The thallus of the lichen Ramalina menziesii Tayl. is comprised of subunits which resemble planar nets. The nets arise upon older nets as buds which develop perforations and expand. Microphotographic study of thallus development in the field shows several features which differ from a recently proposed model of development for this species. New perforations develop continually in young tissue produced at the apical margin of the net. The apical margin is inrolled, and may furcate. This type of branching is developmentally distinct from the formation of new nets from buds borne upon expanded parts of the net. Anatomical study of net apices appears to support assertions that programmed tissue necrosis contributes to perforation formation at sites where gaps are present in the algal layer. Thalli of Ramalina menziesii from different localities show conspicuous morphological differences. Highly reticulate forms are found at inland sites, while fine, dissected forms occur near the coast. Direct observation of net development in morphologically different populations indicate that the basic pattern of bud formation, perforation, expansion, and apical production of new tissue is similar. However, differences in the relative degree and timing of these growth processes can be related to the morphological variation observed. In net buds of the more finely dissected forms found near the coast, precocious perforation and expansion of the youngest tissue within the apical margin creates fragile nets whose growing margins often become contorted and destroyed while nets are still small. This disrupts the production of new reticulations. Two types of deviation from the basic pattern of bud development were also observed sporadically in several populations. Some nets may develop fundamentally three-dimensional reticulation patterns, due to secondary fusions of young tissue within the inrolled apical margin. In other nets, the apical margin loses its inrolled appearance and becomes planar. Such net apices cease production of new reticulations and mature as part of the reticulum. Preliminary developmental studies involving reciprocal transplants resulted in vigorous growth of inland thallus nets at the coastal sites, but little or no growth of coastal thalli at inland sites. The new tissue produced by the nets of inland thalli growing at the coast appeared to maintain the morphological characteristics typical of inland thalli. The results do not appear to support the hypothesis that environmental differences directly control morphological differences between the populations.