1094 THE STRUCTURE OF EVOLUTIONARY THEORY
p. 1063 and Fig. 10-12). Experiments then confirmed this precise, and rather odd,
prediction: "Indeed, organs in these two whorls are very much like vegetative
leaves—they develop with stipules, are green and covered with branched hairs, and
senesce slowly, all characteristics of leaves but not of floral organs" (Weigel and
Meyerowitz, 1994, p. 203). By the same logic, triple mutants should grow all floral
parts in leaf-like form—as they do: "In triple mutants that lack A, B, and C activities,
all floral organs resemble leaves" (pp. 203- 204 —and Fig. 10-14), thus supporting
(Pelaz et al, 2000, p. 202) "the theory that flower organs are simply modified leaves."
Theissen and Saedler (2001, p. 470) add, with specific homage to Goethe: "combined
loss-of-function of class A, B, and C genes results in a transformation of all floral
organs to leaves, corroborating Goethe's view that leaves are a developmental ground
state."
Moreover, gain of function mutations also confirm the model by imposing inner
floral expression upon outer parts, thus resembling the action, for a different
symmetry of radial whorls, of classical homeotic mutations of Drosophila, expressed
in a linear, anteroposterior array. Over expression of C genes, for example (1994, p.
206), represses A functions in whorls 1 and 2, "with carpels where sepals are usually
found, and stamens in the places ordinarily occupied by petals" (p. 206).
Later work has revealed some of the upstream regulators of this system. For
example, Pelaz et al. (2000) identified three genes (named SEP1/2/3) required for the
action of B and C genus that regulate the inner three whorls of petals, stamens and
carpels. In triple mutant Aribadopsis plants that suppress the action of SEP1/2/3, all
floral whorls develop as sepals (which are regulated by A genes). (See Honma and
Goto, 2001, for later data on the even
10 - 14. Mutations that delete activity of all ABC genes cause all floral organs to develop as
leaves. Ordinary flower at A; triple mutant with all flower parts replaced by leaves at B. From
Weigel and Meyerowitz, 1994.