PubMed · 14800298
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W H SCHOPFER. 1950. [Morphogenesis and vitamins].. https://pubmed.ncbi.nlm.nih.gov/14800298/
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A morphodynamic model for phyllotaxis based upon an axiomatic approach is presented. We show that the helical forms of alternate phyllotaxis can be derived from the assumption of the rudiment's growth and movement on the cylindrical embryo surface in the absence of a longitudinal displacement. This leads to the repeating transition of tetragonal packaging of the rudiments into hexagonal packaging and vice versa. Under these conditions, sequences of rudiments produce either left-handed or right-handed helices, the number of which at the circumference of the cylinder corresponds to adjacent numbers of the Fibonacci series. Cross-opposite phyllotaxis forms are defined as superior with respect to the alternate ones, and verticillate phyllotaxis forms as superior with respect to the cross-opposite ones. Different phyllotaxis forms can be interpreted as a result of stretching of crystalline structures of the embryo formed by dense packing of rudiments. The superior phyllotaxis forms can be considered as the additive summation of lower forms. Morphodynamic mechanisms underlying the formation of multiple forms of helical phyllotaxis are discussed.
In the last few years, somatic embryogenesis and organogenesis of white spruce (Picea glauca) have been used as model systems to investigate biochemical and physiological events related to morphogenesis. This has been possible because studies conducted in vitro allow for manipulations of the culture conditions, in which changes in morphogenetic events can be easily related to physiological alterations. De-novo synthesis, salvage, and degradation of both purine and pyrimidine nucleotides are operative at all stages of somatic embryo maturation and germination. Fluctuations in the activity of these pathways delineate important morphogenic events. The early phases of embryo development are accompanied by a decreased salvage activity of purine nucleotides, which reflects a reduction of cell proliferation and the initiation of organized growth. Activities of the salvage enzymes are present throughout the maturation period, and also during the imposition of the drying period, which is required for successful embryo germination. The operative salvage pathway in dried embryos is needed for the enlargement of the nucleotide pool necessary to sustain the reactivation of the overall cellular metabolism at germination, before the reactivation of the de-novo pathway, which is a later event. Manipulations of the culture medium which improve the germination frequency of the embryos also result in increased salvage activity. Similar changes in nucleotide synthesis were also observed during the initiation of shoot development from epicotyl explants of white spruce and cotyledons of radiate pine. Results from these studies can be used for improving growth and development in culture.
Both the physiological and the pathological morphogenetic processes that we can meet in embryogenesis, neogenesis and degenerative dysgenesis present common features: they are ruled by three different kinds of mechanisms, one related to cell migration, the second to cell differentiation and the third to cell proliferation. We deal here with an application to the cambial growth which essentially involves the third type of mechanism. Woody plants produce secondary tissue (secondary xylem and phloem) from a meristematic tissue called vascular cambium, responsible for the radial growth of a tree. This paper focuses on the formation of secondary xylem, considered in two dimensions in a cross-section framework. A new discrete modelling approach is used, based on the cellular scale, in order to attain a more accurate understanding of how the elementary microscopic behaviour of each cell takes part in the macroscopic morphogenesis. The mathematical model essentially uses an occurrence method simulating the main features of radial growth with simple geometric rules, such as Thom's division rule (Thom,1972)to account for the cell proliferation. The study applies to concrete instances in which the changes made in the geometrical cellular patterns of the vascular cambium clearly affect the shape of the tree, as in Pinus radiata (D. Don.).