EXPERIMENT STATION BULLETINS. 425 



yield, and commimicability of the etiological factor by contact or through the 

 intervention of insects, mainly the Aphididae. Wilting, chlorosis, leaf drop, 

 canker formation, are symptoms specific for certain mosaic diseases but are 

 not generally applicable to the group. 



Histological Lesions in RHatcd Diseases 



Quanjer (4) has found and described a definite necrosis of the phloem 

 tissues in potato plants affected with leaf-roll. His findings have been con- 

 firmed by Artschwager (5). No investigation of the phloem tissues of mo- 

 saic plants has so far been reported, and apparently the significance of the 

 association of necrotic tissues in a disease so closely related to mosaic as leaf 

 roll has not been appreciated. In disease phenomena, cause and effect are 

 usually closety associated, so that it seems reasonable to assume that the 

 finding of certain areas specifically affected by disease indicates also the 

 location of the inciting cause. The location in the phloem of the agents 

 which cause the mosaic diseases is further indicated by observations on mode 

 of transmission. 



Modes of Trditsiiiissioii 



The mosaic diseases, and others, like potato leaf-roll, are transmissible by 

 insects. Plant lice are mainly responsiljle for the widespread dissemination 

 of the infectious material. The feeding habits of these insects offer valuable 

 evidence as to the probable nature and location of the causal organisms. 

 Those species known to transmit these diseases take their food material from 

 the phloem tissues of the host plants, and after feeding for some time upon the 

 veins and parts of the plants close to the phloem they are capable of trans- 

 mitting the disease to healthy plants. Other species which do not attack 

 the food-conducting tissues have not been shown consistently to be carriers 

 of the infectious agents. 



The fact that some of the mosaic diseases are contagious indicates that 

 the causative agents are generally distributed in the plant. Juice expressed 

 from any aerial part of the plant is usually effective in reproducing the 

 disease when inoculated under favorable conditions into healthy plants. A 

 cytological study employing modern improved methods, in a search for pos- 

 sible causative organisms in the phloem of diseased plants has not been re- 

 ported. Investigators have reported meagerly upon observations of tissues 

 in mosaic plants but no exhaustive search has been made of the phloem, 

 apparently the vulnerable point of attack in seeking for the solution of the 

 true nature of the mosaic organisms. 



PRESENT INVESTIGATIONS 



Bean mosaic investigations have been in progress at the Michigan Agri- 

 cultural Experiment Station since 1919. Most of the work has been con- 

 cerned with variety and species susceptibility, and with insect transmission. 

 Some cytological work was clone during 1921 but was not pursued to any con- 

 clusion. As soon as mosaic material was available in the summer of 1922 

 an intensive histological study was projected, using all of the common killing 

 and fixing solutions and botanical stains. The main object of the investi- 



