ABSTRACT
The understanding of the effects of physical and chemical agents on chromosomes can be attributed principally to the pioneering works of L. J. Stadler in maize, F. Oehlkers on snapdragon and A. F. Blakeslee and A. Avery on Datura. A systematic attempt to explore the properties of different chemical agents was first made by A. Levan and his collaborators. The meristematic or the dividing zone represents a heterogeneous mass of cells, in which the nuclei are at different stages of division. The most widely used chemical for securing a large number of metaphase plates is colchicine. It causes metaphase arrest by inhibiting the operation of the spindle mechanism. Chromosome breakage can be induced by several chemical agents. Fragmentation, followed by translocation of some fragments, may bring about a new patterning of chromosome segments resulting in heritable phenotypic difference categorized as subnarcotic effect.
