ABSTRACT
As areas of cultivation of single crops enlarged, the build-up of pathogens increased in importance and man quickly learned to refrain from farming the same annual crop in the same ground for several years in succession. The grower has always recognized the importance of eradicating weeds that compete with his crop. The concept that there is a specific but progressively changing target status for each cell throughout the life-span of its development is central to present thinking on the differentiation and organization of plant form and function. Abscission zone cells of dicotyledons all exhibit the characteristics of Type 2 cells in that they can enlarge and separate in response to ethylene, and the ethylene response is repressed by auxins. The cells of an actively dividing meristem encounter internal and external signals throughout the cell cycle. When a chemical is applied to a plant an array of different factors assumes significance in determing the eventual morphological and biochemical changes that occur.
