Fig. 8. Diagram illustrating how petal length and width, and sepal length and width are combined to form an ideograph. Above, Iris virginica; below, I. versicolor.
Photograph of three-dimensional model showing the precise geometrical relationship in petal and sepal size and proportions of Iris virginica (left), I. versicolor (center), and I, setosa (right). In the model, Iris versicolor is placed two-thirds of the distance between the two putative parents, since their chromosomal contributions to the hybrid are in the approximate ratio of 2 to 1.
Anderson, E. (1936) ‘The species problem in Iris’, Annals of the Missouri Botanical Garden, 23(3), pp. 457–469–471–483. [link]