Phasic Development of Plants

Summary 1. Investigations on the cause of the failure of winter plants to ear in the sowing year have led to a theory of plant development based upon a discrimination between growth and development. The latter is regarded as a sequence of qualitative changes or phases, each consisting of complex internal readjustments which lead to eventual reproduction. The developmental phases proceed in a strict rotation and a subsequent phase cannot begin until the preceding phase has been completed. 2. Although growth has no causal connexion with development, the true relationship between development and growth has not been conclusively investigated, while some circumstantial evidence suggests that development can be maintained only with a certain minimum of growth. 3. As development is at least relatively independent of growth, the embryo can be induced to develop at a much reduced growth rate before sowing and even before complete seed ripeness. This is known as vernalization. Although vernalization has no direct effect on the vegetative period after sowing, subsequent development proceeds under relatively different conditions. The effect of vernalization is thus analogous to that of a change in time of sowing. 4. The first three developmental phases and a possible transitional phase between the first two have already been established; the first two and the transitional phase being connected with the initiation of floral organs and the third with gametogenesis. 5. For completion of each of the developmental phases a different but definite complex of environmental factors is required. It is essential to discriminate between factors which do or do not affect the progress of a phase. In addition, among factors which affect progress, those which are indispensable for a phase must be distinguished from those which affect the progress of a phase only in the presence of indispensable factors. 6. For vernalization of the first phase a definite balanced complex of temperature, moisture and aeration is required by a physiologically potent embryo. This complex varies widely within and between species. The effect of light and darkness has not been convincingly demonstrated, although the indirect effect of day length is not excluded, particularly for a plant which is distinct from the embryo in a seed as regards the mode of nutrition. A critical thermoperiod can be established, below which the vernalizing temperature becomes ineffective. 7. The complex of factors required for the second and subsequent phases has not been fully studied. In environmental studies particular attention must be given to the optimal blend of day length and temperature. Long‐day plants require light to complete the second phase and can only tolerate darkness if the latter is not in excess, while short‐day plants show the opposite relation. The darkness requirement of short‐day plants can be deciphered as an inhibitory effect of light upon the second phase, if the intensity of light is above a definite maximum. The relation of cereals at the third phase seems to be in conflict with Lysenko's ecological concepts. 8. The physiology of plant development has received little study. Not all the changes pertaining to development are detectable morphologically. The changes constituting a phase are gradual, quantitative, additive and irreversible, and their completion causes a qualitative change in the internal environment of a plant. Thereby the properties and relation of a plant to its environment are also changed. 9. The changes pertaining to an advance in development are elaborated and retained in the promeristem and transmitted only through cell division, further elaboration taking place in the daughter cells. The entire body of the embryo is sensitive to temperature, whereas the response of the promeristem to photoperiods is effected through some activities in the leaves. In the latter a catalytic substance is secreted, which on being transmitted to the tip participates in the developmental processes. 10. The nature of the changes constituting the first phase has received little attention. The hormonal theory of vernalization is inconsistent with the theory of phasic development and has not been substantiated. Of the enzymes studied, only catalases and peroxidases were suggestive. Presumgbly the enzymes and hormones are concerned more closely with the rate of growth than of development. The endosperm, aleurone layers or integuments do not participate directly in the developmental process, which is confined to the embryo alone. Investigations of the physical and chemical changes in the protoplasm in relation to phasic development were more suggestive and provided a method of diagnosis of the first two phases. The nature of the second and subsequent phases has not been studied. 11. In environmental studies the biochemical method of diagnosis should be employed whenever possible, as the conceptions based upon the “after‐effect of vernalization” are not always reliable. The possibility of vernalizing the embryo during seed ripening has necessitated a thorough revision of the conceptions hitherto formed regarding the length of developmental phases and earliness or lateness in various ecotypes. 12. The genetical conceptions announced by Lysenko differ but little from the orthodox conceptions, greater stress having been laid on the effect of the environment. The cytogenetical conceptions require further study, while the changes in the genotype of a plant induced, as it is claimed, as a result of adaptation to a new environment (training of plants) must be studied cytologically.