores the difference between organisms having prokaryotic and those having Eukaryotic cells.