References

4 How Can Plants Choose the Most Promising Organs? 61
formation of auxin occurs in all parts of a developing shoot, but especially
in young expanding leaves (Ljung et al. 2001). Though the quantitative data
are meager, available knowledge about auxin synthesis and vascular differentiation
suggest that its synthesis is enhanced by light. Since neighboring
branches shade one another, competition is environmental, not only internal
(Sachs et al. 1993). Yet if shade influences auxin formation both types of
competitioncouldreflectthesameinternalmechanismbymeansofcompetitive
vascular orientation. All this is in accordance with the hypothesis
that auxin is a signal that integrates the information about the location, size,
environment and rate of development of a growing branch (Sachs 2004).
Are there other possible mechanisms of branch competition, in addition
to auxin-induced orientation of vascular differentiation? These mechanisms
need not be mutually exclusive and it is possible, or even likely, that
more than one has a role in a process as central as the relations between
plant organs. An alternative to oriented differentiation is the adaptation of
plant tissues to the higher level of auxin that is supplied by the stronger
branch (or other signals that a shoot may produce). The cambium thus
responds to the “best” branches – the ones that are the strongest sources
of auxin – and “ignores” the weaker branches, whose vascular tissues deteriorate
without being replaced. This possibility is plausible and it would
account for evidence that interactions between branches need not require
vascular reorientation (Snow 1937). However, there appears to be no concreteevidenceaboutitscellularbasisandthewayadaptationtohighauxin
couldspreadthroughplanttissues.
4.4
Conclusions and Future Prospects
A general conclusion goes beyond the relations between tree branches. Similar
processes of developmental selection could have a large role in biological
pattern formation (Edelman 1987; Sachs 1988, 1991, 2002; Frank 1996,
1997). This selection actually generates information about the location of
the different structures, such as branches, during development itself. In
this it differs from programs or prepatterns in which detailed information
precedes actual differentiation. Developmental selection shares principles
with the Darwinian mechanism of evolution (Frank 1997), though there
is an important difference. The various branches of a tree are genetically
identical and the outcome of their selection is an adaptive form, not an
evolution of a new genetic system. Conflicts of interest between alternative
branches or other structures of the same organism do not arise.
The specification of form by stochastic variation followed by selection
appears to be counterintuitive, but it is supported by direct observations of