In einzelnen Blüthen und an einzelnen Pflanzen kann jedoch das Verhältniss, in welchem die Formen der Reihe gebildet werden, nicht unbedeutende Störungen erleiden.
In individual flowers and on individual plants, however, the proportion in which the forms of the series are produced can suffer some not insignificant disturbances.
proportion = Verhältniss See p. 3, s. 8
produced = gebildet from bilden, to shape, form, or educate, again witrh developmental connotations.
not insignificant = nicht unbedeutend Mendel is using a double negation here to emphasise that deviations from the expected ratio can be quite pronounced.
forms = Formen See p. 3, s. 8.
disturbances = Störungen Bateson has “fluctuations”, Sherwood “deviations”; see p. 5, s. 7.