The Standardization of Smog

Of the plants commonly found in Los Angeles County whose responses to smog 2 have been under extensive observation, Poa annua (annual bluegrass) was selected as one worthy of intensive study. It was early reported as one of the most sensitive of the plants observed in the field (11). Because it has a regular habit of growth, can be easily and quiekly grown in small containers, and because the band of damage caused by smog is measurable, Poa annua lends itself to quantitative work. It was therefore thought that it might be standardized to serve as a biological indicator of smog concentrations. A plant indicator has a unique value, since it measures smog in terms of the damage potential to the living cell. It also gives a visible record of the effects of smog conditions at the time and place at which it was exposed. The technique of filtering from the atmosphere its plant damaging components by means of activated carbon filters was devised in 1950, in a project sponsored jointly by the California Institute of Technology, the University of California, and the Los Angeles County Air Pollution Control District (4). Comparison could then be made between plants grown in air containing smog, and those grown in the same conditions in pure air. It became evident to many observers that certain portions of the mesophyll tissues were acted upon selectively by smog, and that damage was usually found in areas of rather young tissue. Noble (12) described the pattern of damage found in many species, including Poa annua, with special attention to the location of damaged areas as a function of the maturation of the leaf. Bobrov (3) made an anatomical study of Poa annua, in which she showed by photomicrographs the relation of susceptible portions of the blade to cells in the young-mature stage. Older cells were shown to be more heavily suberized. Loftfield (9) in 1921 had shown that alfalfa was susceptible to sulfur dioxide only during those hours in which the stomata were open. Middleton, Kendrick and Schwalm (11) described the microscopic appearance of damaged areas on the leaves of a number of broad-leaved crop plants, noting that damage occurred especially in the mesophyll in the region of the stomates. Bobrov (2, 3), using sections of fresh