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of a single character classification is likely to produce groups containing
members whose relationships are heterogeneous except for the single character.
The principle of overall similarity (see definition p. 20), besides giving
a better reflection of the overall gene content of the cultivar, also minimizes
the effect of a character that may not properly reflect the natural affinity
of the cultivars. The power of this type of similarity measure has been demonstrated in several applications: Wirth et al. (1966), Orchidaceae; Irwin and Rogers (1967), Cassia, Leguminosae; Prance et al. (1969), Chrysobalanaceae; Hawksworth et al. (1968), Arceuthobium, Viscaceae.

For any single character, the character states we have selected serve to divide the study into parts (or, in other words, will make a single-level classification). A character that has a large number of states carries more information than one with few states, and will partition the study into more discrete parts. On the other hand, if the states do not reflect a valid gene expression, or if the states are so finely delimited that it is difficult or impossible to assign a specimen with confidence to one particular state, very little useful information will be represented. In this study, cultivars rather than specimens are being compared and classified. For instance individual specimens within a cultivar might have distinguishable but very fine differences in the color of the petiole of the leaf. But the circumscription of the state assumed by the cultivar must be expressed in a manner that contains or subsumes these more exact and particulate color refinements.

The characters chosen to group and classify the cultivars of M. esculenta had to be comparable over the whole of the species and to be at least relatively stable, the latter being the most difficult restriction in regard to a collection of cultivars. The subsequent characters have been observed in sufficient frequency to recognize that they are stable within and between cultivars and the only ones consistently observable in a species whose cultivars may not