![]() ![]() Shoot apical meristems (SAMs) of seed plants are small groups of pluripotent cells responsible for making leaves, stems and flowers. The expression of this mutation in genetic mosaics demonstrates that its effect does not diffuse laterally within the leaf primordium. The Okra mutation acts early in the development of a leaf and appears to accentuate a developmental pattern that is also responsible for heteroblastic variation in leaf shape. Cells in the marginal region of the developing cotton leaf contribute more to the growth of the lamina than they do in tobacco. Clonal analysis demonstrates that both the rate and duration of cell division are fairly uniform throughout the leaf. Allometric analysis of leaf growth suggests that leaf shape is determined during the initiation of the primordium rather than during the expansion phase of leaf growth. The effect of these factors was investigated by allometric analysis, cell lineage analysis, and by studying the expression of the leaf shape mutation, Okra, in genetic mosaics. Leaf shape in cotton is regulated by the developmental age of the shoot and by several major genes that affect leaf lobing. A short account each on heteroblasty, heterophylly, senescence and evolution of leaf is also provided. This article also discusses the ontogenetic and genetic bases of the differences between simple and compound leaves. The genetic basis for shoot apical meristem (with indeterminate growth)-leaf primordium (with determinate growth) boundary is also discussed. This chapter deals with the genetic network that operates during various phases of leaf ontogeny. The leaf primordia are initially with a leaf axis from which the lamina, petiole and phyllopodium regions of the mature leaf arise. The various theories to explain this pattern are briefly described. Leaves are arranged on the stem with very characteristic and non-mutagenic phyllotactic pattern characteristic of each plant. They are always associated with shoot apical meristems from which they arise. Leaves are the most important organs of plants and carry out very vital physiological activities such as photosynthesis, respiration, transpiration, photoreception and synthesis and supply of signal compounds, including growth regulators. ![]()
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