General

The term ‘growth factor’ has been defined in two ways:

1. A compound that is required for growth in small amounts but excludes those compounds that function as structural material and are not used for energy (Cochrane, 1963).

2. A compound that in minute amounts is necessary for, or stimulatory to, growth and does not serve merely as an energy source (Fries, 1965).

The second definition clearly encompasses a broader range of compounds than the first. Fries (1965) in amplifying his definition pointed out that it includes certain organic compounds, e.g. amino acids, nucleotides, fatty acids. These compounds would not be covered by the first definition because they or carbon skeletons based on them are incorporated into structural material. Equally, the compounds are usually required at higher concentration (10–1000 pg cm−3) in contrast to 0.01–1 pg cm−3, the concentration at which vitamins (the bulk of those compounds embraced by the first definition) are required by fungi. By vitamins I mean those compounds as understood by other biologists to come under this title, i.e. the compounds have a catalytic function in the organism as a coenzyme or constituent part of a coenzyme.

Nevertheless, it is difficult to make a clear separation between the two categories. Cochrane (1963) pointed out the arbitrary nature of the definition that he used. Thus, riboflavin is included under it but one cannot separate adenine's cofactor-like role in ATP from its part in nucleic acids.