EVIDENCE RELATING TO THE DIFFUSION‐REACTION THEORY OF MORPHOGENESIS

already determined by some feature in the inherent organization of the cell or tissue. Turing (1952) has proposed a diffusion-reaction theory of morphogenesis, based on well-known laws of physical chemistry, in which he has indicated how, in an embryonic tissue with an initially randomized or homogeneous distribution of diffusible reacting substances, a regular, stable, patternized distribution of metabolites may nevertheless be produced; the inception of this pattern being the initial phase of the ensuing morphological and histological developments. A simplified account of this theory has been given by the writer (Wardlaw, 1953 a), and its application to morphogenesis in plants discussed. Turing's theory, if it can be justified, may account for such phenomena as polarity, whorled and fibonacci phyllotaxis, the radiate differentiation of tissues in the root-stele, and so on. As thus far developed, the theory relates to relatively simple and symmetrical patterns. Its further elaboration to more complex patterns will undoubtedly be difficult; and its mathematical presentation is likely to be beyond the non-mathematical botanical investigator. Nevertheless, once the central idea of the theory has been grasped, one can understand that a reaction system, which initially gives rise to relatively simple patterns, is likely to become more complex and to give rise to more complex ones as development proceeds. Moreover, asymmetries in the environment of the reaction system, i.e. the adjacent organs and tissues, may induce characteristic asymmetries in the system and in the patterns to which it gives rise. Turing's theory is evidently of great importance in the investigation of morphogenesis, for it has a very general application to problems of organismal form and structure; and even if it cannot be justified, it certainly seems to point in the right direction and the work expended on it may lead to the formulation of an alternative and more appropriate