The influence of neighbours on the growth of modular organisms with an example from trees

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Abstract
The growth and form of a modular organism is determined by the rigid rules of iteration (branching) and the differential response of each growing point to the local conditions around it. The degree of response of each individual module is itself dependent on the degree of physiological integration of the whole organism. Morphological continuity is a requisite but not a guarantee of physiological integration. In general, ‘phalanx’ growth-forms show more physiological integration than ‘guerrilla’ growth-forms. Trees, as an example of morphologically integrated modular organisms, show a variety of responses to the presence of modules both of the same and of different species. When two modules interact, three extreme responses are possible: (i) both modules stop growing or change their orientation, or both; (ii) one module is inhibited while the other continues its growth; (iii) neither is affected by the presence of the other. The first case produces a clear separation (‘shyness’) between neighbouring modules both within and between trees. The second case produces a hierarchy of dominance-suppression. Finally, failure to ‘recognize’ the presence of a neighbour module may result in physical damage by abrasion of both participants. Under certain circumstances, this can also produce a visual impression of ‘shyness’. The importance of this ‘recognition’ mechanism is discussed for both modular animals and plants.