FOUR BASIC QUESTIONS OF PLANT BIOLOGY

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Abstract
The 4 questions discussed are the nature of life, the mechanism of heredity, the mechanism of development, and the problem of evolution. The basic property of life is organization, but no single property, present in all living things, distinguishes them from nonliving systems. Life results from integration of various properties, which at the individual level are principally replication of specific substances and response to stimuli, but at the level of the population in time the most significant property of life is its ability to generate unpredictable systems of variability. Heredity is determined primarily by the DNA‐RNA‐protein system, which must be regarded as a single functioning unit, since its elements are functionless when separated from each other. In addition, 4 different kinds of mechanisms for regulating gene action are postulated. Our full understanding of heredity must include a thorough knowledge of these mechanisms, as well as of the reasons why organisms differ in base pair composition of their DNA, and in the total amount of DNA present in the nucleus. Development in higher plants does not involve changes in basic heredity, but rather semi‐permanent alterations in the physicochemical conditions of cells. Modern research on cell, tissue, and organ culture is beginning to elucidate the nature of these alterations, and to relate them to gene‐controlled enzyme activity. The translation of physicochemical differences in cells to morphogenetic behavior is principally via 4 processes, which are beginning to be understood: differential rates of cell division and cell enlargement; orientation of the mitotic spindle by cell shape, cell growth, and cytoplasmic structure; intracellular polarization of cytoplasmic elements followed by differential mitosis across the gradient thus produced; and intercellular induction of mitosis. In regard to evolution, the basic processes controlling it have been identified and are reasonably well known. Future research should be on the nature of hybrid inviability and sterility, the comparative evolution of genetic systems, and the question of whether new morphological characteristics require the origin and activity of genes with new enzymatic activities, or are brought about by repatterning previously existing sequences.