The to rate of growth of plants is relatively easy measure and often provides an index of the toxic effects of pollutants. A reduction in the rate of growth can lead to stunted development and dwarfism, a response that can be observed in the leaves of trees exposed to sulphur dioxide and other aerial pollutants. There is also the question of the action of chemicals which can affect growth regulation. The so-called plant growth regulator' herbicides (see sections 1.2.10 and 7.4.10) can cause characteristic growth disturbances in plants. Included here are the widely used phenoxyalkanoic acids MCPA, 2,4-D, CMPP and 2,4-DB. They cause distorted growth, malformed leaves and severe epinasty (downward curvature) of stems and les. They cause unequal growth of young rapidly developing tissues near the meristem. Effects of this kind will be familiar to gardeners who have used them as selective herbicides to control broad-leaved weeds (dicots) in lawns.