Comparative histology of healthy and psorosis-affected tissues ofCitrus sinensis

does not appear. First page follows. Introduction The general characteristics and effects of psorosis (scaly bark) have been described by Fawcett(18) and Fawcett and Lee.(21) The first evident expression of the disease is the cracking or rupturing of the outer bark, with the formation of raised scales, pustules, or pimples. As the disease progresses, the wood underlying the affected bark is also commonly involved. The bark symptoms constitute the most conspicuous form of the disease and suggested the name psorosis originally given it by Swingle and Webber.(53) In ensuing years other effects were occasionally noted on small twigs, water-sprouts, mature leaves, and rarely on fruit. More recently a mosaic-like symptom on young leaves has been discovered by Fawcett,(20) suggesting a virus disease. Most of the histological work of this paper was completed before the mosaic-like symptom was discovered, and considerable attention was given to searching for the presence of possible microörganisms in the diseased tissues. This paper deals especially with the differences in structure of normal and diseased bark and wood. Some work in comparing normal and diseased twig and leaf tissues is also included. Material and Methods Specimens showing various stages of psorosis on Valencia orange trees nine years old were collected at different seasons of the year at the Citrus Experiment Station, Riverside, California.