Historical Constraints and the Evolution of Development 1093
pathways in higher plants" (p. 1647), and that suppression by the lee mutation
therefore causes reversion to a ground state—which, as Goethe proposed so long ago,
most closely resembles a stem leaf in basic form. (Biologists with a zoomorphic bias,
including the author of this book, may be confused by a claim that embryonic
features might thus be conceived as departures from a ground state. The directionality
of bilaterian ontogeny, with embryonic features as transient and formative, leads us to
equate embryonic forms with any sensible concept of a "ground state." But plants
maintain embryonic tissues throughout life as restricted and persistently specialized
regions on differentiated foundations that animal biologists might tend to regard as
"adult." Therefore, a botanical rationale for viewing these foundations as a ground
state, with embryonic tissues as a specialization, can easily be defended.)
Meinke (1992, p. 1649) concludes: "The phenotype of leafy cotyledon suggests
that the difference between leaves and cotyledons in Arabidopsis is controlled by a
single regulatory gene (LEC) expressed only during embryo-genesis." Then, in a
statement strikingly evocative of Goethe's archetypal theory, he portrays (1992, p.
1649) the ordinary stem leaf as a ground state, with all its serial homologs (to apply
this zoomorphic term to cotyledons and, putatively, to flower parts) as specializations
thereupon: "The preferred model is that LEC functions to activate a wide range of
embryo-specific pathways in plants. Loss of gene function disrupts embryonic
maturation and returns mutant cotyledons to a basal developmental state. The leafy
appearance of mutant cotyledons was unexpected because there was no evidence that
cotyledons defective in maturation should be transformed into foliage leaves.
However, this observation is consistent with the origin of cotyledons as specialized
leaves during plant evolution and the homology of embryonic cotyledons and
vegetative leaves."
For the more complex organs of inflorescence at the other end, Weigel and
Meyerowitz, in their classic review (1994) of the ABC model (see pp. 1063-1065) for
floral development in Arabidopsis (and many other angiosperms, though perhaps not
all, see Kramer and Irish, 1999), posed a first key extension beyond the model's basic
elucidation: "The ABC model left one complication, though: what happens in the
absence of all organ identity activity" (p. 203). Weigel and Meyerowitz then turned to
Goethe for the classic prediction based on notions of the archetypal leaf: "Goethe
(1790) had proposed that floral organs represent modified leaves, suggesting that a
vegetative leaf is the ground state of floral organs."
Weigel and Meyerowitz presented striking evidence to confirm this Goethian
prediction that suppression of all ABC activity should cause presumptive floral parts
to approach the ground state of stem leaves. The sequential action of ABC genes
permits a simple formulation of tests for this hypothesis. AC double mutants, for
example, should knock out determinants for the outermost sepals of whorl 1
(triggered by A genes alone) and the innermost carpels of whorl 4 (C genes alone),
but impose less effect upon the petals and stamens of whorls 2 and 3, which also
require the influence of B genes (see