Drawbacks of five kingdom system of classification:
1. The kingdoms monera and protista are still heterogenous groups of organisms. Both include photosynthetic and heterotrophic organisms with cellwall and without cellwall.
2. In this system of classification phylogenetic relationship particularly of lower organisms are not fully reflected, e.g., some green algae and some photosynthetic bacteria get hydrogen from sources other than H2O but these are grouped under different kingdoms.
3. Each kingdom is polyphyletic. For example, many different protistian lives gave rise to multicellular group and none of the these higher kingdoms contains the single ancestor of all its members. Multicellular lives originated from protists many times.
4. Unicellular green algae, such as Chlamydomonas, volvox have not been grouped in the kindgom protista due to their resemblence with the other green algae in structure, reproduction and pattern of life cycle.
5. The slime moulds are quite different from the other protists with which they have been combined.
