38 
BULLETIN 879, IT. S. DEPARTMENT OE AGEICTJLTTJEE. 
tious 12 hours earlier than that from leaves at more distant points. 
In these tests cucumber plants were inoculated either at the base or 
tip of the stem, and other plants were then successively inoculated 
with fragments of leaf tissue from various points on the plant first 
inoculated. These later inoculations were made every 12 or 24 hours. 
The results of this work are given in Table XIY. The fact that mo- 
saic diseased plants possess the power of infection before the appear- 
ance of visible symptoms is a most important factor in its relation to 
disease dissemination and control. Efforts to control cucumber mo- 
saic in the field by the eradication of all diseased plants as soon as 
they appeared have met with little success, largely because of the 
fact that a great number of plants were constantly serving as a source 
of infection during the period when their juices were infectious, but 
before visible symptoms had appeared. 
Table XIY. — Occurrence of mosaic virus in inoculated cucumber plants prior to the 
appearance of risible symptoms. 
Plant 
No. 
Date of first 
inoculation. 
July 18, 
: do 
do 
do..... 
do....! 
1916 
,1911 
Point of inocu- 
lation. 
Tip of stem. . 
Base of stem . 
Tip of stem.. 
Base of stem. 
Tip of stem.. 
Base of stem. 
Date first 
symptoms 
were noted. 
Part used for secondary 
inoculations. 
Date of first 
successful 
secondary 
inoculation. 
Julv 22,1916 Leafattipofstem July 22,1916 
.1 July 24,1916 do July 23,1916 
J do do | do 
f do Leaf at base of stem j do 
' \ do | Leaf at tip of stem do 
/July 25,1916 j Leaf at base of stem 
" \ do Leaf at tip of stem 
Aug. 15, 1917 Tip leaf of snoot 6 inches 
from point of inocula- 
tion. 
Tip leaf of main stem 30 
inches from point of 
inoculation. 
Aug. 16,1917 
Julv 24,1916 
July 23,1916 
Aug. 13, 1917, 
6.45 a. m. 
Aug. 13, 1917, 
4 p. m. 
Time 
virus 
was 
present 
before 
first 
symp- 
toms 
appeared 
(hours). 
4S 
METHOD OF DISTRIBUTION OF THE VIRUS. 
The distribution of the virus throughout the plant is probably 
effected either by cliifusion through the parenchymatous cells or by 
conduction through the vascular elements. THiile the question is 
difficult to determine definitely, it seems more probable, in the 
writer's estimation, that the vascular system is most important, 
although diffusion may also be a factor. When inoculation is made 
at the base of the stem, the first symptoms, which invariably appear 
in the young leaves, develop as rapidly as when inoculation is made 
directly into these leaves. It thus appeal's that the distribution is 
rapid, since the young leaf tissue may contain the virus within three 
days after inoculation in sufficient amount to produce infection in 
other plants. It would seem that diffusion alone would be too slow 
to allow the virus to reach all parts of the plant in as short a time as 
