Tree forms have been classified according to the aspects of their development that follow relatively invariable programs. However, a prominent variability or plasticity is apparent in the duration and rate of the development of individual shoot apices and the probability that a branch, once formed, remains on the tree. Even though overall form may be predictable, these variable processes have a major role in the detailed structure of tree canopies. It follows that there must be mechanisms that constrain or limit chance events after, rather than before, they have been expressed in tree development. Constraining mechanisms include the inhibition of the development of a branch by another branch that had already started growing. This inhibition is due to both internal signals and to mutual shading. It results in any available space in a canopy being occupied by one or more branches, even though the precise location of these branches is not specified. The development of leading branches is also constrained by a progressive differentiation and growth limitation which is a function of the distance of the apices of these branches from the roots. At both the cellular and the organ levels unpredictable details are a characteristic of advanced, late evolutionary groups. Their prevalence means that architectural models stress an important, but partial, picture. There could be no ideal models nor a correct tree classification; there is no alternative to comparing tree forms in relation to major characteristics chosen according to the problem at hand.
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