THE MOSAIC DISEASE OF CUCURBITS. 19 
NO VISIBLE CAUSAL ORGANISM DISCOVERED. 
CULTURAL STUDIES. 
No parasitic organism has been found to be constantly associated 
with mosaic plants, although many cultures have been made from 
roots, stems, leaves, and fruits. In attempting to isolate a causal 
organism a great variety of media of all types, both vegetable and 
synthetic, were employed, particular use being made of those con- 
taining the juices of the cucumber plant itself. Tissue cultures 
from all parts of the plant remained sterile in most cases, and where 
growth occurred it resulted only from contaminations, no single 
organism being constantly present. Variations in temperature and 
oxygen supply and in acidity of the media were also tried, but these 
experiments were equally unsuccessful. 
The possibility of an increase of the virus in the culture medium 
without visible growth has been tested in many cases by inoculation 
from the medium itself, but no infection has ever appeared. The 
juices of mosaic plants may produce infection after passing through 
a Berkefeld filter, and attempts were therefore made to make cul- 
tures from these filtrates on various media, but without success. 
Similar filtrates of the juices of healthy plants when inoculated from 
infectious filtrates have never shown evidence that the virus could 
increase on such a medium. All attempts to cultivate the virus 
from the filtered juice of mosaic plants are also complicated by the 
fact that the juice loses its infectious character within two to three 
days after it is expressed from the tissues. 
ABSENCE OP ORGANISM IN DISEASED TISSUES. 
The absence of a visible causal organism is further supported by 
the fact that stained sections made from all parts of mosaic plants 
have given no indication of the presence of any parasite in the tissues. 
No differences have been detected between the stems and roots of 
healthy plants and those affected with the mosaic disease. The 
leaves and fruits show some morphologic differences, but no lesions 
of any sort are present and no visible organisms have been found in 
either the fruit or leaves of mosaic plants. All data so far collected, 
therefore, indicate thajt the disease is highly infectious, but no causal 
factor can as yet be associated with infection. On the other hand, 
the expressed juices of mosaic plants possess definite properties 
related to their power of infection. 
NATURE AND PROPERTIES OF THE MOSAIC VIRUS. 
The study of the nature of the infective principle of mosaic dis- 
eases has been most extensive in the case of tobacco, many workers 
having contributed to a knowledge of the properties and character 
of the virus concerned. While the writer's experiments with the 
