After comparing sixteen aspects of their biology it is clear that despite wide differences in botanical affinity bryophytes and lichens are outstandingly similar in their response to SO2. This suggests that these lower plants can only colonize inhospitable habitats by adaptations which carry with them an inherent susceptibility to air pollution. Extensive laboratory and field work has emphasized the controlling influence of habitat on the survival of species. Among the strongest modifying influences are shelter, the pH and buffer capacity of the substratum and nutrient flushing. There is an apparent connection between survival and age of substratum. Water relations were found to have little effect. The extreme sensitivity of most epiphytes and the problems raised by resistant species are discussed.