Introduction

Early biologists found it convenient to classify all living things as either animals or plants. To many people today, this grouping still seems perfectly adequate. However, examination of the life-forms that exist on Earth shows that this classification is unsatisfactory. Although there is a superficial resemblance between green plants and the fungi, these two groups are divided by profound biological differences. Unlike green plants, fungi cannot manufacture their own food from water and carbon dioxide by the process of photosynthesis. Rather, they require a supply of organic matter from which they can derive their energy. Fungal cellular composition is dissimilar from that of green plants, and the structural polymers of their cell walls are markedly different. Fungi are therefore now accorded their own status as a third kingdom. Furthermore, for many years the classification of microscopic organisms proved to be difficult. Photosynthetic microbes behave very differently from higher plants. It was therefore proposed towards the end of the nineteenth century that microscopic life forms should be classified as a fourth kingdom. This was the kingdom Protista, proposed in 1866, at a time when the scientific study of microbiology was in its infancy. This was, however, almost 200 years after Antonie van Leeuwenhoek described ‘animalcules’ following his development of the optical microscope.

During the twentieth century there have been many advances in microscopy, including the development of the electron microscope.