20 BULLETIN 40, U. S. DEPARTMENT OF AGRICULTURE. 



two leaves of every plant were inoculated in exactly the same manner 

 by injecting the virus into each leaf at two points, one on each side 

 of the midrib. These punctures were made toward the apex of the 

 leaf, about two-thirds the distance from the base to the tip. 



By grasping each leaf near the apex and snapping it off quickly at 

 the point where the petiole joins the stem, the two inoculated leaves 

 were carefully removed from a row of 10 plants on each successive 

 day beginning with March 29. Using this method of detaching the 

 leaves, accidental infection could not be conveyed by the hand to the 

 exposed surface of the severed petiole. 



By removing these two inoculated leaves on successive days the fol- 

 lowing results were obtained : 



Row 1. Two leaves removed March 29 ; all plants remained healthy. 

 Row 2. Two leaves removed March 30 ; all plants remained healthy. 

 Row 3. Two leaves removed March 31 ; all plants remained healthy. 

 Row 4. Two leaves removed April 1; two plants only developed the disease. 

 Row 5. Two leaves removed April 2 ; eight plants became diseased. 

 Row 6. Two leaves removed April 3 ; eight plants became diseased. The 

 symptoms began showing in some of these plants on April 3, the day the 

 leaves were removed. 

 Row 7. The inoculated leaves of row 7 were not removed, since eight of the 

 plants were already diseased on April 4. 



The results in row 3 indicate that the virus had not passed into 

 the stem from the inoculated leaves which were removed three days 

 after inoculation, since none of these plants developed the disease. 

 Allowing the inoculated leaves to remain on the plant four days from 

 inoculation in some instances gave the virus time to pass into the 

 stem. All removals later than this failed to prevent the development 

 of the disease in the plants, showing that the virus had already passed 

 from the leaves into the main stem. 



Experiments have shown that the sap obtained from infected plants 

 between the time of inoculation and prior to the appearance of the 

 first observable symptoms will produce the disease in healthy plants. 



During the writer's experiments the disease developed throughout 

 a series of controls following inoculation with the sap of a sup- 

 posedly healthy plant. Since this plant itself very soon developed 

 symptoms, it is clear that the sap having been obtained during the 

 incubation period of the disease in this plant contained the mosaic 

 virus, the disease coming into expression later. 



DURATION OF THE DISEASE IN AFFECTED PLANTS. 



As the result of their observations of the mosaic disease. Beije- 

 rinck and Woods have mentioned that plants under certain con- 

 ditions sometimes appeared to recover from the disease. 1 Lode- 



1 Some practical growers have claimed that the disease can be checked if taken at its 

 first appearance by pulling affected plants until they are loosened from the soil. There 

 is little in this view, however, to recommend its general adoption. 



