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Plants are an advantageous group for the consideration of the development of biological form. Plants share most aspects of cell biology with other organisms, yet their embryonic development continues throughout their life, their cells do not move relative to each other and their structure is relatively simple. The chapters in this book are centred around the structure of tissues and its purpose is to try and predict what should be looked for at a molecular level so as to account for observable forms. Each chapter deals with a defined problem such as the role of hormones as correlative agents, tissue polarization, apical meristems and cell lineages. The final chapter develops an alternative approach to the problem of the specification of biological form, that of 'epigenetic selection'. The chapters are centred around the structure of tissues, an intermediate and neglected level between overt morphology and biochemistry, and will be of great interest to all those engaged in attempting to understand the principles behind plant development.

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The paper deals with the general problem of the physiological basis of branching, and the roles of known and unexplored factors in sensitivity to apical dominance. It is shown that when pea seedling shoots are completely or partially inhibited by other shoots on the same plant auxin can promote their elongation, even though it does not have this effect on inhibited buds. This influence of auxin is only exerted on internodal elongation and not on apical growth. When kinetin in a solution of alcohol and carbowax is applied directly to the lateral buds of pea seedlings, it releases them from inhibition by the growing apex. It is shown that the role of alcohol in this solution is to act as a surfactant, permitting good contact with the buds, while that of carbowax, being hygroscopic, is to maintain a thin film of solution over the buds. Buds thus released from apical dominance by kinetin do not elongate as much as do uninhibited control buds. Such kinetin‐treated buds can, however, be made to elongate normally by the application of auxin locally to their apices. It is concluded that growing shoots are relatively insensitive to correlative inhibition because they synthesize two types of growth substances, namely, auxin, which antagonizes the inhibitory effect on internodal elongation, and cytokinins, which permit the apex itself to develop. In the discussion it is brought out that many cases of branching, which appear at first to bear little relation to one another, can be understood on the basis of two principles, namely: (1) Any reduction in the growth rate of a dominant apex reduces its inhibitory effect on other apices, and (2) once an apex starts growing it becomes less sensitive to inhibition by other apices These generalizations and the experimental results are tentatively interpreted in terms of an interaction between the syntheses of auxin and of cytoldnin.