[…] vowel changes […] correlate with meaning changes. They cannot be expressed by morphophonemic formulas, since these formulas state the alternate forms of a single morpheme, whereas 'take' and 'took' are not the same morpheme, having different meanings. Such vowel changes are usually described as special kinds of morphological modification, though they may alternate with additive suffixes like '–ed' ‘past time’.
- Harris (1942a), a pag.170