150 DISEASES OF GLASSHOUSE PLANTS. 
mosaic disease have shown that it contains a number of 
separate diseases, each of which has a limited host range. 
Thus tomato mosaic may readily be transmitted to the 
petunia, tobacco, bittersweet, and black nightshade, and 
with difficulty to the potato. It is also possible to cross- 
inoculate any of these from the others. The mosaic 
disease of the cucumber has not been transmitted experi- 
mentally to. any of the above plants. 
Carrier Plants 
Inoculation experiments conducted with large num- 
bers of plants have added facts of considerable value to 
our knowledge of this disease. It is frequently observed 
that a limited number of plants remain apparently 
healthy. They do not develop characteristic lesions, and 
one might expect them to be healthy. When, however, 
the juice from such plants is extracted and pricked into 
healthy plants the latter frequently become diseased. 
This phenomenon has been examined critically, and it has 
been shown that plants may become infected without 
showing any outward signs of the disease. In other 
words, “‘ carrier” plants exist—a fact which has also 
been observed in susceptible weeds, such as the black 
nightshade. The identification of sources of infection 
thus become increasingly difficult, and their complete 
elimination impossible. 
The Effect of Environmental Conditions 
The influence of environmental conditions upon the 
incidence of other disease has proved to be considerable, 
and it is not strange to find that the same occurs in the case 
of mosaic disease. The temperature of the air is im- 
portant in this respect, the development of the symptoms 
being retarded by low temperatures and increased by 
higher ones, in proportion to the extent to which the rate 
of growth is decreased or increased. Thus in the case of 
